Stephen W. Kahler

Recent work based on nitrate abundances in ice cores has shown that large solar energetic (E > 30 MeV) particle (LSEP) events during the spacecraft era of observations (1960–present) are diminished in comparison with those of some preceding eras and that LSEP events have occurred during low cycles of solar activity. McCracken et al. (2004) have reported an inverse correlation between those LSEP events and the magnitude of the associated reconstructed heliospheric magnetic field (HMF). A physical explanation by McCracken (2007a, 2007b) is that the lower HMF and coronal magnetic field B imply that fast coronal mass ejections (CMEs) produce shocks with enhanced Alfvenic Mach numbers M_A and higher compression ratios r, leading to more numerous and energetic LSEP events. From a possible decline of the HMF over the next several solar cycles, he has warned of a return to the environment of the more intense LSEP events preceding the current spacecraft era. We formulate and discuss seven questions using recent published observational, modeling, and theoretical work to assess the assumptions and the validity of his explanation and watch. We conclude that a return to more intense LSEP events is certainly possible, but (1) the inferred large increase in HMF characterizing the spacecraft era is in doubt; (2) there is no good evidence to connect more intense LSEP events with weaker HMFs; (3) a new Gleissberg minimum, with lower HMF, may not be imminent; (4) a lower active region B should result in slower CME speeds V less likely to produce shocks; (5) the average CME V increases significantly with SSN; (6) a lower coronal B likely results in a lower V_A as the explanation requires; and (7) the lower coronal B leads to weaker, not more intense, LSEP events because of decreased spectral cutoff energies.

Since February 2003, the Solar Mass Ejection Imager (SMEI) has been observing interplanetary coronal mass ejections (ICMEs) at solar elongation angles ε > 20°. The ICMEs generally appear as loops or arcs in the sky, but five show distinct outward concave shapes that we call V arcs. We expect to observe some V arcs, formed by trailing edges of ICME flux ropes or by leading ICME edges sheared by solar wind (SW) speed gradients at the heliospheric current sheet. We characterize the properties of these V arcs and compare them with average properties of all SMEI ICMEs. The typical V arc speeds argue against a slow MHD shock interpretation for their structures. We estimate the V arc solar source locations and their opening angle dynamics as tests for SW shearing. The first test contradicts but the second supports the SW shearing explanation. The implications of the small number of V arcs observed with SMEI is considered. The point P approximation used to determine the V arc locations and inferred solar source regions is critically examined in Appendix A.

Properties of near-relativistic (E ≳ 30 keV, NR) and relativistic (E ≳ 0.3 MeV) electron events produced near the Sun and observed within 1 AU are reviewed. Observations suggest the CME-driven shocks are the sources of many events, but flares are often sources for NR events.

The search continues for coronal environmental factors that determine whether a fast coronal mass ejection (CME) results in a solar energetic particle (SEP) event at 1 AU. From a plot of peak 20 MeV SEP intensities versus associated CME speeds we select for comparison two groups of fast, wide, western hemisphere CMEs observed with the LASCO coronagraph from 1998 to 2002. The SEP‐rich CME group produced the largest observed SEP events, and the SEP‐poor CME group produced the smallest or no observed SEP events. The major differences are that the SEP‐rich CMEs are brighter and more likely to be streamer blowouts and to follow colocated CMEs within 12 or 24 hours. The SEP‐poor CMEs are fainter and less likely to interact with streamers or to follow preceding colocated CMEs. Thus we confirm the recent result that the SEP event peak intensities are higher when the associated driver CMEs are preceded within a day by wide CMEs at the same locations. However, the enhanced brightness, and therefore mass, of the SEP‐rich CMEs appears to be their most dominant characteristic and suggests that either large longitudinal and latitudinal extents or high densities are needed for fast CMEs to produce SEPs.

The solar sources of near-relativistic 30 keV) electron events observed at 1 AU are poorly understood. In general, the solar injection times deduced from the observed 1 AU onset times and assumed 1.2 AU travel distances yield injection times about 10 minutes after the associated flare impulsive phases and type III radio burst times. One interpretation is that the apparent delays occur in the interplanetary medium, probably due to scattering of the electrons. If the injection times are delayed from the impulsive phases, the electron acceleration might take place in CME-driven shocks. Here a large number of electron events observed with the UC/Berkeley 3DP detector on the Wind spacecraft are compared with CMEs observed by the Lasco coronagraph on SOHO and with type II bursts observed by the 40 to 800 MHz radio receiver at the Astrophysikalisches Institut Potsdam (AIP) and by the 20 kHz to 14 MHz WAVES instrument on the Wind spacecraft. The acceleration of at least some of the electron events is not consistent with the shock hypothesis.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html