XRT Picture of the Week (XPOW)

XRT only sees the hottest regions of the sun's atmosphere (0.5-20 Million degrees) while Hα images show relatively cool plasma (~10,000 degrees) in the solar chromosphere. The above composite image shows how the hottest areas on the sun relate to the cool plasma below. Some of the interesting regions in the composite image are define below.

• Active Regions (AR) are regions where the sun's magnetic field is strong and hot plasma is trapped in the magnetic field. Active regions are bright both in x-rays and in Hα so hot and cool plasma gets trapped in the strong magnetic field.
• A filament (or filament channel, a.k.a. prominence) is cool, thick plasma that is suspended above the surface of the sun. Filaments develop over regions where the magnetic field is changing direction (in/out) of the sun. They don't emit any x-rays and are dark in Hα so they must be pretty cool. Notice how it it looks like the filaments are shrouded in x-ray's.
• A coronal cavity is a tube-like hole in the corona that typically sits above filament channels. Although we see x-ray clouds above prominences, the clouds are actually above these holes. Coronal cavities are best viewed on the solar limb.
• Prominence arcade loops are the hot coronal loops that prevent prominences from erupting. These are the loops that shroud prominences and cover coronal cavities. When a prominence erupts it has to tear these loops. This usually causes a coronal mass ejection and a solar flare.
• The south polar coronal hole is visible. These are regions on the sun where weak magnetic fields exist of the same polarity. The fast solar wind originates in coronal holes.
• The filament sigmoid is a highly unpredictable filament that is likely to erupt. They are usually near or inside active regions. Scientists who try to predict space weather pay close attention to sigmoids.