A snaky filament lifts off
posted: June 13, 2014

Solar filaments are clouds of ionized gas above the solar surface squeezed between magnetic regions of opposite polarity. Being cooler and denser than the surrounding plasma, they appear as dark lines when seen on the solar disk using special filters. At the solar limb, the same features shine bright and are called prominences.

Early June, a long filament rounded the east limb, towering 150.000 km over the solar surface. Because of its length, about 1.5 times the Earth-Moon distance, space weather forecasters knew they had to keep an eye on this feature. Indeed, such long filaments are known for becoming unstable and may eventually erupt. Exactly this scenario unfolded on 4 June when part of the filament slingshot itself into space at an estimated speed of around 500 km/s. A movie of the event can be seen here.

The eruption was not associated to an obvious x-ray flare, but a disturbance was noted in the EUV imagery, parallel to the original position of the erupted filament on both the east and west side (see annotated image above). The disturbance propagated through the corona at a speed of 2-5 km/s. Just as the expanding flare ribbons ("parallel ribbons") and the post-flare coronal loops that often can be seen after a solar flare, also this phenomenon is an 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. It is not an EIT wave, characteristics of which were described in a previous newsletter (see here for more details). The footpoints of some faint coronal loops can be seen embedded in the expanding disturbance in the combo movies.

Dubbed in the popular literature as a "canyon-of-fire", many of these features have been observed during this solar cycle. Typical examples include the events of 24 and 29 September 2013, and the one from 24 February 2012.

The eruption was associated to a coronal mass ejection (CME), which was actually an asymmetrical halo, meaning that despite the bulk was ejected south of the Sun-Earth plane, Earth could still expect a glancing blow from this CME. And so it happened, as a shock was observed in the solar wind parameters during the afternoon of 7 June. Short-lived negative excursions in the magnetic field strength were observed (up to -17 nT), leading to a moderate geomagnetic storm on 8 June.

As of this writing, the filament seemed to be in the process of gradually reshaping itself. This can be seen in these images from the GONG/NISP network, displaying the activity in the chromosphere (lower atmosphere of the Sun). So far, no further eruptions from this filament have been observed.

Credits - Data and imagery for the movie clips were taken from the GONG H-alpha network, SDO, SOHO/LASCO, and (J)Helioviewer.