'The Exploration of the Earth's Magnetosphere" can be used in several ways, described in more detail further below:

• (a)   As source material for the high-school or college curriculum,
  especially in physics. astronomy and their history.
  
• (b)   As a source of supplemental projects by high school students
  or by undergraduates.
  
• (c)   As material for independent studies, In particular by
  motivated students who seek to know more about space
  than is provided by regular curricula.
  
• (d)   As the outline of a one-semester introductory undergraduate
  course for non-science majors, which could be titled
  "Space Science for Poets."
  

General Suggestions

1. Any teacher contemplating an extensive use of "Exploration" might find it useful to obtain a self- contained copy (see next item below). The source files can be copied from the browser and stored on hard disk (they also fit 5 HD floppies), and can then called by the browser--e.g using "Open File" on the "File" menu of the Netscape browser.
   With this material on hard-disk memory, "Exploration" can be used without any connection to the internet and it runs quite rapidly. Of course, the many links to other net documents requfre a net connection in order to work.

2. "Exploration" can be freely copied for non-commercial use. Find out here about copying the entire "Exploration" package as a TARed and GZIPed file (about 5 Mb) to your own computer.

3. "Exploration" is best read sequentially, like a book; sections are accordingly numbered, and each is linked to the next one in sequence. Those wishing to look up specific topics are advised to use the index file, accessible from the end of any "Exploration" file. The index file displays a list of titles, from which any file of "Exploration" can be directly reached: an additional notation (+H) at the end can also be clicked, and brings up the attached history file.

4. For a folding 3-dimensional paper model of the magnetosphere, link here

Specific Uses

Relevant files are listed as they appear In the Index file. "S" stands for supplemental files, indented in the index list; "+H" means, include the history file.

(a)   Classroom Material in Physics and Astronomy

Topics listed below, relevant files in parentheses.
Also look up related summary files.

General

(c)    Self Study

Students with a more technical background may want to go beyond the introductory level of "Exploration." A suitable place to start is "A Brief History of Magnetospheric Physics During the Spaceflight Era" which is also included. Space scientists and graduate students, in particular, will find leads to many key articles in the extensive bibliography of that work.

(d)    "Space Physics for Poets "

"Exploration" can provide the outline of a one-semester course on a similar level, which might well be called "Space Physics for Poets." A teacher presenting such a course needs a thorough command of physics and some additional familiarity with the Earth-Sun environment. The article 'A Brief History of Space Physics During the Spaceflight Era" included in these web files contains a great amount of relevant material, and more can be found in its large bibliography and in the file Additional Resources" reached from the home page of "Exploration."

Teachers are also advised to look up the article "Space Physics for Poets" in The Physics Teacher, vol. 35, p. 38-9, January 1997.

Happy Exploring!

G l o s s a r y

Adiabatic invariant--An invariant of a motion is a quantity which does not change as time advances. For instance, the energy of a system is often an invariant (for a swinging pendulum, or a planet and the Sun), and knowing that it stays constant is a great help in calculating the motion.
  Adiabatic invariants are quantities associated with approximately periodic motions. They almost do not change, and thus also help in calculating the motion, to a very good degree of accuracy. They are often important in calculating the way ions and electrons move in a magnetic field.

Alpha particle--A type of fast ion emitted by many types of heavy radioactive nuclei, such as uranium. Actually, the nucleus (atom stripped of all electrons) of the gas helium.

Ampere--see electric current.

Argus (project)--A 1958 experiment by the US military, to create artificial radiation belts by exploding small nuclear bombs above the atmosphere.

Astronomical unit (AU)--The mean Sun-Earth distance, a unit of distance widely used in expressing distances in the solar system. 1 AU = 149,600,000 km = 92,957,000 miles.

Attitude(of a satellite)--The direction in which the satellite is oriented in space.

AU -- acronym for Astronomical Unit, mean distance to the Sun.

Aurora (short for polar aurora)--A glow in the sky, often observed in a ring-shaped region around the magnetic poles ("auroral zone") and occasionally further equatorward. The name comes from an older one, "aurora borealis," Latin for "northern dawn," given because an aurora near the northern horizon (its usual location when seen in most of Europe) looks like the glow of the sky preceding sunrise. Also known as "northern lights," although it occurs both north and south of the equator.
   The aurora is generally caused by fast electrons from space, guided earthward by magnetic field lines, and its light comes from collisions between such electrons and the atoms of the upper atmosphere, typically 100 km (60 miles) above ground.

Aurora, diffuse --see diffuse aurora

Aurora, discrete--see discrete aurora

Auroral acceleration--The process by which auroral electrons acquire their energies, typically 1-10 keV. May be associated with parallel voltage drops or with an interaction between particles and plasma waves, and may be related to magnetic reconnection in the plasma sheet.

Auroral electrojet--see electrojet, auroral

Auroral kilometric radiation--intense radio waves whose wavelength is of the order of a kilometer, emitted from regions above the ionosphere where the aurora is (apparently) accelerated. Since the waves are even longer than those of the AM radio band, they are stopped by the ionosphere and do not reach the ground, but they are readily observed from spacecraft.

Auroral oval--the region in which aurora appears at the same time, corresponding to the "ring of fire" around the magnetic pole, often observed by satellite cameras. It resembles a circle centered a few hundred kilometers nightward of the magnetic pole, and its size varies with magnetic activity. During large magnetic storms it expands greatly, making auroras visible at regions far from the pole, where they are a rare occurence.

Auroral zone--the region on Earth where auroras are common--a smeared-out average (over time and distance from the magnetic pole) of the auroral oval. Typical magnetic latitude is 63-65 degrees.

Barium release --the firing from a rocket or spacecraft above the atmosphere of a charge of barium, evaporated by a thermite process. Usually produced shortly after sunset, when the sky is already dark but sunlight still reaches the high altitude where the release occurs. The barium atoms are released as a vapor, they spread rapidly and are readily ionized by sunlight. The ion cloud then moves with the local plasma and is therefore a useful tracer of plasma flows.

Birkeland currents --electric currents linking the Earth´s ionosphere with more distant regions, flowing along magnetic field lines. Named for Kristian Birkeland, a pioneer of auroral research who first proposed such currents around 1900, these currents are often associated with the polar aurora and with substorms.

Boundary layer --a transition layer between two neighboring regions in the magnetosphere. The plasma sheet boundary layer (PSBL) is the transition from the plasma sheet and the tail lobes. The low latitude boundary layer (LLBL), just inside the magnetopause, is the transition between the equatorial magnetosphere and the solar wind (more accurately, the magnetosheath, solar wind slowed down by passage through the bow shock).

Bow shock --a sharp front formed in the solar wind ahead of the magnetosphere, marked by a sudden slowing-down of the flow near Earth. It is quite similar to the shock forming ahead of the wing of a supersonic airplane. After passing near Earth, the slowed-down flow gains speed again, to the same value as the surrounding solar wind.

Chromosphere--a reddish layer in the Sun´s atmosphere, the transition between the photosphere and the corona

Convection (magnetospheric)--large-scale plasma flow, circulating in the magnetosphere and driven by the solar wind. Plasma physics requires such circulation to be associated with an electric field. Assuming that the electric field propagates along magnetic field lines (as it would along good conductors of electricity) and reaches the polar ionosphere, corresponding electric fields should be observed above the polar caps, and such fields exist.
    In the view proposed in 1961 by Axford and Hines, plasma near the flanks is dragged tailwards by the adjoining solar wind flow, through the action of "viscous-like forces"; in the view suggested that same year by Dungey, plasma travels tailward on "open" field lines following reconnection. Evidence suggests both processes contribute. In both models the plasma returns earthward in the plasma sheet near midnight, a process which could be not continuous but intermittent, associated with substorms.

Corona--see solar corona.

Coronal mass ejection (CME)--a huge cloud of hot plasma, occasionally expelled from the Sun. It may accelerate ions and electrons and may travel through interplanetary space as far as the Earth´s orbit and beyond it, often preceded by a shock front. When the shock reaches Earth, a magnetic storm may result.

Cosmic rays --A steady drizzle of high energy ions arriving at the solar system from the distant universe. Their energies are enormous, ranging from 1-2 billion electron volts to perhaps 100,000,000 that much, though the higher energies are rare. Their total energy flow is comparable to that starlight. The origin of their huge energies is uncertain, but may come from expanding shock fronts created by supernova explosions.

Crab nebula --a cloud-like nebula observed in the Crab constellation, the remnant of a supernova explosion observed in China in 1054. It contains a very rapidly rotating (and hence, young) pulsar, which is probably the remnant of the supernova. The emissions of radio waves and light from this nebula suggest the presence of high energy particles.

Cusps (of the magnetosphere)--two regions of weak magnetic field, on the sunward boundary of the magnetosphere, one on each side of the equator. They separate magnetic field lines closing on the front from those swept into the Earth´s magnetotail.

Diffuse aurora--a spread-out glow often covering much of the auroral oval. It is not seen by the eye but can be observed quite well by satellite cameras. See discrete aurora.

Dipole--a compact source of magnetic force, with two magnetic poles. A bar magnet, coil or current loop, if their size is small, create a dipole field. The Earth´s field, as a crude approximation, also resembles that of a dipole, located near the Earth´s center.

Discrete aurora (or "auroral arcs") are the typical ribbon-like structures of aurora observed from the ground. From space they may appear as brighter spots in the diffuse aurora.

Drift--A magnetically trapped ion or electron moves as if it were attached to a magnetic field line. Drift is one of the features of such motion, namely its slow shift from one guiding field line to its neighbor. In the Earth´s magnetic field, such drifts gradually move particles all the way around Earth. Viewed from far above the north magnetic pole, ions drift around the Earth clockwise, electrons counter-clockwise, resulting in an electric current circling the Earth, the ring current.

Dynamo process--the generation of an electric currents by the flow of an electrically conducting fluid through a magnetic field. For instance, the magnetic field originating inside the Earth is believed to come from a dynamo process involving the flow of molten iron in the Earth´s hot core. The energy required to create the current is obtained from the motion of the flow.

Earth radius (RE)--the average radius of the Earth, a convenient unit of distance in describing phenomena and orbits in the Earth´s neighborhood in space. 1 RE = 6371 km = 3960 miles, approximately.

Edison effect--the flow of an electric current through a laboratory vacuum, between two metal wires, one of which is heated. The current flows only when the heated wire is more negative, because it is due to free electrons released from the wire by heat. The Edison effect made possible "vacuum tubes" used in radio and television equipment before the invention of the transistor.

Electric charge--that which causes electrons and ions to attract each other, and to repel particles of the same kind. The electric charge of electrons is called "negative" (-) and that of ions "positive" (+). Materials such as glass, fur and cloth acquire an electric charge by rubbing against each other, a process which tears electrons off one substance and attaches it to the other. Electric charges (+) and (-) may also be separated by a chemical process, as in an electric battery. About Ben Franklin's role in studying and naming electrical charges, click here.

Electric current--a continuous flow of electrons and/or ions, through a material with conducts electricity. A currents usually flows in a closed circuit, without beginning or end. In daily life currents are generally driven through wires by voltages produced by batteries or generators. In space plasmas, some currents may be produced this way, but many are inherent to the way ions and electrons move through magnetic fields, e.g. their drifts.

Electric field--the region in which electric forces can be observed, e.g. near an electric charge. As a field, it may also be viewed as a region of space modified by the presence of electric charges.

Electrojets, auroral--two intense electric currents, flowing around the auroral oval from the day side towards the night side and meeting somewhat west of midnight. Associated with Birkeland currents and caused by the unusual electric conductivity properties of ionospheric plasma, the electrojets are responsible for practically all of the magnetic disturbance observed on the ground due to substorms. Their magnitude (derived by analyzing such disturbances) often serves as a convenient gauge of the intensity of substorm activity.

Electromagnet--a magnet powered by an electric current. Usually the current flows in a coil, which may or may not contain a core of iron or of some other magnetic material.

Electromagnetic field (EM field)--the regions of space near electric currents, magnets, broadcasting antennas etc., regions in which electric and magnetic forces may act. Generally the EM field is regarded as a modification of space itself, enabling it to store and transmit energy.

Electromagnetic wave--a combination of oscillating magnetic and electric field, spreading in wavelike fashion through space at a speed of about 300 000 km.sec. James Clerk Maxwell´s theory in 1864 suggested that light was such a wave, and today we know that such waves include all forms of light--also infra-red and ultra-violet, as well as radio waves, microwaves, x-rays and gamma rays.

Electron--a lightweight particle, carrying a negative electric charge and found in all atoms. Electrons can be energized or even torn from atoms by light and by collisions, and they are responsible for many electric phenomena in solid matter and in plasmas. (About the discovery of the electron in 1897, click here.

Electron volt (ev)--a convenient unit of energy applied to ions and electrons, equal to the energy gains when such particles "fall" across a voltage difference of 1 volt. Gas molecules at room temperature have about 0.03 ev, on the Sun´s face about 0.6 ev, typical electrons of the aurora 5000 ev, typical protons in the inner radiation belt 20,000,000 ev, typical cosmic ray protons near Earth 10,000,000,000 ev, and the highest energies of cosmic rays may reach up to 10,000,000,000 times more.

Electroscope--a simple instrument, indicating the presence of electric charge by the spreading-apart of two leaves of metal foil, hanging next to each other inside a glass jar. The rate at which an electroscope in dry air loses its charge is a rough indicator of the prevailing level of ionizing radiation.

Energetic particles--charged atomic particles moving rapidly, often at a significant fraction of the speed of light. They can penetrate matter, ionize the material which they traverse and emit energetic photons (e.g. of x-rays). See also radiation belt, cosmic rays, solar energetic particles.

Energy--loosely, anything that can cause a machine to move. For example, energy is contained in moving water, water raised to a high place, heat or magnetic fields. The energy of fast ions and electrons (measured in " electron volts") is a measure of their speed, and it enables them (for instance) to penetrate matter.

Explorer 1--first satellite launched by the US, around midnight of January 31, 1958. Carrying Geiger counters, Expolorer 1 and the similar Explorer 3 (launched two months later) discovered the existence of a belt of magnetically trapped energetic particles around Earth.

Field line preservation--a property of fluids which are perfect conductors of electricity (including "ideal plasmas"), by which two particles which initially share the same field line, continue to do so into the future. The opposite also holds for such fluids: two particles which start out on different field lines will always be on different field lines.

Frequency--the number of back-and-forth cycles per second, in a wave or wave-like process. Expressed this way, the frequency is said to be given in units of Hertz (Hz), named after the scientist who first produced and observed radio waves in the lab. Alternating current in homes in the US goes through 60 cycles each second, hence its frequency is 60 Hz; in Europe it is 50 cycles and 50 Hz.

Flare--see solar flare

Gamma ray bursts--brief bursts of gamma rays from the distant universe, observed by satellites.

Gamma rays--electromagnetic waves of the highest frequencies known, originally discovered as an emission of radioactive substances.

GCM--guiding center motion

Geiger counter--a simple electronic detector of energetic particles. It consists of a thin straight wire at a high positive voltage (usually close to 1000 volts) relative to a cylindrical electrode surrounding it. Geiger counters can detect ("count") high-energy particles, but they cannot identify their type or distinguish their energy.

Geocorona--the outermost layer of the Earth´s neutral atmosphere, a huge cloud of hydrogen surrounding our planet. Its density diminishes with distance and it has been observed up to distances of 5-6 Earth radii.

GSM coordinates--geocentric solar magnetospheric coordinates, the system in which locations in the large-scale magnetosphere are usually given. The main axis ("noon-midnight") points at the Sun, and the plane of symmetry ("noon-mignight plane") contains the Earth´s magnetic axis, which however needs not be exactly perpendicular to the sunward direction.

Guiding center--An ion and electron in a magnetic field, of suitably low energy, is constrained to circle ("gyrate") around a local magnetic field line, while the center of its circular motion slides up or down along the line and also slowly shifts from one guiding field line to its neighbor, following certain rules. The center of that circle is known as the particle´s guiding center and the entire mode of mottion is called guiding center motion. See gyration, magnetic mirroring, drift.

Gyration--a term used in plasma studies for the circular motion of an ion or electron around its guiding center.

IMF--interplanetary magnetic field (see below).

IMF polarity--the general direction of interplanetary magnetic field lines in a certain location (e.g. near Earth), i.e. whether they head away from the Sun ("away polarity") or towards it ("towards polarity"). The IMF polarity determines which of the polar caps of the Earth is magnetically linked to the Sun and gets polar rain guided towards it. See interplanetary sector.

Inner magnetosphere--the region of the magnetosphere in which ions and electrons are relatively stably trapped. Approximately the region threaded by field lines which cross the equator within synchronous orbit, i.e. within 6.6 Earth radii.

Interplanetary magnetic field (IMF)--the weak magnetic field filling interplanetary space, with field lines usually connected to the Sun. The IMF is kept out of most of the Earth´s magnetosphere, but the interaction of the two plays a major role in the flow of energy from the solar wind to the Earth´s environment.

Interplanetary shock--the abrupt boundary formed at the front of a plasma cloud (e.g. one from a coronal mass ejection) if it pushes its way through interplanetary space much faster than the rest of the solar wind. See bow shock. For an article about the impact of a large shock on the magnetosphere, click here.

Interplanetary sector--a region of interplanetary space in which all magnetic field lines point either away from the Sun ("away sector") or towards the Sun ("towards sector"). The Earth´s orbit typically contains 4 sectors, but 2 or 6 are not unusual; they are caused by waviness of the current sheet separating magnetic field lines from opposite polar regions of the Sun.

Ion--usually, an atom from which one or more electrons have been torn off, leaving a positively charged particle. "Negative ions" are atoms which have acquired one or more extra electrons, and clusters of atoms can also become ions.

Ionic Theory--in chemistry, the theory (by Svante Arrhenius, 1884) which first explained the behavior of acids, alkalis (bases) and salts when dissolved in water. By the ionic theory, each molecule of such materials consists of molecular or atomic groupings charged with positive or negative electricity ("ions"), held together by their mutual electrical attraction. In water these electrical forces are greatly weakened, the groupings often get separated, and if an electric current flows, positive and negative ones migrate with it in opposite directions.

Ionization--the process by which a neutral atom, or a cluster of such atoms, becomes an ion. This may occur, for instance, by absorbtion of light ("photoionization") or by a collision with a fast particle ("impact ionization"). Also, certain molecules (such as table salt or sodium chloride, NaCl) are formed by natural ions (like Na+ and Cl-) held together by their electric attraction, and they may fall apart when dissolved in water (which weakens the attraction), enabling the solution to conduct electricity.

Ionosphere--a region covering the highest layers in the Earth´s atmosphere, containing an appreciable population of ions and free electrons. The ions are created by sunlight ranging from the ultra-violet to x-rays. In the lowest and least rarefied layer of the ionosphere, the D-layer (around 70 km or 45 miles), as soon as the Sun sets the ions and electrons recombine, but in the higher layers, collisions are so few that its ion layers last throughout the night

Lagrangian point--in a system dominated by two attracting bodies (such as Sun and Earth), a point at which a third, much smaller body (such as a satellite) keeps the same position relative to the other two. Theoretically, the Sun-Earth system has 5 Lagrangian points, but only two are important: L1 (L-one), on the sunward side of Earth, about 4 times the distance of the Moon, and L2 at approximately the same distance on the midnight side.

LLBL--low latitude boundary layer. See boundary layers.

Magnetic field--a region in which magnetic forces can be observed. See "electromagnetic field," a more general field also including electric forces.

Magnetic field lines--lines in space, used for visually representing magnetic fields. At any point in space, the local field line points in the direction of the magnetic force which an isolated magnetic pole at that point would experience. In a plasma, magnetic field lines also guide the motion of ions and electrons, and direct the flow of some electric currents.

Magnetic latitude--geographic latitude of a location, in a system of latitudes and longitudes whose axis is not the rotation axis of the Earth but the magnetic axis, i.e. the axis of the dipole at the Earth´s center which best fits the internal magnetic field. The auroral zone, for instance, is near magnetic latitude 65 degrees. See magnetic local time.

Magnetic lines of force --Michael Faraday´s original term for what is now widely called magnetic field lines.

Magnetic local time (MLT)--in the a system of latitude and longitude whose axis is the dipole axis, magnetic local time is the longitude, measured not in degrees but in hours (1 hour = 15 degrees).
   The zero of this longitude is not fixed relative to Earth (the way the Greenwhich meridian is for geographic longitude), but rather relative to the Sun: the line of magnetic longitude facing the Sun always has MLT = 12 hours ("magnetic noon"), and the opposite one has MLT = 0 or 24 hours ("magnetic midnight"). See magnetic latitude

Magnetic mirroring--the process by which an ion or electron, constrained by its guiding center motion to follow a magnetic fields line, slows its advance down that line as it enters a region of stronger magnetic field, and is ultimately turned back ("mirrors") at a certain "mirror point."
    Mirroring is what makes possible long-term trapping of ions and electrons in the Earth´s radiation belts. In the inner magnetosphere, ions and electrons are confined between two mirror points, one north of the equator and one south of it. These turn them back before their motion along the guiding field line reaches the atmosphere, where they might otherwise have been lost by colliding with molecules of air.

Magnetometer--intrument for measuring magnetic fields. Spacecraft often carry fluxgate magnetometers, which measure components of the magnetic field (3 of them are combined to give its strength and direction) but need to be calibrated. Rubidium-vapor and similar instruments measure only the strength, but their reading is absolute, related to atomic constants.

Magnetic poles --A term with two meanings:

• (1) the points on the surface of the Earth towards which the compass needle points. (Several slightly different definitions exist, because the field is not exactly that of a dipole.)
• (2) A concentrated source of magnetic force, e.g. a bar magnet has two magnetic poles near its ends.

Magnetic reconnection--In a plasma, the process by which plasma particles riding along two different field lines can be made to share the same field line (see field line preservation). For instance, following reconnection, solar wind particles on an interplanetary field line, and magnetospheric ones on a field line attached to Earth, may find themselves sharing the same "open" field line, which has one end anchored on Earth and the other extending to distant space.
   Magnetic reconnection can occur when plasma flows through a neutral point or a neutral line at which the intensity of the magnetic field is zero and its direction is not defined. It is an important concept in the theories of energy transfer from the solar wind to the magnetosphere and of energy release in substorms.

Magnetic storm--A large-scale disturbance of the magnetosphere, often initiated by the arrival of an interplanetary shock originating at the Sun.
   A magnetic storm is marked by the injection of an appreciable number of ions from the magnetotail into the ring current, a process accompanied by increased auroral displays. The strengthened ring current causes a world-wide drop in the equatorial magnetic field, taking perhaps 12 hours to reach its greatest intensity, followed by a more gradual recovery.

Magnetometer--an instrument for measuring magnetic fields. Spacecraft often carry fluxgate magnetometers, which measure components of the magnetic field (3 of them are combined to give its strength and direction) but they need to be calibrated.
    Rubidium-vapor and similar instruments measure only the field strength, but their reading is absolute, related to atomic constants.

Magnetopause --The boundary of the magnetosphere, separating plasma attached to Earth from the one flowing with the solar wind.

Magnetosheath--the region between the magnetopause and the bow shock, containing solar wind which has been slowed down by passage through the bow shock. As the magnetosheath plasma streams away from the bow shock, it gradually regains its former velocity.

Magnetosphere--The region around Earth, bounded by the magnetopause, whose processes are dominated by the Earth´s magnetic field.

Magnetotail--The long stretched-out nightside of the magnetosphere, the region in which substorms begin. It starts about 8 Earth radii (RE) nightward of the Earth and has been observed to distances of at least 220 RE. See plasma sheet, tail lobes,

NENL--near-earth neutral line (see below).

Neutral point --A point at which the magnetic intensity is zero. Plays an important role in magnetic reconnection.

Neutral line (NL)--A line along which the magnetic intensity is zero. Like a neutral point, a NL can play an central role in magnetic reconnection, but because of physical reasons, it may be a more likely setting for the actual reconnection process.

Neutral line, near-earth (NENL)--a neutral line which many believe forms in the plasma sheet during magnetic substorms.

Neutral line, distant --A neutral line in the distant magnetotail where (by Dungey´s theory) interplanetary field lines which were split apart by magnetic reconnection when they first encountered the magnetosphere are once more re-united.
   There is little doubt such re-uniting takes place: what is unclear is the exact manner in which it happens.

Northern lights--an older name for the polar aurora.

Orbit--the line followed by a spacecraft or a celestial body. See Sun synchronous orbit, synchronous orbit.

Parallel voltage drops ("parallel electric fields")--voltage drops along magnetic field lines.

Particle--in general, a charged component of an atom, that is, an ion or electron.

Photon --colloquially, a "particle of light." Although light spreads as an electromagnetic wave, it can be created or absorbed only in discrete amounts of energy, known as photons. The energy of a photon is greater the shorter the wavelength--smallest for radio waves, larger for visible light, largest for x-rays and gamma rays.

Photosphere--The layer of the Sun from which all visible light reaches us. The Sun is too hot to have a solid surface and the photosphere consists of a plasma at about 6000 degrees centigrade.

Planetary magnetospheres --the magnetospheres of planets, especially of Jupiter, Saturn, Uranus and Neptune, all of which have dipole-like magnetic fields stronger than the Earth´s. Mercury has a weak magnetic field, Mars and the Moon are magnetized in patches (probably on their surfaces) and Venus, although non-magnetic, has its own interaction with the solar wind, by means of its thick ionosphere.

Plasma --a gas containing free ions and electrons, and therefore capable of conducting electric currents. A "partially ionized plasma" such as the Earth´s ionosphere is one that also contains neutral atoms.

Plasma physics --the study of plasma phenomena--in the laboratory, where it may one day help extract energy from hydrogen fusion, in the Sun and the distant universe, in the Earth´s ionosphere and in the magnetospheres of Earth and other planets.

Plasma sheet --a near-equatorial layer of denser plasma in the tail of the Earth´s magnetosphere. It separates the two tail lobes, the two bundles of magnetic field lines connected to the regions around the Earth´s magnetic poles.

Plasmasphere --A region of relatively dense but cool plasma, surrounding Earth, extending to distances of about 5 Earth radii (RE). The plasmasphere is the upward extension of the Earth´s ionosphere, getting less and less dense with increasing distance, and it shares the Earth´s rotation.

Polar caps --in magnetospheric usage, the regions around the Earth´s magnetic poles, inside the auroral oval. The field lines in these regions extend into the tail lobes of the Earth; they reach great distances and do not close in the magnetosphere.

Polar orbit --a satellite orbit passing over both poles of the Earth. During a 12-hour day, a satellite in such an orbit can observe all points on Earth.

Polar rain --a drizzle of electrons observed inside the polar caps, apparently from the high end of the energy distribution of solar wind electrons. Its origin in the solar corona is revealed by the fact that in general only one polar cap receives it at any time--the one which (depending on IMF polarity) is linked to the Sun.

Proton --an ion of hydrogen and one of the fundamental building blocks from which atomic nuclei are made.

Radiation --a term with two broad meanings:

• In the narrow sense, some type of electromagnetic wave: radio, microwave, light (infra-red, visible or ultra-violet), x-rays or gamma rays are all types of radiation.
  
• Colloquially, the full term is "ionizing radiation" and means any spreading emission which can penetrate matter and ionize its atoms. That includes x-rays and gamma rays, but also high-energy ions and electrons emitted by radioactive substances, accelerated by laboratory devices or encountered in space (e.g. the "radiation belt" and "cosmic rays," also known as the "cosmic radiation").

Radiation belt --The region of high-energy particles trapped in the Earth´s magnetic field.

Radioactivity --Instability of some atomic nuclei, causing them to change spontaneously to a lower energy level or to modify the number of protons and neutrons they contain. The 3 "classical" types of radioactive emissions are (1) alpha particles, nuclei of helium (2) beta-rays, fast electrons and (3) gamma-rays, high-energy photons.

Radio Astronomy--the observation of radio waves from the Sun, planets and the distant universe. In many cases these are signature of energetic particles.

Radio waves--Electromagnetic waves of relatively low frequency.

Reconnection--see "Magnetic reconnection"

Ring current--A very spread-out electric current circling around the Earth, carried by trapped ions and electrons.

Shock--A sudden transition at the front of fast flow of plasma or gas, when that flow moves too fast for the undisturbed gas to move out of its way. Also occurs when a steady fast flow hits an obstacle.

Solar corona--the outermost layer of the Sun´s atmosphere, visible to the eye during a total solar eclipse; it can also be observed through special filters and best of all, by X-ray cameras aboard satellites. The corona is very hot, up to 1-1.5 million degrees centigrade, and is the source of the solar wind

Solar energetic particles--high energy particles occasionally emitted from active areas on the Sun, associated with solar flares and coronal mass ejections. The Earth´s magnetic field keeps them out of regions close to Earth (except for the polar caps) but they can pose a hazard to space travelers far from Earth.

Solar flare--a rapid outburst on the Sun, usually in the vicinity of active sunspots. A sudden brightening (only rarely seen without special filters) may be followed by the signatures of particle acceleration to high energies--x-rays, radio noise and often, a bit later, the arrival of high-energy ions from the Sun.

Solar wind--hot solar plasma spreading from the solar corona in all directions, at a typical speed of 300-700 km/sec. It is caused by the great heat of the corona.

Space tether--see tether, space

Space Weather--the popular name for energy-releasing phenomena in the magnetosphere, associated with magnetic storms, substorms and interplanetary shocks.

Substorm--a process by which plasma in the magnetotail becomes energized at a fast rate, flowing earthward and producing bright auroras and large Birkeland currents, for typical durations of half an hour.

Sun--the star at the center of our solar system. The Sun keeps Earth warm and sustains life on it, and it also emits the solar wind and occasional bursts of solar energetic particles.

Sunspot--An intensely magnetic area on the Sun´s visible face. For unclear reasons, it is slightly cooler than the surrounding photosphere (perhaps because the magnetic field somehow interferes with the outflow of solar heat in that region) and therefore appears a bit darker. Sunspots tend to be associated with violent solar outbursts of various kinds.

Sunspot cycle (or solar cycle)--an irregular cycle, averaging about 11 years in length, during which the number of sunspots (and of their associated outbursts) rises and then drops again. Like the sunspots, the cycle is probably magnetic in nature, and the polar magnetic field of the Sun also reverses each solar cycle.

Sun-synchronous orbit --a near-Earth orbit resembling that of a polar satellite, but inclined to it by a small angle. With an appropriate value for the inclination angle, the equatorial bulge causes the orbit to rotate during the year once around the polar axis. Such a satellite then maintains a fixed position relative to the Sun and can, for instance, avoid entering the Earth´s shadow.

Supernova--a large explosion at the end of the evolutionary process of many stars. (Strictly speaking, all that follows applies to a "type II" supernova.)
   A star such as the Sun is kept "puffed up" to its apparent size by the heat which nuclear reactions create in its core. Once its nuclear fuel is used up, the pull of gravity overcomes all other forces and makes the star contract to a very small size. The star´s atoms or even its nuclei are then crushed, and the process may turn it into a pulsar or black hole.
   An enormous amount of energy is released in this last collapse, blowing off the star´s outer layers as a rapidly expanding cloud of gas. It is widely believed that powerful shock fronts form ahead of this cloud´s advance, and through them some ions get accelerated to the very high energies of cosmic rays.

Synchronous orbit --a circular orbit around the Earth´s equator, at a distance of 6.6 Earth radii. At this distance the orbital period is 24 hours, keeping the satellite "anchored" above the same spot on Earth. This feature makes the synchronous orbit useful for communication satellites: a satellite transmitting TV programs to the US, for instance, will always be in touch with the US if "anchored" above it, and receiving antennas on the ground only need to point to one fixed spot in the sky.

Tail lobes--the two bundles of nearly-parallel magnetic field lines which stretch into the magnetotail, on opposite sides of the plasma sheet. The northern lobe contains field lines entering the north polar region of Earth, while the southern lobe contains lines emerging from the southern polar region.

Terrella--a small magnetized sphere, used as laboratory model of the Earth. About Birkeland's terrella experiments (1896), here

Tether, space--an experiment in which a satellite was released from the space shuttle at the end of a long insulated cable. The plan was for the dynamo process due to the motion of the tether through the Earth´s magnetic field to generate a large current in the cable.

Ultraviolet (UV)--electromagnetic radiation resembling visible light, but of shorter wavelength. UV cannot be seen by the eye, and much of it is absorbed by ozone, a variant of oxygen, at altitudes of 30-40 km. Satellite telescopes, however, can and do view stars and the Sun in UV, and even in the extreme UV (EUV), the range between UV and X-rays.

Vector--a physical quantity having both magnitude (= strength or intensity) and direction. The magnetic field observed at any point in space is a vector; other examples are velocity, acceleration, force and the electric field, which maps the electric force acting on ions and electrons. Equations involving vectors tend to be more complicated, as they have to describe three-dimensional structure.

Voltage--a sort of "electric pressure," gauging the electric force acting on ions or electrons (or more accurately, the amount of energy they might obtain from that force). In electric devices such as are used in the home, increasing the voltage increases the current--just as increasing the pressure driving water through a pipe increases its flow rate. (The scientific term is "potential" or "potential difference".)

X-rays--electromagnetic waves of short wavelength, capable of penetrating some thickness of matter. Medical x-rays are produced by letting a stream of fast electrons come to a sudden stop at a metal plate; it is believed that X-rays emitted by the Sun or stars also come from fast electrons.

Please note!

Listed below are questions submitted by users of "The Exploration of the Earth's Magnetosphere" and the answers given to them. This is just a selection--of the many questions that arrive, only a few are listed. The ones included below are either of the sort that keeps coming up again and again--the danger of solar eruptions, the reversal of the Earth's magnetic field, etc.--or else the answers make a special point, going into extra details which might interest other users.

AND OH!   Please note--this is a fairly long file and may take a while to load.

1. Reversals of the Earth's field (4 queries)
2. Can the Earth's field be used for spaceflight?
3. The Sun's magnetic poles
4. Synchronous satellites
5. Magnetic field lines
6. Alternate theory of the Sun and solar wind
7. The Geiger counter (2 queries)
8. Measuring the Earth's magnetic field
9. The strength of the Earth's field
10. Solar Eclipses
11. Magnetometer for Observing Magnetic Storms
12. Cosmic Rays FAQs2.html
13. Magnetic Shielding
14. Use of solar wind for space propulsion
15. A working model of the magnetosphere?
16. The Van Allen Belt
17. Magnets of different shapes
18. On building an electromagnet
19. Capturing the Energy of the Solar Wind
20. About the Upcoming Solar Maximum
21. Lining-up of Planets
22. Radiation Hazards to Air Crews
23. The Ozone Hole and the Magnetic Field
24. How are Ions produced?
25. About the "Starfish" artificial radiation belt
26. How do Magnetic Reversals affect Animal Migrations?
27. Which is the "True" North Magnetic Pole?
28. Electric and Magnetic Energy
29. Any connection between Solar Wind and Solar Flares?
30. Ozone and the Magnetic Field
31. What if the Radiation Belt Reached the Ground... ?

If you have a relevant question of your own, you can send it to
u5dps@lepvax.gsfc.nasa.gov.
Before you do, though, please read the instructions

1. Question #1-A

   I need to know as much as possible about the reversal of the magnetic field:
   • how it was noticed
   • who discovered the reversal
   • how long ago did it reverse
   • how many times did it reverse
   • more information about the radiation from the sun if the magnetic field reverses.

   Reply

   ABOUT GEOMAGNETIC REVERSALS: This is a huge subject and I cannot do quick justice to it: look up in the index volume of the Britannica under: Geomagnetism, Plate tectonics, Reversals of the Earth's magnetic field.

   HOW IT WAS NOTICED: When lava pours from a volcano, it solidifies to a black rock called basalt. Basalt is slightly magnetic, and it takes on the direction of the surrounding magnetic field at the time it solidifies. Scientists examined lavas for their magnetism early in this century (I believe) to see how consistent the direction of ancient magnetic fields was with the direction we observe now (would compasses point in the same direction?). The directions generally agreed, but there existed reversals of directions which suggested that there were times in the past when the poles were roughly interchanged.

   No one knew what to make of it. Some suggested "polar wandering", that the whole surface of the Earth slid around the interior like a loose shell.

   WHO DISCOVERED: I don't remember. Check a book by Allan Cox, a collection of historic articles.

   But a big change happened in 1963. People noted that while rocks on Earth were magnetized in a disordered way, the sea bottom was magnetized in long strips. Larry Morley (whose article was regarded so speculative that journals would not publish it) and then Matthews and Vine (who managed to publish) suggested that molten rock was spreading out like a conveyer belt from volcanic cracks in the middle of the ocean floor, e.g. the one in the middle of the Atlantic (Azores islands sit on it). Or rather like 2 belts, one moving towards Europe, one towards America, carrying on them the continental plates, so that Europe and America gradually drift apart. As each belt comes out of the crack, its lava solidifes to basalt, causing it to become magnetized, and when the field reverses, its magnetization reverses too. So the bottom of the ocean records the field like the tape of a tape recorder, containing perhaps 50 million years of record.

   HOW LONG AGO: about 700,000 years, according to the "tape recorder"

   HOW MANY TIMES: Many, about half a million years apart on the average.

   RADIATION FROM THE SUN: Sunlight of course is undisturbed. High-energy protons from the Sun are usually diverted by the magnetic field. During the reversal the field probably does not disappear, but becomes complex and weaker, and protons can more easily reach the atmosphere, as they do now within 1000 miles or so of the magnetic pole. On the ground it makes no difference because the thick atmosphere shields us very well, and none of the protons penetrates far into it.

   David P. Stern

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   Question #1-B

   
   

   Reversal of magnetosphere

   We have been studying the magnetosphere and the Van Allen radiation belts in a high school physical science class. It has been brought to our attention that the magnetic poles of the earth reverse on an average of about every 500,000 years. The last change was about 700,000 years ago, so it would appear that we are long overdue.

   What are the implications of this? How significant would the fluctuation of the magnetic field during such a change be on our protection from solar wind?

   Ricky

   Dear Ricky

   Only yesterday a similar question was submitted, so as a shortcut a copy of it [next item below] and its answer are attached below.

   Some people worry that during magnetic reversals the Earth would receive a higher dosage of high-energy ions and electrons ("radiation" in common terms), which might affect us and any living creatures on Earth. This is not so. Even today, the magnetic shield is not effective near the magnetic poles, yet the radiation received there on the ground is only slightly higher than anywhere else. The reason is that our main shield against such particles is not the magnetic field of the Earth but the atmosphere, equivalent to some 10 feet of concrete.

   In any case, during reversal the magnetic field does not go away, it only gets weaker and develops several more magnetic poles, at unpredictable locations.

   ---

   Question #1-C

   Could you tell me when the earth's magnetic poles will change, and what will happen when it does? Will it happen fast (seconds) or slowly? Thank you!
   Sarah

   Dear Sarah

   No one knows when the next field reversal will occur: in the past, they have occured on the average about once in 700,000 years. The change, whenever it occurs, will be gradual and the field will not drop to zero in between--doing so would mean that the magnetic energy of the Earth was somehow converted or dissipated, and all processes we know for this tend to run on scales of thousands of year, if not more. Right now the main (dipole) field is getting weaker at a rate of about 7% per century, and if you draw a straight line through the points you find it reversing between 1000 and 2000 years from now. It might happen, although the trend may well change. The energy of the field, however, has hardly changed. What seems to have happened is that the more complicated parts of the field (equivalent to several magnets in different directions) have got stronger while the main two-pole ("dipole") field lost strength. The complex field is somewhat weaker (it drops off faster with distance from the source, which is the core of the Earth), but we should not expect the field to be ever greatly weakened.

   The polar field of the Sun seems to reverse every 11 years or so, taking about a year or more. But the Sun's magnetism is different, it has foci right on the surface, in sunspots.

   Hope this answers it.

   David Stern

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   Question #1-D

   
   

   Earth's magnetic field weakening--leading to a pole shift?

   I am just a tax paying citizen, interested in astromony all of my life. I am very interested in the physics of our earth which I believe is related to astronomy as it is our home and a part of this solar system.

   My question is: Is the earths magnetic field weakening, heading to zero point? With this, is the base pulse frequency of the earth speeding up causing the magnetic fields to fluctuate so that it interferes with the pilots navigational equipment, so that the navigational charts have to be redrawn periodically and the air strips renumbered? Are the magnetic poles fluctuating? My experience is that they are. I have a quality, liquid filled compass secured to my desk. It has been very still now for the past month but the six weeks or so prior to that, there were consistant fluctuations in its direction, up to as much as 2 1/2 degrees, always to the west.

   My understanding is: I have seen photographs of the sun taken from satellites, showing the sun going through major activity. Repolarizing itself? Causing the earth to repolarize itself? Going through a natural cycle as it has many times in the past with pole shifts? On a scale from 1 to 10, with 1 being the weakest and 10 the strongest, 2,000 years ago it was a 10, today it is a 1. Is it heading for a zero point when a pole shift will occure? The closer it gets to the zero point, the more fluctuations will occur?

   Are the change in the magnetic frequencies causing at times a confusion in migratory animals? Causing cells to mutate, changing the DNA pattern within the cell? Causing certain strains of bacteria such as staph infections to become resistant to our antibiotics and causing new viruses to appear that we have never seen before, being able to survive in a new magnetic frequency?

   I believe these are very facinating times in which we live. The science of all of this intrigues me to no end. I have some taped interviews of scientists and geologists relating to this subject and I read all that I can get my hands on, on the subject also. Your straightforward comments and answers will be most welcomed to help me to understand more, what is taking place. Thank you so very much.

   Michael

   Reply Dear Michael

   A question on reversals appears in a list of questions and answers, at

   http://www.phy6.org/Education/FAQ.html

   and two more are due to be posted there soon. Also, if you look up "Exploration of the Earth's Magnetosphere" you will see that the reason the Earth has a magnetic field is not any polarization, but electric currents flowing in the Earth's core. You will also find there a great deal of material on the Sun's magnetic field and its relation to sunspots and their cycle.

   Now to your questions.

   Is the Earth's field getting weaker? Yes and no. That field is often viewed as being a two-pole ("dipole") structure similar to that of a small bar-magnet at the center of the Earth, inclined by about 11 degrees to the rotation axis of the Earth, so that the magnetic poles are not the same as the geographic ones. But the actual situation is more complicated, and magnetic charts note the fact by mapping deviations between magnetic north and the direction to the magnetic pole, which fit no simple pattern.

   Why? Because the magnetic field is actually more complicated, and it contains additional fields, of more complex nature. All this originates in the Earth's core, about half the radius of the Earth. If we could go to the surface of the core, all the complicated parts would be much bigger. But they weaken more rapidly with distance, so at the surface of the Earth they are already quite weak, while the "dipole" part stands out more (in addition of actually BEING the biggest chunk of the field).

   Are you still with me?

   The magnetic field of the Earth changes all the time, and yes, magnetic charts have to be redrawn from time to time (this was first found in 1641, by an Englishman named Gellibrand). And yes, in the century and a half since the first careful mapping of the Earth's field, the dipole has become weaker by about 8% (the rate may have speeded up in 1970). If you draw a straight line through the points, you will find that perhaps 1200 years from now, the line goes through zero.

   Extending straight lines too far beyond the present, however, is risky business, as noted by no less a scientific authority than Mark Twain. In "Life on the Mississippi" Twain noted that the Mississippi river was getting progressively shorter (mainly by floods--and by people--creating shortcuts through bends in the river) and he wrote:

   "Now, if I wanted to be one of those scientific people, and "let on" to prove what had occured in the remote past by what had occured in a given time in the recent past, or what will occur in the far future by what has occured in late years, what an opportunity is here! ... Please observe:

   In the space of one hundred and seventy six years the lower Mississippi has shortened itself two hundred and forty-two miles. That is an average over a mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the lower Oolitic Silurian Period, just a million years ago next November, the lower Mississippi was upward of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing rod. And by the same token any person can see that seven hundred and forty years from now the lower Mississippi will be only a mile and three quarters long, and Cairo and New Orleans will have joined their streets together, and will be plodding comfortably along under a single mayor... There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment in fact."

   It is not impossible that the magnetic field will go through zero 1200 years from now, but (judging by the past record of reversals) not likely. In any case, the field is not going away: when one uses observations on the surface to reconstruct fields at the core, one finds that while the dipole field is getting weaker, the complicated parts are getting stronger, and the total magnetic energy does not change, within our observational accuracy. That's why I wrote "yes and no."

   I don't know about migrating animals (they may have magnetic organs, sort of built-in compasses), but there seem to exist no magnetic effects on DNA, resistance to antibiotics and so on; those changes seem more related to chemistry.

   Finally, be cautious with compass readings in your house. Houses do contain electric currents and machinery, and these may affect the readings of a magnetic compass. On NASA's satellites the magnetic sensor usually sits at the end of a long boom, to keep it away from interfering electric currents in the satellite's circuits.

   Keep up your interest in science!
   David

   Back to the main list

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2. Question #2

   student question on the strength of the Earth's magnetic field (from Texas)

   Dear Mr. Stern:

   I am an Industrial Technology teacher at a middle school and one of my students is dreaming of a space propulsion system based on magnetic repulsion of the earth's magnetic field. Could you possibly squeeze in a moment for us and provide some information on the strength of this field and how it has been measured and maybe a relative comparison? Tyson, my student, is really excited about the Internet and will be enthralled to have an answer from a NASA scientist. Perhaps you could steer him to other references as I certainly will explain to him how busy a schedule you must have. Thank you.

   Reply

   I am afraid it won't work. First of all, the magnetic field is very weak. Compared to fields in electric machinery, where appreciable forces are exerted, it is a few thousand times weaker.

   But there is a more fundamental reason. Magnetic poles always come in pairs, equal and opposite: if a field attracts an N pole, it repels the attached S pole. Similarly, if we generate the field by a current in a loop of wire --say, shaped like a rectangle--for each side in which the current flows in one direction, there exists a side where it flows in the opposite direction, and the magnetic field exerts opposite forces of equal strength on the two sides.

   From the preceding one would guess that magnetic forces always cancel, and no net force is exerted. So how come magnets are attracted to each other, or pins to a magnet (same thing, really, since each pin in the magnetic field turns into a small magnet)?

   The answer is that the forces on the N and S poles (or on the opposing currents) are not exactly equal, if one pole, or one wire, is closer to the source of the field than the other. This can be put into a mathematical formulation and the bottom line is that a suitably oriented magnet may be attracted by a magnetic field, moving towards the greatest strength of that field. But the force is proportional to the rate at which the field changes with distance, which in the case of the Earth, is very small.

   The idea of magnetism as anti-gravity has come up before. Your student may look up "Gulliver's Travels" by Swift, where in the third voyage, in a spoof on science and learned societies, Gulliver arrives at an island floating in the air, held there by the repulsion of a large magnet. Swift even gives an explanation, except it's all gibberish gobbledygook, as befits a book of satire. (I won't cite here the name of Swift's island, since too many people in Texas speak Spanish!)

   David Stern

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3. Question #3

   Question about the sun

   Dear Mr. Stern:

   I have a question about the sun that I was hoping you might be able to answer for me. A friend of mine recently returned from a new-age conference where it was presented that the magnetic poles of the sun were about to reverse, and cause a number of changes.

   The idea of the sun having magnetic poles seemed counter to what I remember learning about the sun, and your web page seems to dispel the idea that the sun has actual poles. My guess is that the presenter was taking a dose of creative license with the 11 year cycle of sunspot activity.

   Is it true then, that:

   1.) There are no magnetic poles on the sun.
   2.) Is the change in sunspots related at all to a reverse of polarity of magnetic fields?

   Thank you.
   If you can provide reference to a college-level text as a reference, it would be appreciated.

   Reply

   Actually, your friend was right: the Sun does have polar fields, and they do seem to reverse their polarity each sunspot cycle.

   The Sun's most concentrated magnetic fields are of course in sunspots, but people have long suspected there might also exist polar fields, because during a total eclipse of the Sun one often sees streamers coming out from the polar regions, looking very much like the pattern of iron filings near the poles of a magnet.

   But there was no good way of measuring such diffuse magnetic fields: the field of sunspots affects the light emitted from them ("Zeeman splitting") but the effect elsewhere is very weak. Then in the 1950s (if memory serves me) the Babcocks pushed the technique to its limits and found the polar field. This revealed the reversal of the polar magnetic field and suggested this field was somehow coupled to that of sunspots (which also reverse each cycle--they come in pairs, and the leading spot, in the direction of the Sun's rotation, has north or south polarity, in alternate cycles), a sort of a cumulative effect of the distant field of many spots. Theories exist by Horace Babcock and Robert Leighton, though they are somewhat qualitative.

   The fact the magnetic field lines at the poles stick straight out means they do not hinder the escape of the solar wind in any way, and indeed the Ulysses spacecraft which recently passed above the Sun's poles confirmed (as was predicted) that the solar wind there is faster. There seems to exist no great effect of the reversal on Earth, though one might expect a bit more magnetic storminess when the polarity is opposite to that of the Earth.

   For more on the Sun, see:

   • http://seds.lpl.arizona.edu/nineplanets/nineplanets/sol.html

   • Kenneth Philips, "A Guide to the Sun", Cambridge U. books, 1992 (paperback '95).

   Yours,
   David Stern

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4. Question #4

   Synchronous Satellites (query from Barbados)

   Dear Dr. Stern:

   I have been told, and read, that in order for a satellite to remain in a fixed position relative to the earth, it must be in a synchronous orbit, and that this type of orbit is best for communication purposes. All of the other orbits I have read about are used by satellites with goals other than communication. Satellites in orbits other than synchronous are not fixed in position relative to the earth.

   This being the case, it seems to me that all satellite dishes for reception of TV signals would face the equator - south east, south west, or somewhere between. My observation of satellite dishes does not seem to support this concept. Many dishes do have a southerly inclination, but others do not. Further, here, where we are close to the equator, it seems to me that in order to focus on a satellite, the dish should be aimed high - higher, at least, than one located in the USA or Canada where the angle between the earth and the satellite would be smaller than here in Barbados. Many dishes here seem to follow a line of sight that is barely above the horizon.

   So, my question - are all communication satellites in fixed orbits above the equator, and if so, why don't all reception dishes face the same way?

   As I said at the outset, I know that I am being brash in writing you, and I would appreciate your indulgence. Over the past year or so, I have asked at least a half dozen engineers for an explanation, and received little more than a blank stare in response. Access to other resources here is not always easy. If you are unable to take the time for an explanation, perhaps you would direct me to a source that could satisfy my curiosity.

   If you do reply, please bear in mind that I am an accountant, not an astronomer.

   Reply

   I am very glad to hear from you, to find someone as far away as Barbados interested in satellites, but I have no good answer to your question. It is absolutely true that all commercial communication satellites orbit above the equator, at a distance of 6.6 Earth radii, with a 24-hour period, which keeps them above the same station. You know probably that the space shuttle and other low-altitude spacecraft complete one orbit in about 90 minutes: the further out you go, the longer it takes, until you reach the moon, which takes one month. So it stands to reason that somewhere in that range the orbital period is exactly 24 hours. If a 24 hour orbit is inclined to the equator, the satellite does not stay above the same spot but wanders back and forth: so it must be an equatorial orbit. Only then can the antenna on the ground be fixed in one position.

   It is true, however, that not all satellites tracked from Barbados are at the longitude of the island. To receive phone calls from Europe, say, it could be that a satellite is tracked which is orbiting at the longitude of Europe, and then the dish should point towards the southeast.

   I do not know how you reached my name; maybe you were directed by a search engine to the file.

   http://www.phy6.org/Education/wsynch.html

   I hope you are aware that this is only one part in a much larger exposition, the Exploration of the Earth's Magnetosphere, dealing with the Earth's magnetic environment, with its home page at

   http://www.phy6.org/Education/Intro.html

   You might look it up. Also try:

   http://www.phy6.org/Education/wlagran.html

   dealing with satellites keeping a fixed position (or staying close to one) relative to the Earth in its orbit around the Sun.

   David P. Stern

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5. Question #5

   Magnetic Field Lines Mr. Stern,

   I am enjoying your presentation on magnetospheres very much and I am finding it most interesting and informative. However, I have one question that I have not found an answer to yet.:

   If the earth's magnetic lines of force are in fact "lines" upon which electrons and protons can collect like "beads on a wire", what is the spacing between these lines say, at the altitude of the recent Tethered Satellite experiment?

   Some co-workers and myself have had a rather heated discussion on this matter (i.e. whether the magnetic lines are "lines" or a "field"). We would be most greatful if you would enlighten us about these magnetic lines of force around our planet just a little more than you have in your presentation on the Goddard Home Page on the WEB.

   REPLY

   Be very careful here! Magnetic field--space modified by magnetic forces, so to speak--is one thing, and magnetic field lines are something else again. They are a mathematical description of that field, no more tangible than lines of latitude and longitude which describe the surface of the Earth. One never asks how close THOSE lines are; you can draw any number of them, depends how tightly you are willing to space them.

   Magnetic field lines are defined as lines that point everywhere along the magnetic force (in a fluid, a complete analogy is given by flow lines or "streamlines"). They can be described by formulas, in terms of quantities known as Euler potentials or Clebsch functions.

   But there also exist intuitive properties: particles threaded by a common field line, tend to share that field line later on as well. Say we have 10 ions numbered 1... 10 sitting on a common spot on the Sun, and therefore sharing there a field line, and destined to come out in the solar wind one day apart. The Sun rotates, so make a drawing with a circle representing the Sun and 9 radial lines coming out about 15 degrees apart. After 10 days, particle #1 is 2.5 inches along the first line, particle #2 2.25 inches on the 2nd one, and so on, down to particle #10 still on the surface: the line conecting the particles is a spiral, so we expect interplanetary field lines to have a spiral shape, and we derived this from intuitive concepts alone (though the same thing can be derived from formulas).
     The details of this excercise are described in http://www.phy6.org/stargaze/Simfproj.htm.

   The spacing between field lines is not meaningful (though some engineers speak of "density of magnetic field lines" to describe a quantity commonly known as "flux density.") Suppose you draw two field lines of the Earth, reaching Earth 1 meter or 1 foot apart. Each can have electrons or ions trapped around it. The meaningful question is what is the radius of the circle these electrons or ions describe around their guiding line, and that depends on their energy, and how strong the field is (the circle gets larger in the weak fields far from Earth), but it is generally much more than 1 foot or 1 meter. No problem: densities are so low that such ions or electrons rarely collide, and their orbits can easily overlap. The radius of gyration of auroral electrons can be 100 meters, which is why auroral "curtains"are so thin. On the other hand, solar wind ions entering near the "nose" of the magnetosphere have radii of the order of 500 kilometers, or (say) 350 miles, because the field there is much weaker, and that is therefore the order of the expected thickness of the magnetopause, the boundary between the solar wind and the magnetosphere.

   David P. Stern
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6. Question #6

   Solar Wind To David P. Stern:

   Hello David, I liked the WWW article on The Solar Wind-History. In the article it said:

   "Not everyone accepted Parker's theory of the solar wind when it was first published in 1958, and it was debated until observations confirmed it."

   Have you ever heard of the book OAHSPE by John Ballou Newbrough copyright 1882? Oahspe means Earth, Sky, and Spirit. Oahspe contains much scientific information that was discovered many years later, such as the earth's magnetosphere, Van Allen Radiation Belts, the SOLAR WIND, the origin of stars in nebula, the configuration of galaxies, the beginning substance of life(DNA) in nebula, the cycle and age of galaxies and the stars they contain, interstellar and intergalactic Unseen matter(Dark matter). Oahspe says the Sun has a vortex that streches throughout the solar system. Scientist have some knowledge of the Sun's vortex, they call it the "Solar Wind'. .... (most of the letter omitted)

   Tell me what you think about it. Please send me a reply email. Hope to hear from you soon.

   Reply

   I am sorry to have to disagree with you, but just coming out with a statement which is later verified is not a prediction, unless it is supported by cogent arguments, which distinguish it from similar predictions which were not verified. That's the essence of the scientific method: what we believe to be true is based on a tight interlocking web of observed evidence. The first evidence for the solar wind came from comet tails, whose behavior had been explained by sunlight pressure: it took a lot of physics to understand why this process worked on dust tails, but not on ion tails. Parker's theory was based on the high temperature of the corona, discovered in 1939 or so, and again involved some intricate physics.

   The solar wind, by the way, is not a spiral: it flows straight out. Only the Sun's magnetic field lines are spiral, because the sun rotates: they tend to act like continuous strings, and since their "roots" rotate with the Sun, they get twisted into spirals.

   David Stern

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7. Question #7a

   The Geiger Counter

   Hi, I am a 10th grader ...I came across a web site ... in my search for how a geiger counter meter works. I was hoping you could give me a relatively simple yet good explanation of how it works (preferably, really simple). I'd appreciate it very much.

   Thank you.

   Reply

   And I thought "Exploration" did give a simple explanation!

   Imagine a fast ion or electron going through the tube. On its way it hits atoms of the gas in the tube and ionizes them--knocks off electrons and leaves positive ions. Usually, such electrons recombine soon. But if there is a voltage difference (electric field) in the tube, before they can do so, the electrons will start moving towards the positive wire and the ions towards the negative walls.

   As they move, they gain energy. This is particularly true near the wire, where the electric field is concentrated and its force is strong. (One can draw electrical field lines just like magnetic field lines, and near the wire they bunch together, like magnetic field lines near the poles of a magnet.)

   If the energy gained by the average electron is enough to knock out additional electrons from atoms of the gas with which it collides, the number of electrons will multiply. As the electrons move towards the wire and the field gets strong, this process grows quickly: one electron frees up two, two release four, and by the time the wire is reached, many more electrons arrive than were released by the initiating particle, enough to draw a measurable current and create a signal in the circuitry attached to the counter.

   There is much more, of course, e.g. ultraviolet light which spreads the process away from the wire as well, which could cause the current to continue without stopping even after all the initial electrons (and the additional electrons caused by them) have reached the wire. Special gas filling takes care of that.

   The counter is usually charged by just a trickle of current from a high voltage source, so that the current taken by the discharge is easily measured. If too many particles pass through the tube, too much current is drawn, the voltage drops and the discharges get smaller, until the electronic circuits supposed to count them don't do so any more. I think that's what happened on Explorer 1.

   David Stern

   Question #7b

   Building a Geiger Counter

   Hello!
   
   I found your web-site in the internet. But I have still two questions.
   1. Which gas is suitable?
   2. Which pressure is in the metal tube(vacuum?)?
   Please can yuo send me a wiring diagram from a geiger counter.
   
   Best regards
   
   Matthias

   Reply

   Dear Matthias
   
   I did my thesis work with Geiger counters, but that was 40 years ago and my memory of that time is sketchy. If you have a local university with a physics department, it (or at the very least, its library) could provide you with much better and much more up-to-date information. Here is what I remember
   1. You should understand, of course, that if the Geiger counter is a metal tube, the end plugs must be glass, with a tight metal-to-glass seal, because the central wire must be insulated from the tube. Or else, the whole thing is inside glass, and the tube nowhere touches the wire.

   2. When a particle passes inside, it creates ions and initiates a discharge. But you don't want the discharge to get too big. As noted, the counter has two main parts, insulated from each other--a cylindrical metal tube and a thin wire through its middle. The two act like a small capacitor, a device that holds an electric charge when a voltage exists between its two parts. For example, the tube may be connected to the ground (voltage zero) and the wire may be charged through a lerge resistor (which only allows a small trickle of current to flow) to some high positive voltage, say 1000 volts; the exact number depends on the design of the counter. (LI> Now when the counter discharges, the electric charge from the wire jumps over to the tube, and the voltage between the two momentarily drops down--not to zero (the discharge is never complete) but by a few hundred volts. A small additional capacitor can transmit that jump to an external circuit, which registers a "count." Afterwards the counter is "dead" (insensitive) for a small fraction of a second, while the capacitance between the wire and cylinder refills to its operating voltage of 1000 volts.

   3. The trick with the counter, if I remember, was for it not to go into a continuous discharge. The filling, at a pressure of a few tens of milibars, was either alcohol vapor or halogen. Alcohol stopped the discharge by itself, but was gradually broken up by the discharges, so that the counters had a certain lifetime. Halogen--I don't remember what it was--chlorine?-- lasted longer but needed an external electric circuit to give it a large negative pulse and shut it off, after each discharge. In either case, the voltage was critical--too big, it went into a continuous discharge, too small, the pulses were small, too, in what is known as the "proportional counter" regime. Today many people prefer proportional counters, since the electronics can amplify the pulses very well. But 40 years ago it was good to have a detector whose output pulse was big enough without any amplification. The pulse was carried out through a small capacitor, which let the pulses through to whatever counting circuit was used, but kept out the high voltage.

   I hope all this is useful. Sometimes simple questions lead to complicated answers!

   Yours
   David P. Stern

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8. Question #8

   Measuring Earth's Magnetic Field

   I am doing a sixth year studies project on magnetism in and was delighted to find the question and reply page with topics similar to what I had thought of studying.

   I was wondering if there was a practical method for measuring the strength and irection of the Earth's magnetic field at different geographical locations. Any help or inspiration would be greatly appreciated.

   Reply

   Is your "sixth year" in school or 6th year in college? It is not easy to tailor an answer to fit either level!

   In any case, the electronic gizmos nowadays used in space are too complicated for a quick discussion, so let me instead describe earlier, simpler methods.

   The direction of the magnetic field is of course given by the compass needle: but that is just the horizontal part of the force, Actually the magnetic force also points i n t o the Earth (or out of it, in the southern hemisphere).

   To find the angle at which the force points down ("dip angle") people used a needle similar to a compass needle, but on a horizontal axis, allowing it to swing in the various directions to which the hands of a wall clock might point.

   That is a bit harder to arrange than a compass needle: if one end of such a needle points at an angle downwards, how is one to know whether the magnetic force is responsible, and not, say, that the needle is not quite balanced on its pivot, but that one end is slightly heavier and therefore points downwards? To avoid this problem one starts with an unmagnetized needle, balances it very carefully, and only then magnetizes it. When in 1831 the expedition of John Ross searched for the north magnetic pole, it carried along a dip needle, and when it pointed straight down (while the regular magnetic needle showed no preference for any direction), that was it .

   Measuring the strength of the field is harder. Take a thin long bar magnet and hang it by a thin thread, then wait until it points north-south. After it does, push one tip slightly left or right and let go: it will swing back to north-south, but will overshoot to the other side, then turn back to the right direction, swinging back and forth like a pendulum, gradually quieting down to point steadily. The average length of each swing depends on two things: the strength of the bar magnet and the strength of the magnetic force. With a stopwatch, measure 20 swings or so and figure out how long each swing takes.

   Then put a small compass needle on a table, and put the small magnet nearby, in such a position that it tries to line up the compass to point east-west.The small magnet and the Earth's magnetic force obviously compete fordetermining which way the needle points, and by looking at the actual angle of the needle, and its distance from the small magnet, we again get an observation that depends on how strong are (1) the small magnet and (2) the magnetic attraction of the Earth. Using these two observations and some calculation, the physicist can find both these unknown quantities.

   This method was proposed by Carl Friedrich Gauss in Germany around 1835. It obviously won't work on an orbiting satellite--but how measurements are made there is another story altogether.

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9. Question #9

   The Strength of the Earth's Magnetic Field

   Could you please send me any information regarding the current field strength of the earths electromagnetic field? My data is current as of 1975 which is by far outdated. My reading from that time were 30,000 gammas at the equator. If possible could you please send information on the current decay of the earth's magnetic field.

   Any information would be greatly appreciated.

   Reply

   I am not sure at what type of information you need, or to what use you put it. The most complete information on the Earth's internal magnetic field is in form of a set of coefficients, to be plugged into a mathematical representation--the so-called spherical harmonic expansion. The coefficients generally used are the so-called IGRF set (International Geomagnetic Reference Field) chosen by a committee every 5-10 years and based on the "best available" observations. You can find them on the world-wide web at
   http://fdd.gsfc.nasa.gov/IGRF.html

   Some of these models also include the annual change of the field (but not in the above files). You might like to search the web using (say) the Altavista or Yahoo search engine, on the term IGRF.

   If you just want maps of the field, for instance those describing, the variation of its strength over the globe, try

   http://swdcdb.kugi.kyoto-u.ac.jp/igrf/index-j.html

   The text seems to be in Japanese, for on my computer it does not give anything readable, but the maps are in English. Clicking on the first will show you that the magnetic intensity around the equator varies quite a bit. but 30,000 gamma (or nanotesla, same thing) is a reasonable value.

   The field has been weakening since Carl Friedrich Gauss measured it around 1836, by about 5% per century, recently accelerating to 7%/century. The total energy of the field however is nearly constant, as shown by the late Ned Benton. This means that the field is not really weakening, only reshuffling its energy, reducing the "main dipole" (=north-south bar-magnet pattern, declining as noted by about 7% per century) and reinforcing the more complicated parts.

   These tend to contribute a weaker field, because the magnetism originates in the Earth's core, about half an Earth-radius down: all magnetic fields at the surface are weaker than those in the core, because of the distance, but the more complicated fields decrease faster.

   Whether the main dipole will reverse in about 1300 years is anyone's guess. Geological evidence suggests it has happened in the past, but odds are against it, because the mean frequency of such reversals in the past seems to be about once in 500,000 years.

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10. Question #10

    Solar Eclipses

    I'm working on a science project about the solar eclipse.

    My first question is, how can you figure out exactly when the next eclipse will come?

    The next question is: What are the main theories about the incredible heat of the corona? How can it be so warm when it's so far away from the sun's center?

    Reply

    Dear Lars

    Predicting eclipses is relatively straightforward, you just need to know the motion of the Sun and Moon across the sky, and when they occupy the same area, you get an eclipse.

    By now we have pretty good formulas for the orbital motion of the Earth around the Sun (which determines where the Sun is in the sky) and of the moon around the Earth, and can predict eclipses quite accurately. The journal "Sky and Telescope" usually carries accurate maps of where the eclipse can be seen (if the sky isn't cloudy) and the times when it should happen. The journal also maintains an eclipse page on the web, at:

    http://www.skypub.com/eclipses/eclipses.shtml

    The heat of the corona is still a great mystery. I can describe to you one theory, but it i