The Sun produced an X-class flare on both 24 and 26 October, events that were discussed in the STCE newsflashes here and here. The first X-class flare, an X3.3 event, was produced by NOAA 13869. The X1.8 flare from 26 October was produced by NOAA 13873. These sunspot regions were part of a string of sunspot groups visible in the southeast solar quadrant. They are in the same location on the Sun as another series of sunspot groups just a solar rotation ago. The latter were the source of some of the strongest solar flares so far this solar cycle (STCE newsitem). Underneath is an annotated white light image of the solar disk on 1 October (upper image) and on 28 October (lower image). The numbers refer to the NOAA active regions, the arrows link the old with the returning and/or new sunspot regions. Only for the returning groups NOAA 13872/13869, it is not so clear how much is the remainder of the old sunspot duo NOAA 13839/13844, and how much is from newly emerged sunspots. Different group splittings are possible.
Both X-class flares were associated with Type II and Type IV radio emission (see the STCE SWX classification page), as well as mild radio bursts at all observed frequencies. In both cases, also a Tenflare was observed, i.e. more than a doubling of the solar radio flux at 2800 MHz (at a wavelength of 10.7 cm) above the pre-event background. Values recorded were resp. 5900 sfu and 4200 sfu. The greater than 10 MeV proton flux became mildly enhanced following the X3.3 flare, but finally crossed the alert threshold on 26 October at 19:10UTC, driven by the X1.8 flare. This can be seen in the annotated images underneath of the GOES x-ray flux (top) and GOES proton flux (bottom). The proton event reached a maximum of 364 pfu on 28 October, but was still above the 100 pfu level at the time of this writing. At this level ("S2" on the NOAA scales), satellites may have experienced an occasional single event upset (SEU , a glitch in the software operations), and High Frequency communications (HF Com ; 3-30 MHz) may have been somewhat disturbed over the polar regions.
Both eruptions were associated with a asymmetric halo coronal mass ejection (CME ; see the SOHO/LASCO C3 coronagraphic imagery above), due to the close proximity of the source regions near the southeast solar limb. The CMEs had a speed of resp. about 1300 km/s and 1600 km/s. In both cases, the bulk of the CME was directed away from Earth, but both delivered a glancing blow to the earth environment. Travel time for these interplanetary CMEs was resp. about 59 hours and 45 hours. Shock-like structures were observed in the solar wind as recorded by DSCOVR on 26 and 28 October, with maximum speeds observed of resp. about 500 and 600 km/s (yellow curve in graphs underneath). The arrival of the first ICME on 26 October resulted in a modest spike in the greater than 10 MeV proton flux around 16:30UTC, remaining just below the 10 pfu alert threshold (see annotated graph above). This is called an Energetic Storm Particle (ESP) event, which is a type of solar energetic particle (SEP) events characterized by the usually rapid increase (at least 20% above background) in the greater than 10 MeV proton flux associated with a shock driven by a CME (Ameri et al. 2023). Such an ESP signature was much less obvious from the ICME that arrived early on 28 October, with maybe a hint around 04:45UTC. The resulting geomagnetic disturbance from the ICMEs was all-in-all mild, as could be expected from a glancing blow. The global Kp index reached resp. active and minor storming conditions, whereas the K_BEL was at resp. active and unsettled conditions. The passage of both structures left no obvious fingerprint in the evolution of the Dst index (Disturbance storm time index), which remained at quiet levels (Dst > -30 nT).
