4 Solar Energetic Particles (SEPs)
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.
(from Mewaldt et al., 2005
