Introduction

The Corona

   At an intensely hot one to two million K, the corona is warm enough to glow in x-rays. It is nearly a billion times less dense than the photosphere. For comparison, air is about a thousand times less dense than water. Due to the extremely low density, the movement of plasma in the corona is ruled by the ever-shifting magnetic field. In some regions (most commonly the poles) the magnetic fields are open and plasma travels out along the field lines creating the solar wind. In active regions, the magnetic field is highly concentrated and forms closed loops. Prominences and filaments are the outlined by cooler plasma suspended within the confines of an arched magnetic field. Occasionally, a large amount of plasma is violently expelled from an active region. This event is known as a coronal mass ejection, or CME.

Coronal Mass Ejections

   The few CMEs that intercept earth are known as halo CMEs. Charged particles from the CME follow the lines of the earth's magnetic field, concentrating at the poles and sometimes causing beautiful auroras. If conditions are right the magnetic field of the halo CME will interact in a more destructive way with the earth's magnetosphere. In some cases the protective magnetosphere is pushed back, leaving satellites and astronauts exposed to a potentially fatal rain of highly energetic ions. Processes are set in motion that can lead to induced currents in pipe and power lines. Eventually the damage from these currents can lead to failures of flow rate monitors, and even large portions of power grids. The ability to predict CMEs would be useful in light of our heavy dependence on electric power and satellites.

    Sigmoids

        Often an S or inverse S shape, known as a sigmoid, appears at active regions that produce CMEs. This distinctive shape is easily seen in x-ray images of the sun. A deeper understanding of the processes that contribute to the formation of sigmoids will give greater insights into the mechanics of coronal mass ejections.