posted: July 16, 2015

On 22 June 2015, the strongest proton event so far this year took place, with the greater than 10 MeV proton flux reaching 1070 pfu (proton flux units; Notes 1 and 2). It was only the 5th such event (more than 1000 pfu) so far this solar cycle (SC), and only the 41st since the start of systematic satellite measurements began in 1976. Interestingly, the maximum (19:00UT) coincided with a period of severe geomagnetic storming (Kp=8) from 18:00-21:00UT, and with a a moderate M6.5 flare peaking at 18:23UT. This was nicely illustrated by the overview chart from SWPC (image underneath).

It goes without saying that there are few of this kind of coincidences, not in the least because of the low number of strong proton events. Also, since 1976, there have only been about 500 X-class flares and 6000 M-class flares. Geomagnetically, there have been only about 500 3-hour periods with strong to (extremely) severe geomagnetic storming. So, it is obvious that the instances of strong geomagnetic storming and strong solar activity falling together with a strong radiation storm are quite rare.

Using the NOAA charts and listings of all these events, a quick review covering SC23 and SC24 revealed that only 11 days satisfy the above conditions. These days are displayed in the table underneath, with the intensity of the individual storms as described according to the NOAA-scales (see the second table underneath for a summary).

The Halloween period (29 October 2003) as well as the Bastille Day event (15 July 2000) are both having coinciding storms during and after the maximum proton flux (Smax). The graphs underneath provide other examples of coinciding storms: on the left the events from 6 November 2001, and on the right those from 26 July 2004. Each display the x-ray flux, proton flux, and evolution of the Kp-index. It is clear that when increased activity of these three fall together, the resulting space weather effects can be felt in all domains. A good thing this does not happen very frequently!

Note 1 - 10 MeV = 10 million eV. The eV (electron volt) is a very tiny amount of energy corresponding to about 0.16 billionth of a billionth of a Joule. For comparison, a flying mosquito has a kinetic energy of about a trillion eV (= 1000 billion eV).

Note 2 - pfu: proton flux unit. This is the number of particles registered per second, per square cm, and per steradian.

Credits - Data and imagery were obtained from NOAA's Warehouse, SWPC, NGDC, and (J)Helioviewer.