The 3 X-class flares from NOAA 2087
posted: June 18, 2014

On 10 and 11 June, the not so big but complex sunspot group NOAA 2087 produced three X-class flares in less than 24 hours. On 10 June, there was an X2.2 peaking at 11:42UT, followed by an X1.5 at 11:52UT. On 11 June, an X1.0 peaked at 09:06UT. This brings the X-class flare counter for the current solar cycle to 34. The associated coronal mass ejections (CME) were not directed to Earth, though from the second CME a glancing blow was expected. In the end, no obvious CME-signature was observed in the solar wind data.

The first movie shows last week's active regions NOAA 2080, 2085 and 2087 and their flaring activity in successively higher temperatures: white light (photosphere), transition region (SDO/AIA 304; +/- 80.000 degrees), lower corona (AIA 171; +/- 650.000 degrees), and hot corona (AIA 094; multi-million degrees). It spans a five day period starting 9 June. NOAA 2087 is appearing from behind the southeast limb (lower left), while NOAA 2080 and 2085 are heading for the southwest limb (lower right). All three regions were the source of M-class ("medium") flares over this period.

The second movie focuses on the first two X-class flares from NOAA 2087 starting with a combination movie of the aforementioned SDO/AIA filters. The respective peaks were separated by only 70 minutes. A quick survey of the GOES database reveals this is not so unusual, with every solar cycle having a handful of this kind of flare pairs. The smallest time delay seems to have occurred on 18 August 1979, when only 26 minutes separated the peaks of an X1 and an X6 flare. NOAA 2087's X2 flare was also an impulsive event, barely lasting 8 minutes. Since the GOES-measurements started in 1976, only 21 X-class events did not last 10 minutes (not-including the X2 flare).

Scientists from the PROBA2 Science Centre were able to observe these two X-class flares with imagery (SWAP) and data (LYRA). LYRA observes solar flares just as GOES, but in extreme ultraviolet (EUV) and thus in slightly different wavelengths (LYRA "blue" and "grey" channels). These channels can be calibrated such that they can be compared to the GOES data. Sometimes, LYRA slightly underestimates the solar flares as was the case with the first flare (X2.2). This is related to the formula with which the proxies are calculated, and not to the temporal solution. Both the LYRA and GOES curves peak about 4 minutes after the onset of the first flare (X2.2).

Interestingly, LYRA does *not* underestimate the second flare (X1.5). This is because there is obviously much "cooler" (EUV) plasma involved than in the first flare. The first flare appears to consist mainly of "hot" (soft x-ray) plasma, which is observed by GOES as well as by LYRA. It appears to be impulsive, rises quickly and drops almost as quickly. The second flare keeps on rising in LYRA, after GOES has already peaked, about 3 minutes longer. This is expected, because LYRA also observes in spectral regions in which GOES does not observe (somewhat longer wavelengths).

The SWAP instrument, an EUV telescope, onboard PROBA2 recorded an off-limb EIT wave (image above; see this news item for more details on these features). In the difference movie (one image subtracted from the previous one), the disturbance seems to propagate through the corona at an average speed of about 400-500 km/s.

The flaring activity was also recorded in the radio-frequencies. The radioastronomy observatory at Humain recorded a strong radio burst for the first X-class flare, but nothing obvious for the second one, consistent with reports from other observatories (NOAA,...).

The Nançay RadioHeliograph showed the radio-emission from both flares at 150.9 MHz. The imagery indicates that the radio-emission associated to the first flare had a relatively stationary source, whereas for the second flare this radio-emission seems to be moving away from the Sun.

Credits - Data and imagery were taken from from the Humain Radioastronomy Station, PROBA2/SWAP and LYRA, the Nançay RadioHeliograph, GOES, SDO, and (J)Helioviewer.