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Goals
The student will learn
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About the observation of "coronal holes," by x-rays, also about related fast streams and moderate magnetic storms that recur at 27 day intervals
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About "coronal mass ejections" (CMEs), their effect near Earth and their monitoring from space.
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About high-energy ions and electrons accelerated by solar activity, probably from magnetic energy, and the hazard they pose to spacefarers.
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About NASA's "great observatories," expanding the coverage of the electromagnetic spectrum by astronomers.
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?
- X-rays are not too well reflected from objects the way light is--they usually go through until they are absorbed
- You only see objects around you when they are illuminated by sunlight or by lamps. In normal surroundings there are no X-rays. Unless he used an X-ray flashlight, Superman's X-ray vision would work no better than your own vision in total darkness.
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?
Why?
- Because the corona is very hot, about a million degrees centigrade. In a hot gas ions and electrons bounce around with greater speed, and when they collide, more energy is released. The photons they produce are also likely to have more energy--that is, rather than visible light, they may be in the region of X-rays or the Extreme Ultra-Violet (EUV).
What are coronal holes?
- Dark areas of the corona, seen in EUV or X-rays. The first extensive observation of such "holes" was done from space station "Skylab" in 1973.
What is the connection between coronal holes and the solar wind?
- The solar wind coming from coronal holes seems to move faster.
Any reason suggested?
- Coronal holes probably occur in regions where the magnetic field does not greatly impede the flow of plasma.
(Teacher may supplement)
Strong magnetic field lines can guide the motion of plasma--it moves easily along them, not so easily from one field line to its neighbor.
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?
- They don't avoid the poles, on the contrary--each polar region seems to be a near-permanent "coronal hole." The space probe "Ulysses" passing over them observed fast solar wind streaming out of them, similar to the way it streams from coronal holes.
Spacecraft in the solar wind observe there a weak magnetic field. What is the origin of that field?
- The field comes out from the Sun, and is dragged out by the solar wind.
(Here, if time allows, the class may learn more about the interplanetary magnetic field and draw its configuration. That project is described here.)
What are "Coronal Mass Ejections" (CMEs)?
- CMEs are big bubbles of plasma, threaded by field lines of the Sun, which get blown away by releases of magnetic energy.
Why are space researchers interested in CMEs?
- The can cause magnetic storms if they reach Earth.
- They are big and energetic, but we are still not sure how they are propelled.
- Some move much faster than the solar wind and pile up shock fronts at their boundaries. Such shocks may generate high energy particles and might play an important role in magnetic storms
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.
The additional particles originate on the nightside of the Earth, in the long "magnetic tail" formed there. The same procedure also accelerates electrons along magnetic field that come down in the "auroral zone, " about 2500 km from the magnetic poles. When these electrons hit atoms of the upper atmosphere, around an altitude of 100 km., they produce light--the "northern lights" or "polar aurora. "
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)
- Monitoring satellites are stationed at the L1 Lagrangian Point, an equilibrium position between the Earth and the Sun, at about 1% of the Sun's distance. Any ordinary feature in the solar wind arrives at that point about one hour before reaching Earth. Unfortunately, a shock moving at twice the solar wind speed gives only half that lead time.
--Special spacecraft cameras can actually see CMEs being ejected. When ejected from the side of the Sun they appear as huge expanding bubbles, but of course, in that case they do not move towards Earth. If the spacecraft is near Earth and the CME is coming right at us, the detection of CMEs is difficult, because the Sun is on that same line of sight--although it has been done. In the future, however, spacecraft orbiting the Sun in the same orbit as Earth but located far from Earth (the "solar stereo" mission) should observe such CMEs from points far from their line of motion, and get a much better view.
What is "solar activity"?
- Active solar phenomena, associated with sunspots and their cycle, include flares, CMEs, bursts of radio waves, bursts of X-rays, and emission of high-energy particles from the Sun. All these seem to be powered by the energy of magnetic fields, which are concentrated in sunspots.
(Teacher explains further)
When magnetic energy is released on the Sun, many electrons are accelerated (just as auroral electrons arise on Earth). Electrons are lightweight (they only form about 0.05% of the weight of matter to which they belong) and are therefore held much more tightly by magnetic forces. Because magnetic field lines tend to form arches with both ends on the Sun, they guide the electrons accelerated on them back to the Sun's denser atmosphere. When those returning electrons hit dense matter, X-rays are produced.
How are medical X-rays produced in a doctor's office?
- Same way. Inside the X-ray machine is a glass tube empty of air, at one end of which electrons are emitted by a glowing wire. They are accelerated by a high voltage towards a metal target at the other end, and as they hit the target, X-rays are produced.
Do you think the polar aurora produces X-rays? Give your reason.
- Yes, it does, because here too, fast electrons hit matter--in this case, atoms of the atmosphere. Our atmosphere is too dense to allow them to reach the ground, but they can be observed in space.
(In fact, the "Polar" satellite carries an X-ray camera which takes auroral pictures in X-rays from above. This gives information about the energy of the electrons, which the visible auroral glow itself does not provide.)
Do you think that picture tubes of TVs and and computer monitors produce X-rays? Give your reason.
- Yes, they do, because again, a beam of electrons is stopped by a screen. These X-rays are all inside the tube, however, and the thick glass does not let them get out. Good reasons exist for not watching too much TV, but X-rays are not among them.
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.)
Monitoring satellites, such as the GOES satellites whose main task is weather observation from synchronous orbit, also record solar X-rays. They alert observers to unusual solar activity when the X-ray intensity suddenly rises.
(Optional)
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?
- Because high energy ions associated with flares and CMEs can be hazardous to the health of astronauts.
(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"?
- Each "great observatory" space mission is supposed to open a new range of the electromagnetic spectrum, with a better resolution than that of earlier observations (i.e. distinguishing smaller and fainter objects). By using the new capabilities to survey the sky, new phenomena are often discovered.
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