Spectral Properties of Large Gradual Solar Energetic Particle Events
Wednesday, 17 December 2014: 11:40 AM
Our Sun accelerates ions and electrons up to near-relativistic speeds in at least two ways; magnetic reconnection during solar flares is believed to produce the impulsive or 3He-rich solar energetic particles (SEPs), while diffusive shock acceleration at fast coronal mass ejection – or CME–driven shock waves are thought to produce the larger gradual SEPs. Despite recent advances in our understanding of the properties (e.g., time variations, spectral behavior, longitudinal distributions, compositional anomalies etc.) of large SEP events, the relative roles played by many important physical processes remain poorly understood. These effects include variations in the seed populations, the geometry of the shock, the presence or absence of a preceding CME from the same active region, scattering by ambient turbulence or by self-generated Alfvén waves during acceleration and transport, and the direct presence of flare accelerated material at energies above ~10 MeV/nucleon. Observations from ACE, Wind & STEREO and theoretical studies have indicated that many of these effects are manifest in the spectral properties of H and other heavy elements. In this paper, we present a preliminary survey of the roll-over or break energies observed in the energy spectra of ~0.1-100 MeV/nucleon H-Fe nuclei in >80 large SEP events and determine how they vary with the ion’s Charge-to-Mass (Q/M) ratio. In particular, by combining in-situ SEP, solar wind, and magnetic field data from ACE and STEREO over the course of solar cycles 23 and 24 with numerical modeling, we attempt to understand why some SEP events exhibit spectral breaks that vary strongly with the ion’s Q/M ratio while others do not. We also discuss the potential roles played by the shock geometry and the presence or absence of self-excited Alfvén waves in the Q/M-dependence of the spectral break energies. Finally, we describe how by directly and repeatedly sampling the near-Sun environments where interplanetary scattering and transport effects are significantly reduced, measurements from the upcoming Solar Probe Plus (SPP) and Solar Orbiter missions will unravel the roles of injection, acceleration and transport in producing the Q/M-dependent SEP spectral breaks.