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. As filaments grow longer, they are more likely to erupt, often accompanied by a coronal mass ejection (CME).
An impressive filament eruption took place during the morning hours of 1 November 2014, near the southeast solar limb. It was accompanied by a long duration C2.7 flare, starting at 04:44UT and ending only at 07:05UT. This was a typical example of a "spotless" flare, that is a flare occurring far from any active sunspot region. It is also called a Hyder flare, after the scientist who first studied this kind of phenomena in detail and published two hallmark papers on it in 1967 (see note 1). The filament eruption and flare can be seen in the above images of the Sun's lower chromosphere (H-alpha), taken at 04:10, 04:40 and 05:10UT. SDO's white light image (upper left) shows the absence of any nearby sunspot group. A movie showing the event in various wavelengths can be found here.
Most likely, the filament eruption was also associated to a small proton event (graph above). Proton flux started to increase on 1 November at 17:30UT, that is nearly 12 hours after the flare's peak (05:34UT). The greater than 10 MeV proton flux only briefly reached the event threshold (10 pfu - see note 2) on 2 November at 21:10UT, and occasionally between 22:10 and 22:45UT. In view of the peculiar evolution of the proton flux, it is not excluded that there were other sources for or contributing to this proton event.
The image above was take at 07:30UT through AIA 171 and AIA 304 filters (transition zone and lower corona). The expanding flare ribbons ("parallel ribbons") and the series of post-flare coronal loops ("arcade") that can be seen are the effect of the reconnection higher up in the solar atmosphere. The charged particles get accelerated towards the denser inner solar atmosphere, where they collide with other particles and heat the local chromospheric environment and make it evaporate. The footpoints of some faint coronal loops can be seen embedded in the expanding disturbance.
The coronal mass ejection (CME) associated to this filament eruption had an average plane-of-the-sky speed of nearly 700 km/s, but the bulk was not earth-directed. In the images above, the loops that can be seen are probably part of the same loop system but seen in different wavelengths and temperatures, i.e. SDO/AIA171 and 193, and SWAP174 in EUV (resp. at about 700.000 degrees, 1.3 and 1 million degrees), and LASCO in white light (density driven – images underneath).
This filament eruption bears quite some resemblance to another event just over 2 years ago, when, on 31 August 2012, a filament similar in length and position, but slightly more to the central meridian, erupted. It was associated to a Hyder flare (C8) and a proton event that reached 59 pfu at its maximum. At the time, Earth received a glancing blow from the CME resulting in a minor geomagnetic storm. See this STCE News item from 7 September 2012.
Note 1 - Hyder, Charles L., A Phenomenological Model for Disparitions Brusques followed by Flarelike Chromospheric Brightenings, Part I and II. A comprehensive summary can be found on this IPS webpage.
Note 2 - More info on proton events and pfu (particle flux unit) can be found in the STCE news item dated 23 January 2014.
Credits - Data and imagery for the movie clips were taken from the GONG H-alpha network, SDO, SOHO/LASCO, PROBA2 and (J)Helioviewer.
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