Layer upon layer
posted: May 27, 2015

Last week, the sunspot activity was again all but exciting. Only a few, very small, and mostly inactive sunspot groups were visible in white light. In H-alpha, which is a filter showing the "cold" inner atmosphere of the Sun, the Sun's outlook was dominated by some long filaments. These are local belts of plasma squeezed together by opposite polarity magnetic fields (see last week's newsletter). Moving up to the hot outer solar atmosphere, the extreme ultraviolet (EUV) imagery from SDO showed darkish patches. Some of those patches contain the filaments seen in H-alpha and are called filament channels. Others are indicative of low density areas and correspond to coronal holes, allowing the fast solar wind to escape into space. At even higher temperatures, x-ray imagery show these coronal holes even more prominently, though part of them sometimes correspond to areas where there are simply little active (hot) areas - see this STCE Newsletter of 7 December 2012 for more details.

The image above shows the Sun on 25 May in the 4 above-mentioned filters. All images were taken between 09:10 and 09:16UT, so the locations of the different features correlate very well. The solar outlook changes dramatically when moving from the solar surface (white light) all the way up to the very hot solar atmosphere. This can also be seen in this gif-animation underneath.

All the relevant features can also be brought together on one image, as is done below. The basic image is a white light disk image on which the filaments (red, fat lines) are indicated. The dashed green lines indicate the somewhat dark areas seen in the EUV images. The prominent dark patches seen in x-ray imagery are indicated in blue. The bright areas are indicative of active regions, not necessarily sunspot groups, and are represented by the dashed white contours. The whole has been overlaid on the x-ray image showing the hot outer solar atmosphere. Clearly, the Sun's atmospheric structure is much more complex than a simple white light image would suggest.

Credits - Images were taken from the GONG H-alpha Network, SDO, and GOES/SXI.