Phase Space Density Analysis of Outer Radiation Belt Electron Energization and Loss During Geoeffective and Nongeoeffective Sheath Regions

Abstract

Coronal mass ejection driven sheath regions are one of the key drivers of drastic outer radiation belt responses. The response can however be significantly different based on the sheath properties and the associated inner magnetospheric wave activity. We performed two case studies on the effects of sheaths on outer belt electrons of various energies using data from the Van Allen Probes. One sheath caused a major geomagnetic disturbance and the other had only a minor impact. We especially investigated the phase space density (PSD) of seed, core, and ultrarelativistic electrons to determine the dominant energization and loss processes taking place during the events. Both sheaths produced substantial variation in the electron fluxes from tens of kiloelectronvolts up to ultrarelativistic energies. The responses were however the opposite: the geoeffective sheath mainly led to enhancement, while the nongeoeffective one caused a depletion throughout most of the outer belt. The case studies highlight that both inward and outward radial transport driven by ultra‐low frequency waves played an important role in both electron energization and loss. Additionally, PSD radial profiles revealed a local peak that indicated significant acceleration to core energies by chorus waves during the geoeffective event. The distinct responses and different mechanisms in action during these events were related to the timing of the peaked solar wind dynamic pressure causing magnetopause compression, and the differing levels of substorm activity. The most remarkable changes in the radiation belt system occurred in key sheath sub‐regions near the shock and the ejecta leading edge.

Outer Radiation Belt Flux and Phase Space Density Response to Sheath Regions: Van Allen Probes and GPS Observations

“…No fits to pitch angle or energy distributions were employed in this method, and interpolations were performed to get the K and L * corresponding to the equatorial pitch angles mapped from observations and to increase the energy resolution by adding two artificial energy channels in between the instrumental channels. We note that when investigating RBSP PSD, μ and K were calculated over ranges of the first and second invariants, instead of being fixed to a single value, which allows for a better resolution in PSD (see Kalliokoski et al., 2022). This does introduce fluctuations as more than one point can be found in the chosen ranges at a given time step, but it is not expected to have a significant impact as PSD is averaged in the response analysis.…”

“…The equatorial magnetic field magnitude from the TS04D model was also used to convert the measured local pitch angles to equatorial pitch angles. PSD was calculated from MagEIS data at lower energies (tens kiloelectronvolts to 1 MeV) and REPT data at higher energies (>1 MeV) as described in Kalliokoski et al (2022), where flux was converted to PSD following the formulation in Chen et al (2005). No fits to pitch angle or energy distributions were employed in this method, and interpolations were performed to get the K and L* corresponding to the equatorial pitch angles mapped from observations and to increase the energy resolution by adding two artificial energy channels in between the instrumental channels.…”

“…Sheaths are characterized by large amplitude magnetic field fluctuations and high dynamic pressure, and tend to cause intense wave activity in the inner magnetosphere and strong compression of the magnetopause. Thus, sheaths drive favorable conditions for efficient electron acceleration and scattering loss (e.g., Da Silva et al., 2020; Kalliokoski et al., 2022; Kilpua, Turner et al., 2019).…”

“…ICMEs typically consist of a leading shock and a sheath region followed by the ejecta. The sheath and ejecta have different solar wind properties, which lead to different responses of the radiation belt system (e.g., Kalliokoski et al., 2022, 2020; Kilpua et al., 2015; Turner et al., 2019). Sheaths are characterized by large amplitude magnetic field fluctuations and high dynamic pressure, and tend to cause intense wave activity in the inner magnetosphere and strong compression of the magnetopause.…”

An event of extreme relativistic and ultra-relativistic electron enhancements following the arrival of consecutive corotating interaction regions: Coordinated observations by Van Allen Probes, Arase, THEMIS and Galileo satellites

“…The response of the outer radiation belt electron populations can vary depending on the solar wind parameters, wave activity, and the source and seed electron populations abundance and intensity (Boyd et al, 2016;Katsavrias et al, 2019a;Nasi et al, 2020). Additionally, the response is dependent on the driver of the geomagnetic disturbance, e.g., Interplanetary Coronal Mass Ejections (ICMEs) or Stream Interaction Regions (SIRs) (Shen et al, 2017), even on the properties of the different parts of the drivers (e.g., sheath, ejecta) (Kilpua et al, 2015;Turner et al, 2019;Kalliokoski et al, 2022). Nevertheless, the High Speed Streams (HSSs) that follow an SIR have been shown to be particularly effective in producing multi-MeV electron enhancements, mainly because of intervals of combined southward Interplanetary Magnetic Field (IMF) and solar wind velocity Vsw over 500 km/s, which lead to an enhanced magnetic reconnection rate at the dayside magnetopause (Borovsky and Denton, 2006;Miyoshi et al, 2013;Horne et al, 2018).…”