Pedagoguery

Our sun is a tempestuous star. The eleven year solar cycle periodically bombards earth with tremendous bursts of particles and radiation. How does this happen, and what causes these outbursts? Scientists have made great strides in recent years toward answering those questions.

The primary fact to understand about the sun, is that it is a plasma. That is to say, that a significant fraction of the matter that makes up the sun is ionized; its atoms have one or more electrons stripped off by the energetic nature of the environment. Plasmas therefore contain charged particles, and charged particles in motion create magnetic fields. It is the sun's magnetism that is the underlying cause of both the sunspot cycle, and solar storms.

At the minimum of the solar cycle, the sun's magnetic field is strongest at its poles, and it is relatively uniform. However, since the sun is not a solid body, the equator rotates faster than the poles. Magnetic field lines cannot move freely in a conducting medium, such as a plasma, so they tend to follow the rotation of the sun. As a result, they get stretched, so that eventually, instead of the lines going smoothly from pole to pole, at the equator, they are running perpendicular to the poles. Tension builds and the lines bulge out, creating localized knots of magnetic energy. Because the energy of the magnetic field increases, the local thermal energy decreases to keep things in balance. What we now have is a visible as a sunspot.

Early on in the cycle, sunspots are more common at the lower latitudes, and they are small in size. As the cycle continues, and magnetic field lines get more twisted and contorted, they start getting bigger and appearing at higher latitudes. Individual sunspots evolve as well. Sometimes, their magnetic field lines will straighten out, and the sunspot will fade. Other times, magnetic field lines will cross; and if the crossing lines are of opposite polarity, they can reconnect, leaving a loop of magnetic field free of the sun. The reconnection releases tremendous energy, and the loop is typically ejected, along with the plasma it contains. This is a solar flare.

The reconnection event generally releases large amounts of electromagnetic radiation – much of it as ultraviolet and x-rays. The outer portions of the sun's atmosphere, the corona, factor into what happens with the plasma loop. The corona also has a magnetic field. The corona is also much more diffuse, and astoundingly hot – hot enough to glow in soft x-rays. (How the corona gets heated to this level is still a mystery.) The corona's magnetic field acts as a net, keeping those loops contained. However, if the loop's polarity is opposite to the polarity of the corona's magnetic field, a new reconnection event can occur, and the net rips, propelling the plasma out at high speed. This is called a Coronal Mass Ejection, or CME.

CME's can be dangerous, both to humans and to satellites. The concentration of highly energetic charged particles, can create strong induction currents, frying delicate electronics. The charged particles, can also ionize atoms within human tissue, or even knock atoms out of molecules completely. This can cause dangerous free radicals or even break long molecular chains, such as DNA. Fortunately, the earth's magnetic field shields us from the worst effects of CME's. However, it is not a perfect defense. If a CME is launched in earth's direction, it generally takes one to three days for it to reach us. How the CME affects the earth then depends strongly on whether its magnetic polarity is the same as or opposite to the earth's. If it is the same polarity as the earth's, the dayside magnetic field will be compressed, and the plasma will be redirected to the poles, where it will hit the atmosphere and cause spectacular aurorae. The shock could cause a geomagnetic storm, making the earth's magnetic field vibrate, and causing induction currents like those that shut down Quebec's power grid in 1987. Typically, though, the worst that this type of CME will do is cause spectacular auroral displays.

If the CME has the opposite magnetic polarity to the earth, however, the situation worsens. Not only does it compress the earth's magnetic field, it can actually peel back the outer layers of it, causing the lines to reconnect on the night side. This reconnection releases energy in the form of electric current which travels down the magnetic field lines to the poles, potentially causing large geomagnetic storms.

Fortunately, events such as those described are uncommon, and in general they pose a greater danger to satellites than to us. However, if we ever achieve a significant presence beyond earth orbit, they are definitely something we will have to plan for.

Next issue, I will talk about the amazing possibilities offered by space tethers.