The various highly dynamic processes in the magnetized coronal and interplanetary plasma can cause major acceleration of the charged particle populations. The main locations for electron and ion acceleration are flare sites and shock waves in the corona and in interplanetary space. The energy of these solar energetic particles (SEPs) reaches from a few keV of “suprathermal particles” to some GeV. Sometimes the fastest particles obtain more than half the speed of light, and they arrive at the Earth only a few minutes after the light flash. They are of particular concern in the space weather context since they can penetrate even the skins of spaceprobes traveling outside the Earth’s magnetosphere and blind or even damage sensitive technical systems. The strongest events like the ones in August 1972 or in October/November 2003 produce radiation doses that might be lethal to unprotected astronauts while traveling in space outside our protective magnetosphere (see, e.g. Turner, 2001). For the very largest events, SEP ionization of the polar atmosphere produces nitrates that precipitate to become trapped in the Earth’s polar ice. Ice core analysis revealed that the largest SEP event in the last 400 years appears to be related to the giant flare observed by Carrington in 1859 (Reames, 2004). Forecasting such extraordinary events is still not possible, for two main reasons: 1) We have not yet identified unique signatures for the driving flare that would indicate an imminent explosion and its probable onset time, location, and strength, and 2) the size of the SEP fluxes is highly variable and appears to be only loosely related to the strength of the flare.
For illustration we show what happened during the dramatic Halloween events in late 2003. In Figure 21 (from Mewaldt et al., 2005) the SEP intensities for electrons and protons as measured by the GOES/SAMPEX/ACE satellites in several energy bands are plotted. The largest of the five major SEP events reached its peak intensity during shock passage at Earth on October 29. It had been launched in context with the X 17.2 flare and the associated halo CME about a day earlier. The maximum energy of probably more than 1 GeV was sufficient for the particles to penetrate the whole SOHO spacecraft and cause temporary malfunction of several CCD cameras (see the “snowstorms” in Figures 1, 2 and 3).
Such big events and event sequences do not occur frequently. Since the beginning of regular registrations by NOAA in 1976 there had been only three events with slightly higher proton fluxes: on October 19, 1989 (Reeves et al., 1992), on March 24, 1991, and on November 4, 2001 (according to NOAA records, see http://goes.ngdc.noaa.gov/data/ParticleEvents.txt). Note that these three big SEP events were associated with flares of importance X13, X9, and X1, respectively, while at some much bigger flares (at similarly central positions on the Sun’s disk) the SEP fluxes remained rather low.
The acceleration of particles to such high energies on time scales of seconds or minutes as well as their propagation through space is still not well understood, and active research is going on. The interested reader may wish to study more detailed reviews than the present one can offer, e.g., Kunow et al. (1991); Reames (1999, 2001, 2002); Tylka (2001); Kahler (2001a), and of other authors such as Kallenrode, Lee, Lin, Mason.
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