While the Sun is relatively quiet, we can take a moment and review the four wavelengths of extreme ultraviolet light in which SOHO observes the Sun. The images (from left to right) observe different material above the Sun's surface and into the corona at increasing temperatures, each of which reveal different solar features. All the colors are added so that we can tell immediately which wavelength we are looking at. Remember that these wavelengths are not visible to the human eye nor can they be viewed from Earth since they are blocked by our atmosphere.

The first reddish image (at 304 Angstroms) shows ions of helium at 60,000 degrees C., which are found in the lower corona, not terribly far above the Sun's surface. Next, in the green wavelength (at 195 Angstroms) we see ions of iron at one million degrees C., which are higher up in the corona. The blue image (at 171 Angstroms) sees ions of iron at an even hotter temperature, 1.5 million degrees C. And lastly, we see the upper corona in ions of iron at 2.5 million degrees C.

Yes, counter to what one would expect, the corona gets hotter and hotter as you go out from the Sun. This phenomenon of solar coronal heating has puzzled scientist for years, but they are making real progress on understanding how this happens.

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Previous Picks of the Week

SOHO began its Weekly Pick some time after sending a weekly image or video clip to the American Museum of Natural History (Rose Center) in New York City. There, the SOHO Weekly Pick is displayed with some annotations on a large plasma display.

If your institution would also like to receive the same Weekly Pick from us for display (usually in Photoshop or QuickTime format), please send your inquiry to steele.hill@gsfc.nasa.gov.