Why are SDO images sometimes black and why does GOES-15 not return any x-ray data?

[Marcel de Bont]

With solar and geomagnetic activity being so low now, there is absolutely nothing interesting to report about. There are only two sunspot groups visible but they are very small. This gives us some time to answer a common question during this time of the year:

Why are images from SDO sometimes black and why does GOES-15 occasionally not return any x-ray data?

Around the spring and autumn equinoxes, the Earth or the moon comes in between Earth orbiting satellites and periodically blocks the line-of-sight between them and the Sun. This lasts only a short period every day. The time that the Earth blocks the view of the sun ranges from minutes during the beginning and end of the eclipse season to just over an hour. These so called ''eclipse seasons'' which occur twice a year last for around 45 to 60 days.

SOHO and ACE do not have this problem because they are stationed at Lagrangian point 1 and not in an orbit around Earth like the GOES satellites and SDO. The STEREO spacecrafts orbit the Sun so they also do not experience any eclipse seasons.

If you do encounter dark EIT images from SOHO, it is possible that they are preforming a so-called CCD-bakeout. During these bakeouts they raise the temperature of the CCD for maintenance reasons and no images are being made. This occurs about four times a year and has nothing to do with the eclipse seasons that Earth-orbiting satellites experience.