Solar energetic particles in the inner heliosphere: status and open questions

Abstract

Solar energetic particle (SEP) events are related to both solar flares and coronal mass ejections (CMEs) and they present energy spectra that span from a few keV up to several GeV. A wealth of observations from widely distributed spacecraft have revealed that SEPs fill very broad regions of the heliosphere, often all around the Sun. High-energy SEPs can sometimes be energetic enough to penetrate all the way down to the surface of the Earth and thus be recorded on the ground as ground level enhancements (GLEs). The conditions of the radiation environment are currently unpredictable due to an as-yet incomplete understanding of solar eruptions and their corresponding relation to SEP events. This is because the complex nature and the interplay of the injection, acceleration and transport processes undergone by the SEPs in the solar corona and the interplanetary space prevent us from establishing an accurate understanding (based on observations and modelling). In this work, we review the current status of knowledge on SEPs, focusing on GLEs and multi-spacecraft events. We extensively discuss the forecasting and nowcasting efforts of SEPs, dividing these into three categories. Finally, we report on the current open questions and the possible direction of future research efforts. This article is part of the theme issue ‘Solar eruptions and their space weather impact’.

Space weather: the solar perspective

“…However, gradual events seem to be accompanied as well by an impulsive part which is thought to be related to shorttime magnetic reconnection processes, as observed in flares. Obviously gradual events are caused by both driving agents prolonging the acceleration process but on a less energetic level (see also Anastasiadis et al 2019). Impulsive events are also obtained to be related to those SEP events where the location of the particle accelerator is magnetically well connected to the observer.…”

Application of the Verified Neutron Monitor Yield Function for an Extended Analysis of the GLE # 71 on 17 May 2012

“…This event was extensively studied [e.g. Mishev et al, 2014;Plainaki et al, 2014;Battarbee et al, 2018;Kocharov et al, 2018;Anastasiadis et al, 2019]. A good summary of the observations is given in Papaioannou et al [2014].…”

“…The phases of the event can be quantitatively related to different acceleration stages of SEPs [e.g. Kocharov et al, 2017;Anastasiadis et al, 2019]. An illustration of the time evolution of the derived spectra and PAD throughout the event is given in the supplementary material.…”

Sources of solar energetic particles

“…This picture is mostly consistent with the impulsive-flare-related SEP events being associated with localized sources close to the Sun. Gradual-CME-related SEP events are usually detected at widely separated locations in the heliosphere [16,17]. Remote sensing observations and in-situ measurements from multiple vantage points (SoHO, STEREO, ACE, and Wind) have significantly improved our knowledge about the large longitudinal spread of SEP events and also helped to define the locations of the possible particle acceleration and release sites.…”