Directional anisotropy in solar energetic particle and energetic storm particle fluxes as measured by ASPEX-STEPS and the role of IMF fluctuations

Solar energetic particles (SEPs) accelerated by distant interplanetary (IP) shocks driven by coronal mass ejections (CMEs) are different from energetic storm particles (ESPs), which are generally followed by geomagnetic storms. ESPs are believed to be accelerated by near-Earth IP shocks and often reach their peak intensities at the arrival times of these shocks at the Earth’s location or at the Sun–Earth L1 point. These energetic particles predominantly propagate along interplanetary magnetic field (IMF) lines. However, to the best our knowledge, the influence of IMF fluctuations on the directional anisotropy of energetic particle fluxes has not been investigated. In this study, we address this open question using directionally resolved energetic particle observations from the SupraThermal and Energetic Particle Spectrometer (STEPS) of the Aditya Solar wind Particle EXperiment (ASPEX) payload onboard Aditya-L1 mission. Following its launch on 02 September 2023, Aditya-L1 completed several Earth-bound orbits. During this phase, two of the six ASPEX-STEPS detector units were kept operational. We analyze energetic ion fluxes below 1.3 MeV obtained by these two detectors during a pair of SEP-ESP events observed by ASPEX-STEPS. Our results reveal that the temporal evolution of directional anisotropy in ion differential directional fluxes differs significantly between the SEP and ESP events. Furthermore, fluctuations in the directional anisotropy exhibit periodicities similar to those observed in IMF fluctuations, indicating a strong causal relationship. Number of common periodicities also differs between the SEP and ESP events. These findings are important to understand the transport of energetic particles and space weather impacts. The details of this study will be discussed.