Number 35
posted: September 17, 2014

On 10 September at 17:45UT, NOAA 2158 produced the 35th X-class flare of the ongoing solar cycle. The X1.6 event was extensively covered in this news item. The images underneath show the sunspot region in white light and the corresponding magnetogram about 30 minutes prior to the flare's peak, and the flare at its maximum flux as observed in SDO/AIA 171. The flare took place along the borderline between negative (blue) and positive (red) magnetic polarity.

Graph underneath (Annotated version) shows the evolution of the flare in x-ray (GOES) and EUV (PROBA2/LYRA in W/m2; Solar Demon AIA 094 in counts). One can see that the EUV emissions peak 6-12 minutes later than those from x-ray. This is due to the cooling of the post-flare coronal loops, whose emissions become then better visible in the less energetic EUV passbands. The AIA 094 emissions also show a second peak about 30 minutes after its maximum. This second peak is not visible in x-ray. This "extra" EUV emission does not originate from the original flare site, but most probably from a volume of higher coronal loops. This may indicate there's additional post-flare loop reconnection, but at a lower temperature than during the flare's main peak. This is called the "EUV late phase". No doubt this flare will generate quite a few scientific papers!

The geomagnetic storm from the CME related to this X1 flare did not live up to the expectations, as the CME's magnetic field was directed mostly northward. This prevented a good connection with the Earth's magnetic field, and thus severe geomagnetic storming was not possible. Nice polar lights were observed, but they were confined to the usual places (Scandinavia, Canada, Alaska,...). Nonetheless, some aurora were photographed from Franeker, Friesland (northern part of the Netherlands), as picture underneath demonstrates. It was taken by Vincent van Leijen on 12 September at 23:05LT, when the storm was most intense. See also this web page. Can you see the Big Dipper constellation?

This X1.6 flare was certainly not the strongest one of solar cycle 24 (SC24), ranking somewhere half of the pack. A table with the Top 11 most powerful flares during SC24 can be found underneath. Aside the date, also the hemisphere and source region are mentioned. A little more than half of the X-class flares (20) took place on the northern solar hemisphere. The X6.9 flare, currently the strongest of SC24, is the only one that ranks within the Top 50 of strongest flares since systematic measurements began in 1976 (36th place).

It's also interesting to take a look at the position of these 35 X-class flares on the solar disk. Only one third of these flares took place in the centre half of the solar disk, with most of the other events taking place near the solar limb. In fact, nearly half took place near the east limb, which makes it of course very difficult for any associated CME to have a geo-effective impact.

Finally, this solar cycle will certainly produce quite a few more X-class flares as it further unfolds. Indeed, averaging per year the total number of X-flares over the last 3 cycles, the bar-diagram underneath shows that there is a second peak of "eXtreme" flares about 2-3 years after the year when the solar cycle maximum actually occurred. For example, the Halloween flares (October 2003) occurred about 3 years after SC23 maximum. So, though we have not observed that many X-class flares so far this solar cycle, statistically we are still in for quite a bit of X-class activity in years to come.

Credits - Data and imagery were taken from SDO, Solar Demon, PROBA2/Lyra, GOES, NGDC, STAFF, and Vincent van Leijen.