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. Space weather forecasters keep an eye on these filaments. Indeed, as the magnetic regions suspending the filament may become unstable, the filament can erupt and throw a cloud of charged particles towards the Earth where it can cause geomagnetic disturbances. These eruptions are more likely to occur as the filament grows longer, typically around 200,000 km. Such long filaments usually develop outside sunspot groups.
Two such filaments, each about as long as the Earth-Moon distance, have appeared this month on the Sun's southern hemisphere and eventually erupted on 14 and 20 August. This movie shows their transit over the solar disk and final eruption with accompanying coronal mass ejection. The above images show the same filaments in H-alpha and the subsequent eruptions in extreme ultraviolet (EUV). Interestingly, early August 2012, there was also a big solid filament on the southern hemisphere, eventually erupting one solar rotation later (see the STCE Newsletters 155 and 157 for more information).
Filaments appear all over the solar surface, but long and dense filaments such as the ones described above are not produced that often. In fact, for the ongoing solar cycle, only 14 were identified on the southern hemisphere (the recurring not included). 4 of those (portrayed above) appeared in a relatively small area only about 60 degrees wide (blue area in sketch underneath). They have been appearing every 4 to 6 months since April 2012.
The other group contains 8 filaments, with 6 of them appearing between October 2010 and April 2012. Outlined by the green area in the sketch above, they are located about 10 degrees further away from the solar equator and have heliographic longitudes between 60 and 210 degrees. The most recent big filament (the one that erupted on 20 August) belonged also to this group, as outlined in purple on the sketch. It ended a 10-month drought of long and solid filaments in this area.
Note that the "presence" of these "groups" may be purely coincidental, due to the small number of filaments and the on-sight selection. A more detailed and profound study, taking into account e.g. the wandering of remnant magnetic fields from the sunspot zone to the polar regions, should clear this out.
The two remaining filaments do not belong to one of these groups. Indicated in fluo-green is the filament of August last year, which is located clearly within the sunspot zone close to the solar equator. The other one is the filament that erupted on 14 August (red line in sketch). Though there have been long (but fragmentary) and various small filaments in this longitude band before, it seems this is the first long and solid filament to appear in this area of the solar surface during this solar cycle.
Interestingly, another long -but not as solid- filament was visible this week and erupted early on 29 August. Its location falls right in between the blue and green shaded areas in the sketch, which is nearly opposite to the position of the 14 August filament.
Credits - Data and imagery were taken from Kanzelhöhe Solar Observatory, the GONG H-alpha network, SDO, and SOHO/LASCO.
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