NOAA 3912 was the source of an X2.2 flare that took place on 8 December, peaking at 09:06UTC (GOES). The region was not particularly large, as can be seen in the GONG white light image underneath left taken around the time of the flare's maximum. The middle image shows the magnetic polarity of the regions, indicating some magnetic complexity for the source region. The image on the right shows the flare in H-alpha, a line in the red portion of the solar spectrum, at the time of the flare's maximum. The bright feature is the flare and it corresponds with the location of NOAA 3912.
The imagery above shows the flare in extreme ultraviolet (EUV), as recorded by the GOES/SUVI instrument. It was associated with a Type II radio burst (see STCE SWx Classification page ), which was also observed by the Humain Radioastronomy Station in Belgium, as can be seen in the spectrogram underneath. The horizontal axis represents time increasing from left to right and the vertical axis represents frequency (in MHz) decreasing upward to represent the decreasing density with increasing altitude in the solar atmosphere. The evolution of the solar soft-x-ray flux (GOES) is displayed on top of the spectrogram. A coronal mass ejection (CME) was indeed visible in the SOHO/LASCO coronagraphic imagery starting at 09:24UTC, but not very wide and directed primarily to the west. The CME is not thought to have an earth-directed component (SIDC).
Further radio observations revealed that this was a Castelli-U burst (or U-burst for short). It was named after John Castelli, a military in the USAF (United States Air Force) who worked extensively in the radar and radio domain during World War II and in the 1950-70’s. This "Castelli-U spectral burst" is characterized by a U-shape in a graph depicting the intensity of the burst vs. the observed radio frequencies, i.e. the highest intensities are usually found at frequencies below +/- 1000 MHz and above a few thousand MHz, and a dip in-between (Castelli et al. 1967 - ; Castelli et al., 1977). This can be seen in the graph underneath for the 8 December eruption, based on data from the NOAA/USAF network. The feature requires a specific set of conditions, usually only satisfied with strong flares (see e.g. Chertok et al. 2009 - ; Grechnev et al. 2008). Details, conditions, and further explanations can be found at https://www.spaceacademy.net.au/env/sol/solradp/solradp.htm (at the bottom of the page). Castelli-U bursts were used for predictions of proton events (See https://www.spaceacademy.net.au/spacelab/models/ppm.htm ; Kiselev et al. 2022), but even today -especially when associated with important Type II and Type IV radio bursts- it is sometimes considered as an indication of a pending proton event. Alas, no proton event or enhancement was associated with the X2.2 flare on 8 December. Nowadays, it is also useful in the sense that a U-burst often means that GNSS frequencies (1.1-1.6 GHz ; Global Navigation Satellite Systems such as GPS and Galileo) were amongst the least affected by the eruption, compared to the lower and higher frequencies. Of note is that for this event, the intensity at 245 MHz reached 270.000 sfu. This is not a record-breaking value, but still important enough that it may have impacted some radar or communication applications for a short while.
