(S-6)   Seeing the Sun in a New Light
(S-6A)   Interplanetary Magnetic Field Lines
Part of a high school course on astronomy, Newtonian mechanics and spaceflight
by David P. Stern
This lesson plan supplements: (S-6) Seeing the Sun in a New Light: on disk Sun6new.htm, on the web
(S-6A) Interplanetary Magnetic Field Lines: on disk Simfproj.htm, on the web
The student will learn
Terms: Coronal holes, coronal mass ejections, magnetic storms, solar wind streams, interplanetary magnetic field
Stories, additions and features: A graphic excercise in which the expected shape of interplanetary magnetic field lines is obtained; The "Chandra" X-ray observatory
Starting the lesson(This approach starts with a general discussion of X-rays before bringing up the Sun. Some student might look up beforehand information about the "Chandra" orbiting telescope and describe it to the rest of the class at the appropriate time; see here and here.
If time is short, the teacher can cut this part short and start right away with the solar corona)
Today we will talk largely about X-rays and the Sun. In the "Superman" comic books, Superman not only flies through the air and leaps over tall buildings, but also has the power of "X-ray vision" that allows him to see through walls.
X-rays can indeed penetrate walls, but even with X-ray vision, Superman would never see what is behind them--for at least two good reasons. Any guess?
Suppose you wanted to build an X-ray telescope. How can you focus X-rays? It turns out that they can be reflected if they hit at a very shallow angle--just as a flat stone will bounce off water if it hits at a shallow angle, but will surely go through and sink if dropped vertically.
The "Chandra" orbiting X-ray telescope, launched from the Space Shuttle on July 23, 1999, focuses X-rays in this way. Imagine you cut off the outer tread of a tire, to produce a ring with a slightly concave cross section. The "Chandra" telescope has metal focusing surfaces shaped like the inside of this ring, and by shallow reflections they bring X-rays to a focus. (here is a picture of the remnant of a supernova explosion, taken by "Chandra").
What part of the Sun do X-rays and related wavelengths see best?
What are coronal holes?
What is the connection between coronal holes and the solar wind?
Any reason suggested?
(Teacher may supplement)
Weak magnetic fields, on the other hand, get instead pushed around by the plasma, which modifies their structure. (We'll come to that later.)
Sunspots are sources of strong magnetism. Usually, they come in pairs of opposite polarity--one is like the northward pointing end of a magnet, the other like the southward pointing end--and magnetic field lines seem to connect them, forming arches above the solar surface. Ions and electrons guided along such arches by their strong magnetic fields find it hard to escape.
In between sunspot groups, field lines stick out like blades of grass after a rain, and plasma can ride along them outwards.
So--where do you think sunspots are found--in coronal holes or outside them?
Over the Sun's poles, eclipse photographs show "plumes" sticking out, just like magnetic field lines near the poles of a magnet. (sketch on the board). Does this suggest "coronal holes" avoid the poles, or not?
Spacecraft in the solar wind observe there a weak magnetic field. What is the origin of that field?
What are "Coronal Mass Ejections" (CMEs)?
Why are space researchers interested in CMEs?
Note: an interesting shock-produced event occured on 24 March 1991. A student might read up on it and briefly tell the class about it. See http://www.phy6.org/Education/wbirthrb.html.
(Teacher may explain magnetic storms)
The main feature of a magnetic storm is that it greatly increases the amount of fast ions and electrons trapped in the Earth's magnetic field, typically at distances 2 to 8 Earth radii (RE). All one notes on the ground is a small change in the magnetic intensity--a drop of 1% at the equator is already a big storm--but at synchronous orbit (6.6 RE), so many fast ions and electrons are added that on a few occasions (in big storms) communication satellites have sustained damage.
In the "auroral zone," aurora is no rarity: but during big magnetic storms, the added trapped plasma modifies the Earth's magnetic field in such a way that aurora forms on field lines closer to the equator. Therefore on such occasions people in the middle of Europe and the US also sometimes see auroras.
How can we be warned that an interplanetary shock is approaching?
(teacher supplements the answers of students)
What is "solar activity"?
(Teacher explains further)
How are medical X-rays produced in a doctor's office?
Do you think the polar aurora produces X-rays? Give your reason.
Do you think that picture tubes of TVs and and computer monitors produce X-rays? Give your reason.
Similar processes take place when electrons are accelerated near the Sun.
(The teacher may discuss here the Yohkoh picture of X-rays from a magnetic arch and include some of the material below.)
Magnetic storms occur when CMEs and shocks arrive near Earth, but the direction of the interplanetary magnetic field is also important. If it slants southward, arriving plasma is much more likely to produce magnetic storms than when it slants northward, because its field lines can more easily interconnect with lines of the Earth's polarity. Unfortunately, that direction cannot be sensed remotely. Only when a disturbance passes the L1 point can we tell what its magnetic field might be.
Why would the best time for a manned mission to Mars be the quiet part of the 11-year cycle, when solar activity is rare?
(A student who has read Ben Bova's "Mars" may tell about the chapter in which the book tells about astronauts hiding in a "shelter" during a solar outburst.)
What is the idea behind NASA's "Great Observatories", space missions such as "Hubble" and "Chandra"?
Author and Curator: Dr. David P. Stern