Statistical Results for Solar Energetic Electron Spectra Observed over 12 yr with STEREO/SEPT

Abstract

We present a statistical analysis of near-relativistic (NR) solar energetic electron event spectra near 1 au. We use measurements of the STEREO Solar Electron and Proton Telescope (SEPT) in the energy range of 45-425 keV and utilize the SEPT electron event list containing all electron events observed by STEREO A and STEREO B from 2007 through 2018. We select 781 events with significant signal to noise ratios for our analysis and fit the spectra with single or broken power law functions of energy. We find 437 (344) events showing broken (single) power laws in the energy range of SEPT. The events with broken power laws show a mean break energy of about 120 keV. We analyze the dependence of the spectral index on the rise times and peak intensities of the events as well as on the presence of relativistic electrons. The results show a relation between the power law spectral index and the rise times of the events with softer spectra belonging to rather impulsive events. Long rise-time events are associated with hard spectra as well as with the presence of higher energy (>0.7 MeV) electrons. This group of events cannot be explained by a pure flare scenario but suggests an additional acceleration mechanism, involving a prolonged acceleration and/or injection of the particles. A dependence of the spectral index on the longitudinal separation from the parent solar source region was not found. A statistical analysis of the spectral indices during impulsively rising events (rise times < 20 minutes) is also shown.

Connecting solar flare hard X-ray spectra to in-situ electron spectra using RHESSI and STEREO/SEPT observations

“…As discussed above several energy-dependent effects may alter the spectrum observed in situ eventually leading to spectral breaks, which requires to make a choice between the two different spectral indices. For the events in our sample this choice is often not straightforward as breaks are often found below or around 100 keV, which is in between the mean locations of the two different spectral breaks (60 keV and 120 keV) as reported for instance by, Krucker et al (2007); Dresing et al (2020). We therefore start by using always the lower (δ 1 ) or the upper spectral index (δ 2 ) as observed by SEPT in case of broken power-laws and compare these with the HXR spectral index γ.…”

“…8 If δ 2 and E b are missing, a single power law fit was used Furthermore, especially in the case of more gradual intensity increases, which can also be caused by non-nominal magnetic field orientations resulting in a poor pitch-angle coverage at the SEPT instrument, the 3σ method can yield onset times that are too late. Larger time averaging is often used to overcome these issues, and the time averaging applied for each event has been used as a measure for the uncertainty of the onset time (Dresing et al 2020). This uncertainty has been propagated to ∆t in table 1 in cases when higher averaging was used.…”

Do Impulsive Solar-Energetic-Electron (SEE) Events Drive High-Voltage Charging Events on the Nightside of the Moon?

“…The calculations also ignore photoelectron emission produced by Earthshine on the objects. For the solarenergetic-electron population the fluxes as a function of electron kinetic energy E of Figure 2 of Dresing et al (2020) are used: those differential fluxes F are FIGURE 4 | Rudimentary charging calculations for spherical objects in a bath of energetic electrons using a measured distribution of solar energetic electrons at 1 AU.…”

“…Note for future charging calculations that the electron spectra in Figure 2 of Dresing et al (2020) is the timeintegrated spectra of an SEE event at 1 AU: for actual SEE events there is a systematic time evolution of the electron spectra at 1 AU that must be used in the calculation. And along with the time evolution, there are temporal fluctuations in the intensity of the SEE electrons owing to the fine-scale spatial structure of the magnetic field in the solar wind (Gosling et al, 2004a;Gosling et al, 2004b;Tan and Reames, 2016;Borovsky, 2021).…”

“…Strong solar energetic electron events (>300 keV) occur about 20 times per year (Wang et al, 2012), with concentrations around solar maximum, but with several events per year throughout the other phases of the solar cycle. The survey by Dresing et al (2020) also found on average about 20 events per year at 1 AU that had electrons with energies >150 keV. SEE events may or may not be accompanied by multi-MeV protons in the solar wind (Kallenrode et al, 1992), which occur during "solar-energeticparticle" (SEP) events.…”