THE MEAN IONIC CHARGE STATE OF SOLAR ENERGETIC Fe IONS ABOVE 200 MeV PER NUCLEON

摘要

We have analyzed the geomagnetic transmission of solar energetic Fe ions at ～ 200-600 MeV per nucleon during the great solar energetic particle (SEP) events of 1989 September-October. By comparing fluences from the Chicago charged-particle telescope on IMP-8 in interplanetary space and from NRL's Heavy Ions in Space (HIIS) experiment aboard the Long Duration Exposure Facility (LDEF) in low-Earth orbit, we obtain a mean ionic charge < Q > = 14.2 ± 1.4. This result is significantly lower than < Q > observed at ～1 MeV per nucleon in impulsive, ~3He-rich SEP events, indicating that neither acceleration at the flare site nor flare-heated plasma significantly contributes to the high-energy Fe ions we observe. But it agrees well with the < Q > observed in gradual SEP events at ～1 MeV per nucleon, in which ions are accelerated by shocks driven by fast coronal mass ejections, and hence shows that particles are accelerated to very high energies in this way. We also note apparent differences between solar wind and SEP charge state distributions, which may favor a coronal (rather than solar wind) seed population or may suggest additional ionization in the ambient shock-region plasma.