Superposed Epoch Analysis of Dispersionless Particle Injections Inside Geosynchronous Orbit

Abstract

Energetic particle injections from Earth's magnetotail-characterized as the sudden flux enhancements of tens to hundreds of keV electrons and/or ions-are one of fundamental signatures of substorm activity in the inner magnetosphere. Fractions of the injected electrons and ions play an important role in building-up Earth's ring current during magnetic storms, providing the seed population for MeV electrons in the outer radiation belt, and introducing the source population responsible for the generation and growth of various plasma waves. How the particles of different species/energies can be transported and/or energized and what controls the penetration of injections inside geosynchronous orbit (GEO) where the magnetic field intensity becomes much stronger are a subject of ongoing debate. The dispersionless character of the flux enhancements-so-called dispersionless injection-is known as a manifestation of the particles that are undergoing either a local energization process or a rapid transport process, or both at/near the leading edge of injections (injection region). In other words, a spacecraft at/near the injection region itself should observe a dispersionless injection, and such in situ measurements would provide an opportunity to understand the underlying mechanisms responsible for the injection.Dispersionless injections are categorized into three types: both-species injections (i.e., coincident injections of both ions (mainly protons, H + ) and electrons (e -)), e --only injections, and H + -only injections. Early energetic particle observations from a LANL spacecraft at GEO provided a statistical picture for the three types of dispersionless injections to occur with local time (or spatial) offsets with respect to midnight (Birn et al., 1997;Thomsen et al., 2001). That is, the e --only (H + -only) injections are preferentially observed ∼2 h after (∼3 h before) local midnight, while the both-species injections occur around midnight. The spatially dependent pattern of the three types has been explained in terms of H + and einjection boundaries that have a dawn-dusk separation and move earthward quasi-stationarily.

The Effect of Energetic Ion Dispersionless Injections on the Ring Current Dynamics

“…In our study, we selected proton dispersionless injection events on the night side during substorms from 2013 to 2019. The injections are mainly at pre-midnight, with a peak at 21-22 MLT, which are consistent with previous studies (Birn et al, 1997;Gabrielse et al, 2014;Motoba et al, 2021). We also found that with greater substorm intensity, the observed injections could extend to lower L shells.…”

“…What is more, dipolarization-associated injections are also shown in several studies (e.g., Gkioulidou et al, 2015;Ohtani et al, 1992;Ukhorskiy et al, 2017), and it is found by several observations that after injections, dipolarization is accompanied by increasing dawn-dusk electric field (e.g., Gabrielse et al, 2014;Sergeev et al, 1998). Ahead of dipolarization, H + preceding electron (e -) events and both-species events show a negative dip in the local magnetic field, according to Motoba et al (2021).…”

“…Ahead of dipolarization, H + preceding electron (e – ) events and both‐species events show a negative dip in the local magnetic field, according to Motoba et al. (2021).…”

“…The black and red lines in panel (h) represent the magnetic fields in the H direction (parallel to the dipole axis) in the VDH coordinate system and the total magnetic fields, respectively. Panel (i) shows the B H variations, which are obtained by first calculating the difference between the measured B H and the field in the T89 model ( B H _T89 , see Tsyganenko , 1989) during the quiet time (Kp = 0) to get ΔB H , and then subtracting the pre‐onset level of Δ B H (averaging from t 0 – 15 min to t 0 – 5 min) ( Motoba et al., 2021). After injection onset, there is a dip followed by a sharp and continuous increase in dB H , which indicates the dipolarization of the magnetic fields.…”

Energetic Electron Flux Predictions in the Near‐Earth Plasma Sheet From Solar Wind Driving

“…Suprathermal electrons (∼1 to 100 keV) in the near‐Earth plasma sheet have particular effectiveness at altering the inner magnetosphere (e.g., Garrett, 1979; Turner & Li, 2008; Yu et al., 2019), and they are the seed population for radiation belt MeV electrons (e.g., Allison et al., 2019; Chen et al., 2007; Fu et al., 2011; Horne et al., 2005; Jaynes et al., 2015; Kennel & Petschek, 1966; Kennel & Thorne, 1967; Li et al., 2008, 2012; Malykhin et al., 2021). They contribute to spacecraft surface charging environments (e.g., Davis et al., 2008; Garrett, 1981; Koons et al., 1999), posing a space weather hazard to satellites at geosynchronous orbit (GEO; e.g., Choi et al., 2011; Ganushkina et al., 2021; Matéo‐Vélez et al., 2018; Thomsen et al., 2013); and, they can penetrate deep inside GEO to medium Earth orbit (MEO; Motoba et al., 2021; Turner et al., 2015).…”