HR: 17:45h
AN: SH44C-08    [Abstracts]
TI: Consequences for Ground Level Enhancements (GLEs) from a Reduced Heliomagnetic Field
AU: * Kahler, S
EM: stephen.kahler@hanscom.af.mil
AF: Air Force Research Laboratory, AFRL/VSBXS 29 Randolph Rd., Hanscom AFB, MA 01731, United States
AB: A recent assessment of the long-term records of solar energetic particle (SEP) events by McCracken (Space Weather, 2007) has concluded that the frequency of occurrence of large SEP events is smallest for the most active solar cycles. In particular, the E > 4 GeV fluences were ~ 10 times larger and 4 times more frequent in the 20 years preceding the onset of the space era in 1958. A return in the near future to the solar activity level characteristic of the Gleissberg minima of 1800-1820 and 1875-1910 could introduce an era of much more energetic SEP events with longer fluences than those observed since 1958. The working explanation of this variation of SEP event intensity is that the lower solar activity produces weaker magnetic fields, which decrease the coronal Alfven speeds, which reduce the compression ratios of shocks driven by coronal mass ejections (CMEs). This explanation implicitly assumes not only that other variables such as solar wind density and CME speeds are unchanged through a declining heliospheric magnetic field, but also assumes the existence of the controversial long-term increase in the heliospheric field. We examine these assumptions from two points of view: 1) the relationship of solar magnetic fields to CME and solar wind characteristics, and 2) our best knowledge of the long-term variations in solar wind properties.
DE: 2101 Coronal mass ejections (7513)
DE: 2114 Energetic particles (7514)
DE: 2139 Interplanetary shocks
DE: 2162 Solar cycle variations (7536)
SC: SPA-Solar and Heliospheric Physics [SH]
MN: 2007 Fall Meeting