Solar filaments are clouds of charged particles ("plasma") above the solar surface squeezed between regions of opposite magnetic polarity. Being cooler and denser than the plasma underneath and their surroundings, they appear as dark lines when seen on the solar disk. Special filters are required to observe these features, such as in the Hydrogen-alpha (H-alpha) line in the red part of the solar spectrum at a wavelength of 656.3 nm, or in some extreme ultraviolet (EUV) filters. The annotated H-alpha image underneath (GONG) shows a 15-degrees long filament anchored on one side to the leading sunspot of NOAA 13990. Late on 22 February, it was at the solar limb as seen from Earth. Two days later, it was also close to the southwest solar limb as seen from STEREO-A (EUVI 195). Indeed, that spacecraft is currently preceding the Earth by about 30 degrees, and so it takes the Sun -with its average rotation rate of about 13 degrees per day- 2 days to bring the feature in about the same position as seen from STEREO-A. The H-alpha image is in red and overlaid on the greenish EUV image from STEREO-A. The portion that is not overlapped, shows actually that part of the Sun that cannot be seen from Earth, but only by STEREO-A.
Long filaments are known for their potential to erupt, and so this one was no exception. Starting around 20:00UTC on 24 February, the surrounding magnetic fields became unstable and the filament got ejected into space. The image to the right in the above comparison actually shows the top portion of the filament at the start of its lift-off. It took about 1.5 hours for the complete ejection of the filament, and at that point the post-eruption coronal loops ("arcade") started to rise. As soon as they towered over the solar limb, the GOES soft x-ray flux started to rise with the onset of the associated flare at 21:50UTC. This eventually would become a long duration M3.9 flare, peaking at 23:02UTC and lasting no less than 2 hours and 29 minutes - as shown by the GOES and PROBA2/LYRA curves underneath.
The imagery underneath shows the eruption in extreme ultraviolet, with on the left SDO/AIA 094 giving a multi-million degrees view of the eruption (as seen from Earth), and on the right STEREO-A/EUVI 195 showing the eruption at about 1.5 million degrees. The STEREO-A images give a better view on the filament eruption and the "arcade", whereas the significantly hotter SDO channel provides a good view on the supra-arcade downflows (SAD, discussed in this STCE newsitem), which are the dark streaks flowing over and down towards the rising coronal loops. Note the 2-hours difference between the stills.
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The violent eruption was most likely the source of a minor proton event, which started on 25 February at 00:20UTC and reached a peak of 37 pfu at 02:40UTC. The higher energy proton fluxes (greater than 100 and 500 MeV) remained at their background levels, so except for the increased noise in the EUV images, the impact was all-in-all limited. The eruption was also associated with a solid and fast partial halo coronal mass ejection (CME), which was obviously not directed to Earth in view of the location of the source (behind the solar limb). The CME had a speed near 1100 km/s, reaching "O" ("occasional") on the SCORE-scale for CME speeds (Evans et al. 2013). The clips underneath show the white light coronagraphic imagery from SOHO/LASCO (left) and STEREO-A/COR2 (right) overlaid on respectively EUV-imagery from GOES/SUVI 094 (wider field than SDO/AIA) and from STEREO-A/EUVI 195. Note that in the stills, there's an hour difference between the two and they are not on the same size. In both cases though, no annotations are needed to indicate the associated CME... It really has a good chance to make it into the Top 10 of most spectacular CMEs of 2025!
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