On the Onset Delays of Solar Energetic Electrons and Protons: Evidence for a Common Accelerator

Abstract

The processes responsible for the acceleration of solar energetic particles (SEPs) are still not well understood, including whether SEP electrons and protons are accelerated by common or separate processes. Using a numerical particle transport model that includes both pitch-angle and perpendicular spatial diffusion, we simulate, among other quantities, the onset delay for MeV electrons and protons and compare the results to observations of SEPs from widely separated spacecraft. Such observations have previously been interpreted, in a simple scenario assuming no perpendicular diffusion, as evidence for different electron and proton sources. We show that, by assuming a common particle source together with perpendicular diffusion, we are able to simultaneously reproduce the onset delays for both electrons and protons. We argue that this points toward a common accelerator for these particles. Moreover, a relatively broad particle source is required in the model to correctly describe the observations. This is suggestive of diffusive shock acceleration occurring at large shock structures playing a significant role in the acceleration of these SEPs.

The “SEP clock”: A discussion of first proton arrival times in wide-spread solar energetic particle events

“…To match, the inciter must have a higher speed for relativistic electrons than for the 25 MeV protons they analyzed. While this set of observations already hints towards an interpretation that particle speed dependent cross-eld particle diffusion away from the source location is the likely cause for the onset delays, as recently also shown in the simulations of Strauss et al (2023), the option of having different acceleration regions for protons and electrons driven by an expanding CME shock cannot be ruled out. Based on the observations above, we can add that the onset delays for a single species at different energies bolsters the case for the role of speed-dependent particle cross-eld diffusion.…”

“…This view is seemingly supported by the nding of dropouts in so-called "impulsive" solar particle events (Mazur et al, 2000) that were observed in the range of up to ~ 200 keV. However, solar energetic particle {SEP) events have been observed that reach all solar longitudes (Dresing et al, 2023, Kollhoff et al, 2021, Richardson et al, 2014, and would do so by crossing sector boundaries (Kallenrode, 1993). Therefore, if particles strictly follow magnetic eld lines, the particle acceleration source would have to be nearly equally wide as the observed SEP event.…”

“…The onsets of solar proton events potentially contain information about physical processes acting on the protons, such as acceleration, and scattering, focusing, and drift while interacting with magnetic elds between the acceleration site and the observer. Many authors have used the signature of proton energy dispersion to derive the magnetic eld line length of the rst arriving particles, by using the 1/v method (Dresing et al, 2023), with the underlying assumption that energetic particles, following a simultaneous "release" for particles at all energies, are bound to magnetic eld lines in the solar wind. This view is seemingly supported by the nding of dropouts in so-called "impulsive" solar particle events (Mazur et al, 2000) that were observed in the range of up to ~ 200 keV.…”

“…Here also, if one assumes that particles strictly follow magnetic eld lines, two acceleration drivers would be needed, one for relativistic electrons and the other for ~ 25 MeV protons. However, an alternative view is that there is a single acceleration driver and (1) cross-eld transport in the heliosphere plays a dominant role in shaping onset delays of particle events and (2) that the expansion of particle events in longitude is a function of particle speed (Strauss et al, 2023). To test both views, we analyze the arrival time durations of protons at different particle energies, not for single events, but as an ensemble of events with differing source longitudes with respect to the observer.…”