A beautiful and puzzling full halo CME on July 16


Above is a picture from the LASCO C2 coronograph showing a full halo surrounding the solar disk. This CME left the Sun on July 16 around 16UT. Its origin and direction were puzzling. It was not certain whether the CME was directed straight towards Earth or was directed away from us.

The CME was so fast (~1100km/s) that it could have escaped without leaving a sign of any disturbance on the SoHO EIT images (FeXII 195). One possible source was the Catania sunspot group #16 (NOAA 30), a very complex and extended sunspot group North in the middle of the disk. To comfort the idea that this CME was frontsided were the LASCO images, hitted by cosmic rays a few hours later. The >10Mev component passed above event threshold around 18UT on July 16. This could have then illustrated a type of proton event driven by the CME interplanetary shock wave.

But such event could not be related to any disappearing filament, nor flare, nor even a radio burst. Moreover, in the farside of the Sun, a southern sunspot group likely to be the ex Catania group #95 (NOAA 19) seemed also to be a centered possible source for the CME. Archimedean magnetic field lines connecting this region to the Earth were likely followed by the protons.