Mi. Desai et al., Energy spectra of 50-keV to 20-MeV protons accelerated at corotating interaction regions at Ulysses, J GEO R-S P, 104(A4), 1999, pp. 6705-6719
We analyze here the energy spectra of 50-keV to 20-MeV protons accelerated
at corotating shocks observed at Ulysses in the southern heliosphere during
1992 and 1993. In general, our results are inconsistent with the predictio
ns of two models based on diffusive shock acceleration theory (Fisk and Lee
[1980] and Jones and Ellison [1991]). The main results are the following:
(1) The relationships between the spectral indices and the shock compressio
n ratio show significant departures from those predicted by the two models;
the observed spectra are substantially harder than predicted; (2) the spec
tral indices at the reverse shocks depend strongly on the heliographic lati
tude of Ulysses; the hardest spectra are associated with strong quasi-perpe
ndicular shocks observed between 20 degrees S and 30 degrees S; (3) the spe
ctral indices at the reverse shocks are anticorrelated with the upstream pl
asma velocity V-u' in the de Hoffman-Teller frame of the shock; and (4) the
spectral indices at the forward shocks show no dependence on either the sh
ock parameters or the heliographic latitude. The latitudinal dependence of
the reverse shock spectra is probably related to the tilt (similar to 25 de
grees) of the heliospheric current sheet with respect to the solar equatori
al plane during 1992 and 1993. The negative correlation between the spectra
l indices at the reverse shocks and V-u' indicates that the gradient drift
mechanism plays a crucial role in accelerating particles at corotating inte
raction regions (CIRs). The differences in our observations at the forward
and reverse shocks may be due to the presence of a more energetic seed popu
lation and an enhancement in the level of magnetic field fluctuations upstr
eam of the trailing edges of the CIRs.