Spectral Properties of Heavy Ions Associated with the Passage of Interplanetary Shocks at 1 AU
We have surveyed the energy spectra of ~0.1–100 MeV nucleon^(-1) C, O, and Fe nuclei associated with the passage of 72 interplanetary (IP) shocks observed on board the ACE spacecraft during the period 1997 October–2002 October. Our main results are as follows: (1) The spectral fit parameters are independent of the local shock properties. (2) About 7% of the events exhibit increasing Fe/O ratios with energy; the remaining events have Fe/O ratios that either remain constant or decrease with energy. (3) The Fe/O ratio in the shock-associated particles is typically ~30% lower than in the ambient population. (4) The fractionation pattern of the elemental abundances, the O spectra, and the energy-dependence of Fe/O at the IP shocks are remarkably similar to those of the ambient interplanetary suprathermal ion population. We suggest that the IP shocks studied here reaccelerate energetic particle seed spectra composed of ions from impulsive and gradual solar energetic particle events by systematic rigidity-dependent mechanisms in which higher rigidity ions are accelerated less efficiently than lower rigidity ions.
Additional Information© 2004 The American Astronomical Society. Received 2003 December 5; accepted 2004 April 26. We are grateful to the members of the Space Physics Group, University of Maryland, and the Johns Hopkins Applied Physics Laboratory for the construction of the ULEIS instrument. We thank Adam Szabo for providing the shock analysis program. Work at the University of Maryland was supported by NASA contract NAS5-30927 and NASA grant PC 251428. Funding for C. W. S. and Q. H. is provided by CIT subcontract PC251439 under NASA grant NAG5-6912 for support of the ACE magnetic field experiment. Work at Los Alamos was performed under the auspices of the US Department of Energy with financial support from the NASA ACE project. We acknowledge the use of the lists of Solar Proton Events Affecting the Earth Environment provided by NOAA Space Environment Services Center at http://sec.noaa.gov/ftpdir/indices/2001_events/20011122events.txt and that of CMEs provided by the SOHO LASCO Team at ftp://lasco6.nascom.nasa.gov/pub/lasco/status/LASCO_CME_List_2001.
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