On 1 September around 11:05UT, the STEREO-B spacecraft observed a huge blast on the farside of the Sun (see this movie). The maximum strength of the flare was cautiously estimated to be between M5 and X1.5, giving it a good chance the event belonged to the category of "eXtreme" flares.
The eruption took place in a region that developed on the Sun's far-side. This can be seen when comparing the view in extreme ultraviolet (EUV) of 1 September with the view from Earth 2 weeks before. On the SDO images, no active region at the corresponding location can be seen. Note that the solar axis is currently slightly (about 7 degrees) pointed towards the Earth, and away from STEREO-B over about the same angle. This gives the false impression that the active regions, as seen from STEREO-B, are located on more northern latitudes. Note that another strong flare, albeit slightly weaker, took place only hours later, peaking at 22:15UT in or very near old sunspot group NOAA 2144.
A strong EIT-wave could be seen in difference images from both STEREO-B and SDO, meaning that the wave was strong enough to travel all the way from the back to the earth-facing side of the Sun. This has happened before, such as e.g. on 5 March this year (see this news item for more info on this event and EIT-waves). The arrival of the EIT-wave at the frontside can also be seen in these before/after difference images from PROBA2/SWAP.
The EUV images from STEREO also showed coronal dimmings as well as post-flare coronal loops. Space and ground based radio-instruments recorded a type II radio-burst. This was all indicative that a plasma cloud was ejected into space. This coronal mass ejection (CME) was observed both by SOHO and STEREO-B. The huge CME had an estimated plane-of-the-sky-speed of around 1000 km/s. Images underneath were taken by STEREO-B and SOHO about an hour after the flare. Note STEREO-B is almost at the opposite side of the Sun compared to SOHO.
However, the most amazing effect associated to this eruption was the solar energetic particle (SEP) event. There were so many particles slamming into the camera pixels (creating the white dots in the images) that they saturated the star-trackers onboard STEREO-B, and the spacecraft lost lock on the Sun. This can be seen in the movie, where the solar image started to rotate clockwise from about 16:00UT till about 20:30UT, before resetting itself and providing again images with the correct orientation. More info can be found at SOHO's pick-of-the-week.
The energetic protons also found their way to Earth via well connected magnetic field lines. Around 21:00UT on 1 September, proton fluxes gradually started to rise. This concerned also particles with energies 10 times greater than the ones usually used for characterizing proton events. Nonetheless, fluxes rose not enough to get this enhancement characterized as a genuine proton event, as the maximum flux stayed just below the alert threshold. The effect of the enhanced proton flux was only barely noticeable in the SOHO imagery. More information on the intensity of proton events can be found in this news item. Amazingly, a week after the eruption, the proton flux was still well above the nominal values!
Credits - Data and imagery were taken from SDO, PROBA2/SWAP, SOHO/LASCO, STEREO, CACTus, LMSAL (movie here), (J)Helioviewer, and daily reports from NOAA/SWPC and SIDC/RWC.
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