Once again, sunspot and related solar activity was very low last week. Nonetheless, the period featured two very nice eruptions during the evening hours of resp. 24 and 29 September. See this movie. The images underneath show the eruptions as seen with the SDO/AIA 304 filter in extreme ultraviolet (EUV).
The 24 September eruption
Solar filaments are clouds of ionized gas above the solar surface squeezed between magnetic regions of opposite polarity. Being cooler and denser than the plasma underneath and their surroundings, they appear as dark lines when seen on the solar disk using special filters. At the solar limb, the same features shine bright over the solar limb and are called prominences. Some filaments are long and high enough such that part of it is at the same time visible as a prominence. These are called "filaproms", and the 24 September feature was just such a filaprom. See the top two images of the collage underneath, which are combination images from SDO/AIA 304, 171 and 094.
Around 18:30UT, some reconnection took place rendering the feature unstable and resulting in the eruption. At about 20:30UT, a solid pillar of charged particles towered an estimated 300,000 km above the solar surface. See the two middle images in the above collage.
At the same time, the magnetic field lines were also somewhat relaxing, transforming the solid tower into a wide, rotating fan. Part of the filament being ejected, the rest returned along the magnetic field lines crashing unto the solar surface (bottom two images in collage).
The 29 September eruption
The source of the 29 September eruption was a long and solid filament in the northwestern solar quadrant. It had been transiting the solar disk since about 20 September, and gave the Sun a dark scar. Because of its length and solidness, space weather forecasters were expecting it to erupt.
As shown in the images underneath, the filament became unstable and erupted around 21:30UT. The outlook of the eruption in H-alpha (filter showing the "cold" inner atmosphere of the Sun) was quite different from that in extreme ultraviolet (EUV). In EUV, the event could clearly be seen as an eruption, whereas in H-alpha, the eruption looked more like a sudden disappearance, which is actually the old name for this kind of events. The reason for this difference is in this case probably twofold.
First, the H-alpha filter is a very narrow spectral line. The speed of the ejected material may be high enough to shift it out of the filter line, effectively making the erupting material invisible in H-alpha. It's somewhat similar to your eyes not being able to see the wings of let's say a flying honeybee, though you can see them when the bee is sitting still.
The other reason for the different outlook is that the H-alpha filter has a temperature range that is a lot smaller than the AIA 304 filter. Indeed, H-alpha depicts the Sun's atmosphere close to about 10,000 degrees, but AIA 304 images the Sun at temperatures between 50,000-100,000 degrees. Hence, the heating from the eruption is enough to drive the filament temperature out of the H-alpha temperature range and thus to make the ejected filament plasma disappear in H-alpha images, whereas the erupting filament remained well visible in AIA 304 due to the broad temperature range of that filter.
Further research is required in order to determine which of the two aspects was the dominant one.
In contrast to the eruption 5 days earlier, the 29 September event resulted in a minor C-class x-ray flare (a so-called Hyder flare, albeit a weak one). It concerned a long duration event that started at 21:43UT, so about 15 minutes after the first visible signs of the eruption in H-alpha and EUV. The flare reached its maximum at 23:39UT and lasted 200 minutes (over 3 hours!). Numerous post-flare coronal loops were visible. Another difference was that this eruption was also associated to a moderate proton event. This was only the fifth such event this year.
It comes as no surprise that also this eruption was associated with a CME. This time, SOHO was able to observe the evolution of the spectacular CME in its entirety. It was moving with an estimated plane-of-sky speed of at least 900 km/s (STEREO-B images, core filament directed mainly towards the north). The ejected filament nicely preserved its shape as can be seen from this comparison between a PROBA2/SWAP difference image at 21:55 UT and a SOHO coronagraph image at 02:30UT. For STEREO-B, the eruption showed as a backside halo event, from which one could conclude that Earth might still receive a glancing blow 2-3 days later. As it turned out, Earth already received an impact from the CME during the early morning hours of 2 October, leading to moderate geomagnetic storm conditions. It was the first strong geomagnetic disturbance since late August (see the previous STCE Newsletter).
Credits - Data and imagery for the movie clip were taken from the GONG H-alpha network, SDO, STEREO-B, PROBA2, and SOHO/LASCO.
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