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In recent observations with SAMPEX and SEPICA/ACE an energy dependence of the mean ionic charge (Qm) of Fe has been found in some large, gradual Solar Energetic Particle (SEP) events, with Qm ranging from ~10 at 0.3 MeV/nucleon to ~18 at 40 MeV/nucleon. By combining data from CELIAS/STOF on SOHO with that from SEPICA/ACE measurements can be extended to significantly lower energies (to ~30 keV/nucleon for O and Fe). The ionic charge states of O and Fe were studied for several CME/interplanetary shock related SEP events observed by ACE and SOHO. A typical case from May 1998 is shown above. The mean ionic charge state of Fe at suprathermal energies (in this case ~30 to 80 keV/nucleon) is typically 1-2 charge units higher than observed in the solar wind. In the combined SOHO/ACE data set the Fe charge state shows a clear increase with energy, while the charge state of O does not vary significantly with energy and is consistent with solar wind values. The observed differences in the mean ionic charge of Fe in the solar wind and suprathermal ions may be attributed to charge/mass-dependent acceleration processes and/or stripping of electrons during the acceleration process in the moderately dense coronal plasma. These combined measurements show that the energy-dependent charge states first observed at higher energies also occur at suprathermal energies, providing important constraints on the acceleration and transport processes of suprathermal ions.
Contributed by Alexander Bogdanov and Berndt Klecker, Max-Planck Institut fur Extraterrestriche Physik.
See The SEPICA Home Page and The CELIAS/STOF Home Page for more information on ACE SEPICA and CELIAS/STOF.