The fairest of them all...
posted: Dec 20, 2012

With 2012 nearing its end, it may be a good time to have a quick review of some of the more spectacular events that the Sun produced this year. Though Solar Cycle 24 (SC24) certainly has been weak so far, solar activity was quite interesting at times with the Sun producing some truly amazing events.

A chronological list of 12 of these events can be found underneath. Using Helioviewer and JHelioviewer, a movie was created containing a few clips of each event. Usually, SDO-images were used, occasionally supplemented with imagery from STEREO, PROBA2, SOHO and GOES/SXI.

As this concerns punctual events on the Sun, one will not find here clips from the total solar eclipse, the Venus transit, or the ongoing magnetic reversal at the Sun’s poles. Even then, it was still quite hard to pick these 12 events from all other eruptions, so no attempt was being made to rank them in another way. We leave it up to the reader to determine which is the fairest of them all.

Merry Christmas and a Happy New Year!

Event 1: 19 January 2012 - M3 flare in NOAA 1402
This medium flare had everything: nice coronal loops, a nice cusp (best visible in x-ray), a small proton event, and a full halo Coronal Mass Ejection (CME) that caused a geomagnetic storm on 22 January and that was strong enough to expose some geostationary satellites to the solar wind. NOAA 1402 would repeat itself twice: on 23 January with an M8.7 flare, and a X1.7 flare while rounding the west solar limb. Both events were also accompanied by moderate to strong proton events, and very fast (> 2.000 km/s) CMEs.

Event 2: 24 February 2012 – Filament eruption
An at least 400.000 km long filament in the northeast solar quadrant erupted in the night of 23-24 February, without leaving an x-ray signature. Usually, a shock wave spreads out from the blast side in a more or less round shape. However in this case, because of the length of the filament, the solar tsunami spread parallel from where the filament originally was located. This "canyon-of-fire" as it was soon dubbed on the internet, raced away with speeds up to 20.000 km/h and swiftly covered a transient coronal hole (dark patch to the lower right of the erupted filament) that was generated earlier by the eruption.

Event 3: 7 March 2012 – X5.4 flare in NOAA 1429
The second largest x-ray flare so far this solar cycle was produced at midnight on 7 March 2012 by NOAA 1429. SDO white light images revealed this X5-flare was also a (very rare) white light flare. It was accompanied by the strongest proton storm so far in SC24 ("S3" on the NOAA-scale for radiation storms), and caused airlines to detour their polar flights for lack of communication. It was the largest proton signature registered by the Curiosity spacecraft which at that time was en route to Mars (see this STCE Newsletter). A plasma cloud was also ejected straight to Earth (full halo CME) and eventually resulted in a major geomagnetic storm on 9 March.

Event 4 – 13 March 2012 - M7.9 flare from NOAA 1429
This event took place while NOAA 1429 was rounding the west solar limb. It was particularly impressive in the green light of SDO/AIA 094, reflecting the high temperatures and energy of the event. Though the eruption was not directed towards Earth, the flank of the ejected plasma cloud still sparked a moderate geomagnetic storm on 15 March.

Event 5 – 16 April 2012 - M1.7 flare in NOAA 1461
The visually stunning images of this eruption immediately got worldwide media attention, even making it all the way into the CNN Live broadcast. Apart from the medium strength explosion, there were no space weather related influences for Earth.

Event 6 – 8 July 2012 – M6.9 flare in NOAA 1515
NOAA 1515 was visible on the Sun from 27 June till 9 July 2012. It displayed significant sunspot dynamics, with sunspots continually whirling, splitting and crashing into each other. Hence, it is no surprise that during its transit, this active region produced 30 M-flares and also 1 X-flare (on 6 July). Such a high number of strong flares is a rare "tour-de-force", only performed by the most active sunspot regions. The last M-flare occurred while NOAA 1515 was already very near the west solar limb. The M6.9 flare not only ejected a plasma cloud, but also generated a magnetic reconnection in a trans-equatorial magnetic bridge, promptly inducing another CME. Amazingly, the arch would reshape itself a few hours later and remain visible for another two days – as can be seen in the GOES/SXI clip.

Event 7 – 17 July 2012 – M1.7 flare and bright, solid CME
CMEs come in a wide variety of shapes: pistons, light-bulbs, smoke-rings,… : space weather forecasters have seen them all. The 17 July CME is probably one of the more solid and brighter of 2012. It appeared after a long duration M1.7 flare in active region NOAA 1520.

Event 8 – 19 July 2012 – M7.7 flare in NOAA 1520
NOAA 1520 was the largest sunspot group during 2012, and the second largest so far this solar cycle. This super group was magnetically not so complex and produced only a handful of strong flares. However, the last one was a real beauty. The post coronal loops from the M7 flare were very round, almost as round as the front door of a Hobbit’s house! Some even whisper that the Dark Lord was trying to forge another ring, but that’s probably an entirely different story…

Event 9 – 23 July 2012 – Backside event in NOAA 1520
NOAA 1520 continued to be very active. During the early morning hours of 23 July, it produced another strong flare. As the region was already far on the backside of the Sun, this eruption was not visible from Earth, and no x-ray signature was registered by the GOES15 spacecraft. Interestingly, the accompanying proton storm hit not only STEREO-A, but also Earth – again despite NOAA 1520 being already so far behind the Sun's west limb. Even more amazing was that the speed of the CME was estimated to be around 3.400 km/s by SWRC, making it one of the fastest CMEs ever observed. The CME arrived at STEREO-A only 19 hours after the eruption, a so-called "fast transit event".

Event 10 – 31 August 2012 – Filament eruption
Another magnificent filament eruption occurred on 31 August. This filament was actually making its second appearance, after an earlier and quite dynamic transit during the first half of August. The eruption was accompanied by a C8 flare and a full halo CME causing a minor geomagnetic storm. The core of the ejected filament was not directed towards the Earth, but behind it (see SOHO/Lasco clip).

Event 11 – 5 October 2012 - A very long duration event
The x-ray signature of this relatively small B7.8 flare lasted 7.5 hours (one of the longest so far this solar cycle), but if one includes the pre-flare reconnection and the post-flare coronal loops development, the entire event lasted at least 15 hours! The complexity of the event was discussed in a previous STCE Newsletter. The ejected plasma cloud sparked a moderate geomagnetic storm on 8 October.

Event 12 – 14 October 2012 – The light-bulb loops
CMEs occasionally have the shape of a light-bulb, post-flare coronal loops usually have not. Therefore, the bright loop of hot plasma that accompanied the C4.8 flare made this eruption quite unusual and at the same also visually attractive. The flare occurred in NOAA 1593, a relatively small sunspot region just behind the northeast solar limb.