Another farside blast
posted: October 01, 2014

NOAA 2158, source of the most recent X-class flare on 10 September, rounded the west limb on 17 September. Though sunspot-wise, the region seemed to be decaying, this certainly has not been the case as far as it concerns its flare activity. So, whether this is still old region NOAA 2158 or some new development in and around what used to be NOAA 2158, this area remains interesting since it did not produce eruptions frequently, but several big ones. In particular the events of 1 and 10 September are standing out, as discussed in these news items here, here, and here.

On 24 September, this active area released another strong flare. It was located at the Sun's farside, almost on the opposite side of the Sun as seen from Earth. The flare started around 20:40UT, and was accompanied by post-flare coronal loops ("arcade"), coronal dimming, and an EIT-wave. Unlike the previous two big flares, no enhancement in solar energetic particles was observed this time.

Interestingly, the related EIT-wave made it all the way to the solar north pole and the Earth-facing solar hemisphere. This can be seen in difference images from the PROBA2/SWAP and SDO/AIA instruments. Around 21:40UT, some darkish "movement" can be seen in these images heralding the arrival of the EIT-wave. See this news letter for more info on EIT-waves. Amazingly, the SDO images (LMSAL/Nariaki Nitta) also show the related halo coronal mass ejection (CME) leaving the farside solar hemisphere, despite AIA not being a coronagraph!

A movie showing the halo CME, flare and EIT-wave can be seen here. Imagery was taken from SOHO, CACTus, SDO, STEREO, and PROBA2.