It's alive!... It's alive!...
posted: October 16, 2013

On 9, 11 and 13 October, the Sun produced an M-class solar flare each day. These three medium-sized eruptions were the first since mid-August, ending a nearly comatose period of solar activity.

The 9 October M2.8 flare lasted 33 minutes and peaked at 01:48UT. It originated in sunspot group NOAA 1865. As this sunspot group was still close to the southeastern limb, it was difficult to predict the (strength of the) eruption due to the very oblique angle space weather forecasters had at that moment. The SDO-images underneath show the sunspot groups in visual light, as well as through filters near 80,000 degrees (AIA304, middle) and several million degrees (AIA 131, right). The three images were taken near the time of maximum brightness in x-rays.

The 11 October M1.5 flare lasted 44 minutes and peaked at 07:25UT. It originated in a sunspot group which was at the time still well behind the northeastern solar limb. This suggests that the true strength of the eruption was probably higher than the measured M1.5.

Both flares were visible from the STEREO-B spacecraft, which is currently well on the other side of the Sun as seen from Earth. In particular the two small, transient coronal holes from the 11 October event are obvious as dark patches to the south of the bright active region. These holes are the result from material being flung through the corona during the eruption. By the end of the day, these temporary punctures had disappeared.

Both eruptions were accompanied by coronal mass ejections (CMEs), but none was Earth directed. The 9 October one was not very impressive, in contrast to the CME from 11 October (see SOHO images above). The latter was halo-like as seen by both the SOHO and the STEREO-B spacecraft. Due to the position of the active region at the time of the explosion, the CME was directed towards STEREO-B, and away from Earth. This is called a backside halo CME. See images underneath by SOHO (left, 10:18UT) and STEREO-B (right, 10:10UT).

The 13 October M1.7 flare peaked at 00:43UT and took also place in active region NOAA 1865. Seen in the red light of Hydrogen-alpha, it concerned a parallel ribbon flare running east-to-west along the magnetic neutral line of the sunspot group. See the visual image and magnetogram from SDO (resp. left and middle), and the H-alpha image by GONG (right). The flaring region is at the bottom left in each image. Clearly, the upper ribbon is hovering over the black polarity (inward oriented magnetic field) sunspots, while the lower ribbon is hovering over the opposite, white polarity (outward oriented) sunspots. The CME associated to this eruption was mainly directed towards the south of the Earth.

Credits - Data and imagery for the movie clips were taken from the GONG H-alpha network, SDO, STEREO-B, and SOHO/LASCO.