Prospects for future enhanced solar energetic particle events and the effects of weaker heliospheric magnetic fields

Abstract

[1] 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 (2007aMcCracken ( , 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.

Solar Cycle in the Heliosphere and Cosmic Rays

“…The ice core evidence, however, has been disputed by Wolff et al (2012). In addition, the low inferred HMF values circa 1950 inferred from ionization chambers are not substantiated by more recent reconstructions of HMF based on geomagnetic observations (Svalgaard and Cliver 2010;Lockwood and Owens 2011;Kahler 2008) as well as on cosmogenic nuclides (Steinhilber et al 2010).…”

Space Weather Prediction

“…The IHWU results described here align with and directly support all three prongs of the 6.0 Space Situational Awareness (SSA) Technical Area Plan (TAP), viz: (1) Environmental monitoring capabilities (both ground-and space-based), (2) Assimilation and reporting of space weather conditions and events, and (3) Predictive and climatological models linking the space environment to system effects and impacts. Because the Sun is the principal source and driver of disturbances in the space environment, Space Situational Awareness requires understanding and predicting solar and interplanetary phenomena.…”

“…In addition, the proposed physical model to explain the claimed correlation was also examined and found to be infeasible. The validation results are published in [2,3]. In addition, a study was conducted to see if signatures of heliospheric SEP events could be detected through remote sensing.…”