Abstract
We report spacecraft measurements of the energy spectra of solar protons and other solar energetic particle properties during the 16 Ground Level Events (GLEs) of Solar Cycle 23. The measurements were made by eight instruments on the ACE, GOES, SAMPEX, and STEREO spacecraft and extend from ∼0.1 to ∼500–700 MeV. All of the proton spectra exhibit spectral breaks at energies ranging from ∼2 to ∼46 MeV and all are well fit by a double power-law shape. A comparison of GLE events with a larger sample of other solar energetic particle (SEP) events shows that the typical spectral indices are harder in GLE events, with a mean slope of −3.18 at >40 MeV/nuc. In the energy range 45 to 80 MeV/nucleon about ∼50 % of GLE events have properties in common with impulsive 3He-rich SEP events, including enrichments in Ne/O, Fe/O, 22Ne/20Ne, and elevated mean charge states of Fe. These 3He-rich events contribute to the seed population accelerated by CME-driven shocks. An analysis is presented of whether highly-ionized Fe ions observed in five events could be due to electron stripping during shock acceleration in the low corona. Making use of stripping calculations by others and a coronal density model, we can account for events with mean Fe charge states of 〈Q Fe〉≈+20 if the acceleration starts at ∼1.24–1.6 solar radii, consistent with recent comparisons of CME trajectories and type-II radio bursts. In addition, we suggest that gradual stripping of remnant ions from earlier large SEP events may also contribute a highly-ionized suprathermal seed population. We also discuss how observed SEP spectral slopes relate to the energetics of particle acceleration in GLE and other large SEP events.
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Acknowledgements
This work was supported by NASA at Caltech under grants NNX8AI11G and NNX06AC21G, and under subcontract SA2715-26309 from UC Berkeley under NASA contract NAS5-03131. We appreciate the availability of GOES data at the NOAA website, SOHO CME data at the Catholic University website, and thank Allan Tylka for advice on using HEPAD data. We are grateful for helpful suggestions from both reviewers. Finally, we thank Nat Gopalswamy and Nariaki Nitta for helpful discussions, for organizing the two LWS workshops on GLEs, and for serving as editors for this volume.
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Mewaldt, R.A., Looper, M.D., Cohen, C.M.S. et al. Energy Spectra, Composition, and Other Properties of Ground-Level Events During Solar Cycle 23. Space Sci Rev 171, 97–120 (2012). https://doi.org/10.1007/s11214-012-9884-2
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DOI: https://doi.org/10.1007/s11214-012-9884-2