138. The Ionosphere and radio waves
Hi David,
My question is: What effects do solar x-rays have on HF radio wave propagation?
In case I may have overlooked an answer already posted I apologize. My eyes ain't what theyused to be!
Thanks in advance!
Reply
Dear Floyd
I am not sure I have the best answer to your question, but let me guessand summon what little I remember of my experience as physicist.
The bottom of the atmosphere is transparent to radio waves, but its high layers contain free electrons (and ions), which help reflect or absorb radio signals. These are usually created by sunlight in the ultra-violet, "far ultraviolet" and "soft x-ray" ranges.
The lowest part of the ionosphere is the D layer, about 70 km up, where the atmosphere is rarefied but not TOO rarefied. It is created by ultraviolet light from the sun and its electron density is about 30,000 electrons per cubic cm, which reflects AM frequencies (less than 1 MHz). Because the density of the neutral atmosphere is relatively high, ions and electrons collide frequently, recombine, and are neutralized; this removes the ions, limits their density and drains energy from radio waves. Thus AM stations are received mainly locally, and the same frequency can be used by other stations beyond the horizon. At sunset the ions quickly recombine and the layer disappears: then radio waves are reflected from higher layers, where collisions are fewer and where less radio energy is absorbed, so signals can travel to larger distances. I remember when car radios only used AM, and after sunset many distant broadcasts could be heard, while some local stations shut down to obey federal regulations, so as not to clutter up the broadcast bands.
The F layer around 200 km height has up to a million electrons per-cc and relatively fewer collisions. It reflects short-wave signals with little loss, allowing amateurs, ships and aircraftalso some broadcaststo travel by successive reflections beyond the horizon, in some cases between continents. The E-layer in between produces some loss and is important mainly because horizontal electrical currents of the ionosphere travel there best. Above the F layer the ionosphere extends with gradually lower density to a height of a few thousand kilometers. However, radio waves from Earth are reflected back before they reach there; it was in fact only mapped after satellites with transmitters (like the Canadian Alouette in 1962) bounced radio signals from the "top half" of the ionosphere, on their side of the greatest electron density. A (tiny) graph showingthose ion densities is on my web page where space plasmas are discussed.
X rays from the Sun penetrate deeper into the atmosphere and create ions there. Usually their intensity is too small and ions are few, because deeper layers have more frequent collisions. However, when a flare on the Sun emits X-rays at a rate much larger than usual, they generate a high ion and electron density even in the D-layer or deeper belowsay a million or more electrons per cc. At such times short-wave radio signals are reflected from those layers before reaching the F-layer, but like AM signals at quiet times, they are strongly absorbed and don't travel far. You then get a "signal fade-out." When the X-ray emission ends, the extra ions recombine and the ionosphere quickly recovers.
I hope that answers your question. You will find more in books and on web sites.
139. What is a Plasma?
I am an engineering student in my last semester, and am working on a project about arc plasma gasification and its technologies, gathering information about the latest research on those techniques and also how they are applied in new plants for treatment of urban waste around the world. I am just starting and think it would be very important in an introduction to tell "what is the plasma". I was searching information online and I found out your name, so my question is, could you tell me the name of the book where you wrote about the plasma so I can write it in my bibliography.
Thank you very much and best regards from Spain!
Reply
Long ago I wrote an introduction to plasma at http://www.phy6.org/Education/wplasma.html and in the web page on fluorescent lamps linked from it (new fluorescent lamps, like the spiral ones used in homes, contain a transistor circuit and operate in a more complicated way). All files also have Spanish translations, by Jesus Mendez living near Bilbao.
Plasma is essentially a gas so hot (or subject to strong electric fields, or radiation such as short-wave ultraviolet light) that some (or all) of its atoms have one or more electrons removed, leaving behind a "positive ion". The negative electrons float around until some ion recaptures them, but usually stay near the positive ion because of electric attraction.
Plasmas are a complicated gas, subject to electric and magnetic forces and also to gravity. See "Exploration of the Earth's Magnetosphere" where the behavior of plasma near the Earth is discussed. Be aware that plasma is a very complex gas. For instance, it conducts electricity, but where it is very rarefied, it conducts much better in the direction of magnetic lines of force than perpendicular to them. It has many types of wave, but their spread again depends on the magnetic direction. See
http://en.wikipedia.org/wiki/Plasma_(physics)
By the way, the word "Plasma" was introduced in the early 1800s by the Czech medical pioneer Jan Purkinje, who applied it to the clear liquid of the bloodstream ("blood plasma"). A century later the same name was given to a hot electrically conducting gas by Irving Langmuir.
140. Magnetic shielding of spaceships
I am an aspiring science fiction writer and would like to make my writing as scientifically accurate as possible. I thought that people on a spaceship could protect themselves from dangerous solar radiation by generating an artificial magnetic field around the spaceship. If this technology could work but we don't yet have it, I could include it in my writing. If it's not scientifically accurate, I'll have to think of something else. Astronauts of the future would have to deal with this problem, and I would like to use an idea that would be possible someday.
Reply
Your idea has been raised before, but is not actively pursued because it demands extremely strong magnetic fields. The Earth's magnetic field deflects ion streams from the Sun, because it has a large magnetic moment M, a measure of the strength of a magnet. Given two magnetic poles of strength M a distance D apart, the magnetic moment is MD, proportional both to the strength and the size of the magnet. A magnetically shielded spaceship can create strong M, but D is of the order of the dimensions of the spaceship, not enough to deflect very fast ions.
Also, such ions can still arrive if they get attached to magnetic field lines leading to the magnet. This can be avoided by a doughnut with a shielding coil, with the astronauts inside the doughnut, which however would be inconvenient, and rather heavy.
Radiation bursts from the Sun are relatively rare and short, and the events observed have often been too weak at Earth orbit to create a health risk. In general, it seems better to hide the crew in a shielded zone inside the spacecraft, behind stores of fuel and supplies. See also
http://www.phy6.org/Education/FAQs5.html#q72
141. Travel to distant stars
Hello Mr. Stern.
I happened upon your page and found it very interesting. I would like to ask a question or two myself. First: I once read an article in a science magazine that claimed it would be possible to use the solar wind for propelling a starship. The authors claimed a device much like a sail could catch the solar wind and move the ship away from the sun. They went on to claim that very high velocities could be achieved close to half the speed of light. Do you agree or is it not possible?
Also I once read a publication about possible spaceflight and one problem of moving at high speed addressed by the authors was that space wasn't totally empty but had particles moving through it. A collision with a ship moving at a large fraction of the speed of light would be fatal to the crew. They had no solution except to limit the speed of the ship to prevent damage from such collisions. But I wonder if a strong magnetic field or multiple fields could deflect such particles. I once read a book for laypeople on science and it claimed that even neutrons, though they have no charge, have a magnetic property called spin.
So, could a strong magneticfield provide protection?
Reply
Dear Robert
It is possible to propel a spacecraft by sunlight, reflected from a stretched thin "solar sail." Sunlight pressure is very gentle, so any practical payload requires a huge but very light sail, for which new technology is needed. In any case, velocities attained would be far below the speed of light c (unless the Sun's light is somehow beamedsee sci. fi. book mentioned on the above web page), because as the payload accelerates, its distance from the Sun increases and the pressure drops to very small values.
The solar wind won't work. Its particles are very sparse, and they move at perhaps 0.14% of the speed of light (5 days from the Sun to Earth).
We do not know enough about distant space. Dust hitting a fast spacecraft may perhaps punch pinholes (and if the velocity is a good fraction of c, go right through), but I am not sure how much to expect. A magnetic field will not deter neutral matter. Neutrons and atoms act like tiny magnets, but a magnetic field only shifts their rotation axis and does not move it in space. See discussion of Nuclear Magnetic Resonance near the end of
http://www.phy6.org/stargaze/Lprecess.htm.
For other web pages see http://www.phy6.org/readfirst.htm
142. Energy from the ionosphere or solar wind ??
Hello,
Why are the world's scientists not working towards harnessing the ionosphere or the IMF for global power generation? It is rather clear that this is possible technically a massive engineering solution not a "can it be done" question.
I do not know a honest scientist worth his salt who will deny there is not massive power in the ionosphere and IMFso why is it not being aggressively perused so that in one or two human generations it can be tapped and the global fossil-fuel energy industry shut down?
Reply
Hold it there, Bob
The ionosphere is produced by short-wave sunlight, a tiny part of sunlight. Much more energy than is in that part is conveyed by sunlight reaching the ground--and there is no problem of bringing it down from 60-150 miles up, or of collecting it from many hundreds of square miles. It is much more practical tapping sunlight that falls on Arizona.
The IMF itself is not a source of energy, though the solar wind which drags it out from the space around the Sun could be one. But again, the scale involved is tens of thousands of miles, the energy is diffuse and not all that plentiful, and we have no extension cord long enough to reach the solar wind, or a fixed hook to anchor it there.
143. Shielding Earth from high-energy solar ions
Greetings, Dr. Stern,
I am not a professional physicist or astronomer. I am curious about recent data (2-3 years) about the Earth's magnetosphere. It seems that the theoretical understanding of the protective function and operation of the magnetosphere is changing. Can you refer me to any current information about the Earth's magnetosphere?
Have you seen the recent report of the discovery of a giant breach in the Earth's magnetic field
http://science.nasa.gov/science-news/science-at-nasa/2008/16dec_giantbreach/
Is this a credible source?
Reply
Hello, Richard
As the French saying goes:
Plus a change, plus c'est la même
or as we would say, "The more things change, the more they stay the same." The main magnetic field of Earth only changes very slowly. Its main north-south component is weakening by about 0.1% per year (the trend is irregular), and as far as I know the "protective function of the magnetosphere" is not changing. If it changed significantly, say by 50%, the shielding of high energy particles would be somewhat reduced, but the atmosphere would still protect us as much as 10 feet of concrete. Even now there exists little magnetic shielding near the magnetic poles, and its lack does not bother the Inuit of the far north.
144. Can sunspots lead to earthquakes? (part 1)
Dr. Stern:
I've noticed quite a bit of discussion on the Internet regarding whether sunspots cause earthquakes. (In my field, economics, there are stories about one statistical model that says earthquakes cause sunspots!)
My 12 year old son and I thought that researching this question would be a great science fair project. Our state's curriculum guidelines require that a science fair projects should test a hypothesis. So I thought, this is great! We have a hypothesis and, thanks to USGS, NASA, and the European Space Agency, we have tons of data.
I have two issues that I hope you can help me with. First, my son has the worst science teacher in the world. She seems to think that the only way to test a hypothesis is to run a laboratory type experiment. One of her ideas of an "outstanding" project was when--ugh--students "test the amount of water or other substances like coke or fertilizer that they water plants with." Can you help me convince her that this is a cool and doable project?
The second issue is a bit more complicated. Do you have any tips on how a 12 year old can test his hypothesis. I am economist and statistician, so I can use all types of fancy-pants statistical models. However, it seems from your website that you have certain panache for making these topic understandable to the masses.
Reply (continued in next Q&A)
Dear Eric
Your son's proposed investigation does not sound promising, and will probably lead to a zero result. Sunspots are magnetic phenomena, and while they are associated with magnetic changes and plasma flows in interplanetary space, it beats me how they could affect tectonic plates on Earth.
A more logical question might be, does the position of the Moon in the sky affect earthquakes? By raising tides in the oceans and also in the solid Earth, the Moon periodically deforms our globe and exerts a non-trivial force on it. I vaguely recall this question has been asked before, but not what was found. For this project, your son will need a list of earthquakes and their times, a calendar tracking the Moon in the sky, and probably a computer code to derive for each earthquake the relative position of the Moon. It's not a quick job.
As an alternative, let me propose a project that is altogether different. [It follows in the next item.]
145. Proposed Science Fair Experiment: The Grease-Spot Photometer.
An alternative science fair proposal, continuing the reply to the previous question #144
We are told that a 13W compact fluorescent lightbulb (CFL) emits as much light as a 60 W bulb with an incandescent filament, which uses 4 times as much electric power. But is that true?
Your son will require two table lamps or equivalents, an incandescent 60W bulb and two CFLs of 13W. He will also need a metric yardstick, a homemade paper screen (mounted over a large hole in a cardboard screen), a smidgen of margarine or cooking oil, and probably a handheld calculator.
The experiment would use a grease-spot photometer, like the one described in
http://www.phy6.org/outreach/edu/greaspot.htm. Since it is his project, I will not spell out the set-up for getting his result--let him do it.
Having two CFLs allows other question to be answered, e.g. how does the intensity of light from the side of a CFL compare to that from the top? How does the intensity of the light of a CFL switched on 5 minutes ago compare to that of one turned on just now?
The experiment will also teach your son that measurements generally involve comparison, such as comparing an unknown weight in one pan of balance scales to known weights placed in the other pan. (Often this comparison is conducted when an instrument--say, spring scales in a bathroom--is manufactured or set up, and is then known as its calibration.)
I think your son's teacher will approve. And BTW, I doubt she is the worst science teacher in the world--there exists too much competition for that distinction!
If your son does this, please e-mail me a copy of his project