Contribution of Kinetic Alfvén Waves to Energetic Electron Precipitation From the Plasma Sheet During a Substorm

Abstract

Energetic (≳50 keV) electron precipitation from the magnetosphere to the ionosphere during substorms can be important for magnetosphere‐ionosphere coupling. Using conjugate observations between the THEMIS, ELFIN, and DMSP spacecraft during a substorm, we have analyzed the energetic electron precipitation, the magnetospheric injection, and the associated plasma waves to examine the role of waves in pitch‐angle scattering plasma sheet electrons into the loss cone. During the substorm expansion phase, ELFIN‐A observed 50–300 keV electron precipitation from the plasma sheet that was likely driven by wave‐particle interactions. The identification of the low‐altitude extent of the plasma sheet from ELFIN is aided by DMSP global auroral images. Combining quasi‐linear theory, numerical test particle simulations, and equatorial THEMIS measurements of particles and fields, we have evaluated the relative importance of kinetic Alfvén waves (KAWs) and whistler‐mode waves in driving the observed precipitation. We find that the KAW‐driven bounce‐averaged pitch‐angle diffusion coefficients Dα0α0 ${D}_{{\alpha }_{0}{\alpha }_{0}}$ near the edge of the loss cone are ∼10−6–10−5 s−1 for these energetic electrons. The Dα0α0 ${D}_{{\alpha }_{0}{\alpha }_{0}}$ due to parallel whistler‐mode waves, observed at THEMIS ∼10‐min after the ELFIN observations, are ∼10−8–10−6 s−1. Thus, at least in this case, the observed KAWs dominate over the observed whistler‐mode waves in the scattering and precipitation of energetic plasma sheet electrons during the substorm injection.

Red Line Diffuse‐Like Aurora Driven by Time Domain Structures Associated With Braking Magnetotail Flow Bursts

“…Figures 1i and 1j show that the flow bursts corresponded to enhanced broadband electrostatic fluctuations (or TDSs) with frequencies ranging from tens of Hz to several kHz. Additionally, sporadic electromagnetic wave power enhancements, mostly below ∼20 Hz, indicate the presence of kinetic Alfvén waves (KAWs) (e.g., Chaston et al., 2012; Malaspina et al., 2018; Shen et al., 2023).…”

“…Thus, TDSs are likely linked to red‐line‐dominated auroras, especially when the plasma sheet state is prime for such low‐energy precipitation, like quiet times with low temperatures. In fact, to reliably identify TDS‐driven diffuse auroras, non‐substorm conditions are more suitable, as it is often challenging to disentangle aggregated effects from different, concurrent drivers during substorms that preclude direct linkage to flow bursts and TDSs (e.g., Sergeev et al., 2012; Shen et al., 2023).…”

The high-energy tail of energetic electron precipitation: solar wind drivers and geomagnetic responses

“…Overall, the BLC fluxes are an optimized estimate based on known physics and available instrumentation. Still, the pitch angle diffusion theory by Kennel and Petschek (1966) assumes steadystate conditions, which can lead to uncertainties in geomagnetic active periods (Shen et al, 2023). Moreover, a recent idealized model study by Selesnick et al (2020) points out that the sensitivity of the 0°detector outside the nominal field of view may cause it to be susceptible to quasi-trapped or trapped electrons during periods of weak pitch angle diffusion, hence exaggerating the fluxes.…”

Energetic Electron Flux Dropouts Measured by ELFIN in the Ionospheric Projection of the Plasma Sheet

“…Also, field-line curvature scattering is likely not significant during dipolarization when the magnetic field line curvature radius increases (Lukin et al, 2021), especially for electrons at high pitch angles (Buchner & Zelenyi, 1989;Delcourt et al, 1995). The kinetic Alfvén wave has recently been found to potentially contribute to the precipitation of >50 keV electrons from the substorm plasma sheet, but this scattering is most effective when KAW power is strong enough (Shen et al, 2022(Shen et al, , 2023, which is not always observed. Furthermore, the perpendicular anisotropy will be continuously fed by freshly injected electrons during a substorm, thus its lifetime is expected to be consistent with substorm timescales.…”