The intensity of proton flares

Solar activity seems to have shifted into higher gear. Indeed, 2014 has hardly started, yet we have already recorded 2 proton events (see previous STCE Newsletter). A proton is the positively charged nucleus of the hydrogen atom. For reasons solar scientists do not fully understand, these protons are sometimes released during (usually) strong solar flares. The particles travel at very high speeds (several 10.000 km/s!) and can bridge the Sun-Earth distance in a matter of hours. For example, during the Bastille Day event (14 July 2000), the first protons arrived already within 30 minutes after the peak of the solar flare (images underneath).

Because of their high speed, protons can have very high energies. As such, they constitute a radiation threat to astronauts, in particular during their extra-vehicular activities (space walks). They can 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. No wonder that space weather centers keep an eye on these malicious little creeps!

During the nineties, NOAA developed a scale to categorize the intensity of these events. This S-scale), short for solar radiation storm, has 5 levels going from minor (S=1) to extreme (S=5). In order to be categorized as a solar proton event (SPE), one looks at the number of protons having an energy of at least 10 MeV (a unit of energy – see note 1). If the intensity of these protons is at least 10 pfu (a unit of flux intensity – see note 2) at geosynchronous altitudes, then it is categorized as an S1-event. The interesting part is that this scale is actually logarithmic. Indeed, in order to have a moderate proton event (S2), the proton flux needs to reach 100 pfu (=10*10), not 20! And for a major event (S3), a 1000 pfu (=10*10*10) is required. Analogous steps for a severe (S=4) and extreme (S=5) proton event.

To get an idea what these numbers mean, let's take a look at what SOHO's LASCO/C3 coronagraph experiences when it is bombarded by all these energetic protons. For a scale of S=1 to S=4, the figure above depicts how the coronagraph's field of view is affected. The increasing number of white dots and stripes are all impacts of protons on the CCD’s pixels. No wonder that during a severe event, a satellite's star tracker can no longer distinguish between true stars and impacts from the protons, and can thus become disoriented. A satellite can direct itself away from the Sun, such that sunlight no longer reaches the solar panels and thus depriving itself of the precious energy.

So far this solar cycle, the Sun has produced only four S3 events and zero S4 events. The strongest was the 7 March 2012 event when flux levels reached 6530 pfu. During the previous solar cycle, six S4 events were produced, the strongest reaching 31.700 pfu on 6 November 2001 following an X-class solar flare 2 days earlier (see the image above).

No S5-event has been recorded since measurements began in 1976, but one of the strongest SPE of the space age took place early August 1972. This was right in between the Apollo-16 and -17 missions which took place resp. in April and December of the same year. The event accounted for about 70% of the total 10 MeV fluence for the complete solar cycle (SC20)! Since this large event made such a dominant contribution to the total solar cycle fluence, in some research scientists decided to separate it from the other events, and to class it as an anomalously large (AL) event, in contrast to the remaining ordinary events. It is commonly accepted that if an Apollo mission had flown during the August 1972 event, the astronauts would have received severe (and potentially lethal) doses of radiation (Note 3 for reference and further reading).

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.

Note 3 – Further reading at SPENVIS, ESA's SW web server, and Science at NASA (here, here and here).

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