Ready... Aim... Fire!...
posted: Mar 22, 2013

The Sun took a good aim at Earth this week.

Indeed, on two occasions, an eruption took place very near the central meridian of the Sun. The central meridian is a fixed, imaginary line on the Sun running from the north to the south solar pole. Scientists use this line all the time to unambiguously determine positions of sunspot groups and other solar features on the Sun. The distance to this central meridian is called the "Central Meridian Distance" (CMD). The CMD is negative to the east of the Sun, and positive to the west, and varies between -90° and +90°. If an eruption occurs with a CMD of about 0°, it means the eruption took place very near the central meridian and usually also that it is directed straight towards Earth. Solar orientation grids can be found at various websites, e.g. at SIDC/Soteria, Solar Monitor and at the MEES Solar Observatory. Underneath an example from the SIDC/Soteria page, with the central meridian added as a green dashed line.

The 12 March eruption was associated with a C2-flare that started at 10:17UT and ended only at 12:27UT, making this a long duration event (LDE). The filament belonged to a small sunspot region NOAA 1690. The eruption was at relatively northern latitudes (+/-25°), but part of the ejected material was still directed towards the Earth. From STEREO-B data, it was determined that a coronal mass ejection (CME) had left the Sun with a speed of at least 530 km/s. As predicted, Earth received a glancing blow from this CME resulting in unsettled geomagnetic conditions during the early morning hours of 15 March.

The second eruption occurred on 15 March and started at 05:46UT. It ended at 08:35UT, making it again an LDE. The flare was associated with a filament eruption located very close to sunspot region NOAA 1692, which had already erupted previously during the night of 12-13 March (impulsive C3-flare located about -40° CMD) However, the 15 March flare concerned a M1-flare (weak proton event) located only 10° east of the central meridian, and also a lot closer to the solar equator.

This movie shows both eruptions. There is a slight interruption in the clips of the second flare, due to the Earth passing in-between the Sun and SDO. Images underneath depict the respective flares in the combined light of SDO/AIA 335/171, 193, and 304 (corona and transition zone). They show the flares resp. on 12 March at 10:35UT (onset; eruption in progress) and on 15 March at 9:29UT (post-flare coronal loops).

The eruption was accompanied by a considerable solar radio-disturbance that was recorded by the Humain Solar Radio Observatory (image below). It concerns a typical Type IV burst, which is long lasting radio noise due to energetic electrons being trapped in the post-flare loops forming in the aftermath of the eruptive event. Note that the diagonal stripes near the bottom of the figure are due to ground "interference". Indeed, at Humain, the Sun was still at a low elevation in the sky at the time of the event. As such, radio emissions reached the antenna directly and by reflection on the ground, creating these "fringes".

The associated CME had a speed of at least 950 km/s and was of the halo-type, meaning it was heading straight towards Earth. The CME struck our planet in the early morning hours of 17 March, with the speed of the solar wind jumping from about 400 km/s to 750 km/s. More importantly, the CME's magnetic field was quite strong and oriented southward, favoring moderate geomagnetic storming for a good part of the day. The region of good visibility of polar light was limited to the usual places (Scandinavia, Canada, Alaska and northern USA).

Credits - Data, images and movies for this topic were taken from SDO, PROBA2/SWAP and LYRA, SOHO/Lasco C3, SIDC, STEREO-Behind, ACE and Humain Solar Radio Observatory).