The East‐West Asymmetry of Particle Intensity in Energetic Storm Particle Events

Abstract

Solar energetic particles (SEP) accelerated by the coronal mass ejection (CME) shocks are known as gradual SEP events. During gradual events, the enhancements of particle intensity associated with the passage of the interplanetary shock near spacecraft are referred to as energetic storm particle (ESP) events (Bryant et al., 1962). The properties of ESP events observed at 1 au, and their correlations with shock properties and upstream conditions have been widely studied. For instance, M. I. Desai et al. (2003) found significant correlations between the interplanetary shock abundances and the ambient superthermal ions. Reames (2012) suggested that the shock speed correlates best with the particle intensities in ESP events. Additionally, Ebert et al. ( 2016) examined seven multiple-spacecraft ESP events and found that the peak intensities near the shock nose are larger than at the flank of the shock. More recently, Santa Fe Dueñas et al. (2022) showed that heavy ion peak intensities and spectra at 1 au are organized by longitude relative to their source flare location, which appeared to have an East-West asymmetric distribution of the peak intensity. This asymmetry refers to the difference of particle intensities between the eastern and the western shock flank. Unlike the East-West asymmetry of particle intensity in large SEP events, which is affected by the extended shock acceleration and the transport effects (e.g., Lario et al., 2006;Strauss et al., 2017;Xie et al., 2019), ESP events are typically interpreted as a direct consequence of a local shock acceleration process. Thus, the East-West asymmetry of the particle intensity in ESP events may provide important insights into the underlying acceleration mechanism.