Solar Energetic Particle Events with Protons Above 500 MeV Between 1995 and 2015 Measured with SOHO/EPHIN

Abstract

The Sun is an effective particle accelerator producing solar energetic particle (SEP) events during which particles up to several GeVs can be observed. Those events observed at Earth with the neutron monitor network are called ground level enhancements (GLEs). Although these events with a high energy component have been investigated for several decades, a clear relation between the spectral shape of the SEPs outside the Earth's magnetosphere and the increase in neutron monitor count rate has yet to be established. Hence, an analysis of these events is of interest for the space weather as well as the solar event community. In this work, SEP events with protons accelerated to above 500 MeV have been identified using data from the Electron Proton Helium Instrument (EPHIN) aboard the Solar and Heliospheric Observatory (SOHO) between 1995 and 2015. For a statistical analysis, onset times have been determined for the events and the proton energy spectra were derived and fitted with a power law. As a result, a list of 42 SEP events with protons accelerated to above 500 MeV measured with the EPHIN instrument onboard SOHO is presented. The statistical analysis based on the fitted spectral slopes and absolute intensities is discussed with special emphasis on whether or not an event has been observed as GLE. Furthermore, a correlation between the derived intensity at 500 MeV and the observed increase in neutron monitor count rate has been found for a subset of events.

Assessment of spectral and angular characteristics of sub-GLE events using the global neutron monitor network

“…Candidates for sub-GLEs should exceed SEPs energy of about 300 MeV/nucleon (Atwell et al, 2015;Kühl et al, 2017). There are several candidates for sub-GLE events, now included in the GLE database (gle.oulu.fi) .…”

Real‐time prediction of the occurrence of GLE events

“…Note that the latter version has the best false alarm ratio, compared with the other proton‐based versions (see Table ). This improvement was obtained taking into account some findings in recent studies [ Sandberg et al , ; Kühl et al , ], which report that the pre‐event background measured by EPHIN is an order of magnitude lower than those given by GOES. That is, an extreme proton flux fluctuation in EPHIN data is probably more related to an actual proton enhancement; consequently, the probability of a randomly produced “1” associated with the EPHIN proton flux is lower.…”

“…Regarding the reason for the early proton flux enhancements measured by the EPHIN integral channel and the HEPAD‐P11 channel ( E > 700 MeV) shown in Figure and their similarity, we mention some recent studies that could probably explain them. Kühl et al [] carried a GEANT 4 simulation of the EPHIN instrument and found that there is a possible electron contamination of the EPHIN high energy proton measurements; the increased flux above 700 MeV is believed to be caused by electrons above 10 MeV associated to the same SEP event. Since relativistic >10 MeV electrons arrive first than protons, this contamination could explain the early EPHIN proton rises in both GLE events.…”

“…It is important to note that although the GOES‐based version had one more hit than the EPHIN‐based version, all proton‐based versions presented in Table predicted a common set of GLE events (i.e., GLE‐59, GLE‐60, GLE‐65, GLE‐66, GLE‐67, and GLE‐70). A careful study about the reason why these events are predictable using proton flux data is beyond the scope of this paper; as mentioned earlier, recent studies have proposed a possible contamination of the >700 MeV proton fluxes by >10 MeV electrons [ Kühl et al , ; Heber , ].…”

Book of Proceedings: Ninth Workshop, Sunny Beach, Bulgaria, May 30 - June 3, 2017

“…Namely, in Fig. 3 we present the scatter plots between the ~116 MeV SOHO/ERNE protons and protons detected by the GOES >100 MeV as reported by [Papaioannou et al 2016], left plot, and SOHO/EPHIN >500 MeV as given by [Kühl et al 2017], right plot. The log 10 -log 10 Pearson correlation coefficients and the bootstrapping uncertainty (based on 1000 calculations) are also given on each plot.…”