Space weather forecasters were surprized to see the proton flux (GOES) going up early today 23 July, as there was no particularly strong solar flare or filament eruption observed on the Earth's facing solar hemisphere (GOES ; chart). The fluxes of the more energetic protons followed suit. The greater than 10 MeV proton flux exceeded the event threshold of 10 MeV at 03:00UTC, and is still close near its maximum of about 24 pfu. The greater than 100 MeV proton flux exceeded the 1 pfu threshold at 02:00UTC, and reached a maximum of 1.6 pfu. As a result, no effect from these particles on the Earth's surface have been observed (no Ground Level Enhancement).
At 00:24UTC, a halo coronal mass ejection (CME) became visible in coronagraphic imagery by SOHO/LASCO C2. CACTus revealed a projected (plane-of-the-sky) speed around 600 km/s. LASCO images showed noise from the energetic protons impacting the camera's pixels (LASCO C3 clip underneath). No important radio bursts were observed around the time of this event (OVSA, Learmonth).
So, all this was indicative that the event had taken place on the farside of the Sun. But how intense was it? Once again, Solar Orbiter came to the rescue. As it happens, this spacecraft is currently observing the Sun's farside, being separated by 159 degrees from the Earth (map). Its Spectrometer Telescope for Imaging X-rays (STIX) is a hard x-ray (HXR) imaging spectrometer (see this STCE newsitem for a brief description), and recorded a strong flare on 22 July at 23:56UTC. Though STIX' HXR measurements can of course not directly be compared to the GOES' soft x-ray (SXR) recordings, given enough statistics the STIX readings can be correlated and used to provide an estimated value of the GOES SXR intensity. Thus, based on the STIX observations shown underneath (STIX data Center), the eruption reached an estimated GOES SXR intensity between X6.8 and X30, with X14 the most likely value. This is slightly stronger than the 20 May event (X12, also on the farside), and would make it the strongest solar flare so far this solar cycle (SC25), far and front side combined.
Thus remained the question of where exactly the eruption had occurred. Enter Solar Orbiter's Full Sun Imager (FSI), which is part of the EUI instrument (Extreme Ultraviolet Imager - https://www.sidc.be/EUI/intro). Only 2 images in each wavelength -30.4 nm (temperature around 80.000 degrees) and 17.4 nm (around 1 million degrees)- are available for 23 July so far, one at midnight and one at noon, but that was enough to pinpoint the location of the eruption near the Sun's disk centre as seen by Solar Orbiter. The images underneath show the Sun in extreme ultraviolet (EUV): once by FSI 304 near 00:00 UTC - which is at the time of the eruption-, and a comparison in FSI 174 at 00:00UTC and at 12:00UTC showing the evolution of the flare. Back-tracing based on the observed position reveals that the source region is NOAA 3738, which was also responsible for 2 X-class flare on 14 and 16 July (see this STCE newsitem).
From the observed direction of the halo CME (away from the Earth), it may be worthwhile to keep now also an eye on SolO's MAG instrument, as the CME is heading straight for Solar Orbiter. When the interplanetary CME passes the spacecraft -most likely tomorrow 24 July-, the Bz value observed by the MAG instrument may give an indication of the strength of the ICME. Just for reference: during the grand 10-11 May 2024 geomagnetic storm, Bz reached -50 nT (ACE/DSCOVR)...
