Solar prominences 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 bright blobs when seen near the solar limb and as dark lines when seen on the solar disk (then they are called "filaments"). 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 (656.28 nm), or in some extreme ultraviolet (EUV) passbands such as GOES/SUVI 195. Filaments are structures that only survive by the stability of the surrounding magnetic fields. Hence, longer filaments (from about 20 degrees in length onwards) are prone to erupt. Indeed, as the magnetic regions suspending the filament may become unstable, the filament can erupt and throw a cloud of ionized 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.
Last week, several filaments were visible in H-alpha (GONG). On 12 and 13 April, 3 of these filaments erupted. The smallest filament ("Fil 1" in the annotated H-alpha image above) was about 10 degrees in length and located in the northern hemisphere near the central meridian. It was the first to erupt around 11:00UTC on 12 April. The associated coronal mass ejection (CME) was narrow and directed to the north. It was quickly followed by the quiet dissipation of the southern half of a 50-degree (= more than half a million km!) long filament ("Fil 3") in the southern solar hemisphere. No obvious CME seems to have been associated with this event. Between 21:00 and 23:30UTC, another filament ("Fil 2") erupted, as shown in the H-alpha imagery underneath. It was 20 degrees in length and located to the east ("left") of the very long filament. This eruption was associated with a slow partial halo CME that was thought to have an earth-directed component. The best was saved for last, with the violent eruption of the remainder of "Fil 3" early on 13 April, between about 04:30 and 07:00UTC (clip ; still). It was associated with a fast partial halo CME, as shown in the combined GONG/H-alpha and SOHO/LASCO C2 coronagraphic imagery (still). The SIDC space weather forecaster expects the combined arrival of these CMEs for 15 or 16 April. The clip also shows a comet disintegrating as it approached the Sun from the southeast ("lower left") between 14:00 and 19:00UTC on 12 April.
As a bonus, the imagery also shows an active region close to the west limb. This is active region NOAA 4055 which started its development on 9 April, and started to produce M-class flares from late on 11 April onwards. By noon on 14 April, NOAA 4055 had already been the source of 21 M-class flares, the strongest being an M4.2 flare peaking at 06:50UTC on 14 April. NOAA 4055 is also, together with NOAA 3961, the largest sunspot group to have appeared so far this year, about 5 times the total surface area of the Earth. The imagery underneath is from SDO/HMI and shows the evolution of this active region in white light from 9 April / 18:00UTC until 13 April / 18:00UTC.
