A.J. Tylka, P.R. Boberg, J.H. Adams, Jr., L.P. Beahm, W.F. Dietrich, T. Kleis
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 LDEF in low-Earth orbit, we obtain a
mean ionic charge
= 14.2 +/- 1.4. This result is significantly
lower than
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
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.