Proton events have already been discussed in earlier news items, such as on 23 January 2014 and on 16 August 2012. It concerns solar eruptions associated with a strong increase in the flux of particles with energies of 10 MeV or more (Note 1). Once this proton flux exceeds the pre-established threshold of 10 pfu (Note 2), it is considered a proton event, if not it is labeled a proton flux enhancement.
As the related particles can have very high energies, they constitute a radiation threat to astronauts, in particular during their extra-vehicular activities (space walks). They can also increase the radiation dose of the crew and passengers on transpolar flights, and can cause communication problems over the polar areas (the so-called "Polar Cap Absorption"). These particles also give satellites a hard time. They can create malfunctions in the onboard electronic circuitry, degrade solar panel efficiency, and increase the noise in star-tracking systems.
So far this year, no proton events had been observed. This changed on 18 June. In response to an M1.2 flare peaking at 01:27UT, the proton flux started to increase 3 hours later, and passed the event threshold at 11:35UT. The maximum was reached at 14:45UT, and concerned a minor event, as the flux attained only 16 pfu. The event ended the next day at 02:30UT. The graph above displays the evolution of the proton flux and the x-ray flux as measured by the GOES-satellites.
The source of the proton event was NOAA 2365, a decaying sunspot region that had just rounded the southwest limb. Despite its location, the magnetic field lines were well connected to Earth, allowing easy spiraling of the energetic particles to our planet. The associated coronal mass ejection (CME) was not directed to Earth, but was quite impressive in extreme ultraviolet (EUV) imagery. This movie first provides a full sun clip and a zoom in SDO/AIA 304 (temperature of about 80.000 degrees).
It is followed by a multi-temperature SDO view (AIA 171 in yellow, AIA 131 in blue) overlaid on the wide field-of-view from the PROBA2/SWAP camera. This gives a view of the event in hotter temperatures around 1 million degrees. There's also a zoom on the complex arcade, i.e. a complex series of post-eruption coronal loops (see images above). The movie concludes with a clip of the CME as seen by SOHO's coronagraphs.
This was not only the first proton event in 2015, it was the first event since 11 September last year. In between, there have been quite a few proton flux enhancements, such as e.g. on 16 March 2015 (8 pfu). On 2 November 2014 (11 pfu) and on 23 December 2014 (14 pfu), the event threshold of 10 pfu was actually passed, but the flux increases still did not qualify as true proton events. Indeed, the start of a proton event is defined to be the first of 3 consecutive data points with fluxes greater than or equal to 10 pfu. In both cases, the flux never satisfied that criterion of 3 consecutive data points, with resp. a few separated data points and one data point above 10 pfu (see graphs below). The full list of proton events can be found at the NOAA-website.
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 for the movie clips were taken from SDO, SOHO/LASCO, PROBA2/SWAP and (J)Helioviewer.
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