Reanalyzing Inferred High Energy Ionic Charge States for Solar Energetic Particle Events from 2005-2016 with ACE and STEREO

Abstract

We have estimated mean high-energy ionic charge states of solar energetic particles (SEPs) using the Sollitt et al. (2008) method. In our analysis, the method applies to abundant elements (e.g. N, O, Ne, Mg, Si, and Fe) in SEP events at the energy ranges covered by the STEREO/LET instrument (e.g. 2.7-70 MeV/nuc for Fe) and the ACE/SIS instrument (e.g. 11-168 MeV/nuc for Fe). The method starts by fitting SEP time-intensity profiles during the decay phase of a given, large SEP event in order to obtain energy-dependent decay times. The energy dependence of intensity decay times for a given element in an SEP event is combined with its assumed average atomic mass and an assumed average charge state of 5.9 for carbon as a reference element. The mean charge state for each element is then estimated from the relationship between the energy dependence of its decay times to that for carbon and all other elements in the event. The method has yielded charge states for 8 SEP events using ACE data, 5 SEP events with STEREO A data, and 4 SEP events with STEREO B data through 2015. Calculations for events before 2005 by Sollitt et al. using ACE data also incorporated helium time intensity profile fits, with an assumed charge state of 2, but later calculations dropped helium as a reference element for simplicity, as the calculations were first generalized to later events and to STEREO data. In this paper, we restore helium in order to refine the calculations and compare with the earlier results. In particular, we will focus on the 8 March 2012 event with ACE/SIS data and the 28 September 2012 event with STEREO/LET data. Additionally, other charge state measurements using a geomagnetic rigidity cutoff technique yielded correlations between charge states and Fe/O ratio as well as to observed source solar longitudes. We will use the inferred charge state reanalysis to investigate whether these correlations are also present in ACE and STEREO data.

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