On the Nature of Intense Sub‐Relativistic Electron Precipitation

Abstract

Energetic electron precipitation into Earth's atmosphere is an important process for radiation belt dynamics and magnetosphere‐ionosphere coupling. The most intense form of such precipitation is microbursts—short‐lived bursts of precipitating fluxes detected on low‐altitude spacecraft. Due to the wide energy range of microbursts (from sub‐relativistic to relativistic energies) and their transient nature, they are thought to be predominantly associated with energetic electron scattering into the loss cone via cyclotron resonance with field‐aligned intense whistler‐mode chorus waves. In this study, we show that intense sub‐relativistic microbursts may be generated via electron nonlinear Landau resonance with very oblique whistler‐mode waves. We combine a theoretical model of nonlinear Landau resonance, equatorial observations of intense very oblique whistler‐mode waves, and conjugate low‐altitude observations of <200 keV electron precipitation. Based on model comparison with observed precipitation, we suggest that such sub‐relativistic microbursts occur by plasma sheet (0.1 − 10 keV) electron trapping in nonlinear Landau resonance, resulting in acceleration to ≲200 keV energies and simultaneous transport into the loss cone. The proposed scenario of intense sub‐relativistic (≲200 keV) microbursts demonstrates the importance of very oblique whistler‐mode waves for radiation belt dynamics.

Relativistic Microburst Scale Size Induced by a Single Point‐Source Chorus Element

“…Considering the energy range and the co‐streaming nature of this part of electrons, this feature of the precipitation is due to Landau resonance with the chorus waves. This is consistent with recent studies showing that Landau resonance with oblique chorus waves can induce intense sub‐relativistic microburst precipitation (Artemyev et al., 2022). Note that such strong sub‐relativistic precipitation is not seen in the southern hemisphere because of the co‐streaming nature of Landau resonance, which, in this case would only show up in the northern hemisphere, whereas the southern hemisphere will only exhibit the energetic electron precipitation portion.…”

Temporal Scales of Electron Precipitation Driven by Whistler‐Mode Waves

“…It has similar profile and remains limited to energies less than ∼300 keV. This suggests that the plasma frequency to gyrofrequency ratio f pe / f ce , which controls the latitude of resonance and the energy of precipitating electrons (e.g., see Agapitov et al., 2019; Artemyev, Zhang, et al., 2022; L. Chen et al., 2022), probably did not change significantly over 8 min at most locations.…”

“…Zhang, Artemyev, et al., 2022). However, for nonlinear Landau resonance, the latitudes of trapping and escape from resonance (as well as initial and final electron energies) are controlled by magnetic field configuration and background plasma density (e.g., Agapitov et al., 2014; Artemyev, Zhang, et al., 2022) and, thus, are not expected to change within the short time interval between ELFIN A and B measurements. Accordingly, for both cyclotron and Landau resonances, the chorus wave power at the latitude of resonance is the only quantity affecting the precipitating‐to‐trapped electron flux ratio J prec / J trap , and the latter quantity should be a monotonic function of the former.…”

“…The spatial and temporal localization effects can be appreciated from the three equatorial satellite measurements but the important point here is that large variations in these parameters can be observed at a single point in time due to plasma density and velocity variations. Since f pe / f ce controls the energy range of electrons resonating with whistler‐mode waves, it determines both the wave generation at the equator (hence the observed wave power) (e.g., W. Li, Bortnik, et al., 2011) and the resonance energy range of scattering (hence the precipitation efficiency) (Agapitov et al., 2019; Artemyev, Zhang, et al., 2022). Moreover, such mesoscale density variations may cause whistler‐mode wave ducting (R. Chen et al., 2021; Hanzelka & Santolík, 2019; Hosseini et al., 2021; Ke et al., 2021), allowing them to propagate along the field line without strong damping to higher (middle) latitudes (Bortnik et al., 2007; L. Chen et al., 2013), where these ducted waves can scatter relativistic electrons (e.g., L. Chen et al., 2022).…”

“…However, during the observed precipitation bursts, efficient precipitation (with J prec / J trap > 0.1) is limited to low energies (<200 keV). It suggests that these precipitation bursts could be driven by electron scattering via Landau resonance with very‐oblique chorus waves, which is expected to result in a similar low‐energy microburst precipitation (Artemyev, Zhang, et al., 2022; X.‐J. Zhang, Artemyev, et al., 2022).…”