Mechanism of rapid localized MeV electron acceleration in the plasma sheet burst

Identify the physical mechanism or mechanisms responsible for rapidly accelerating sub-200 keV plasma sheet electrons to at least 2–3 MeV and promptly scattering them into the atmospheric loss cone within a ~3 Earth radii localized region of the deep magnetotail during the 17 July 2021 substorm event, given that consideration of adiabatic betatron and Fermi processes and several non-adiabatic wave/turbulence scenarios did not yield a unique attribution.

Background

The event featured a burst with a hard spectrum extending to ~3 MeV and intensities exceeding the outer radiation belt, mapped to ~33–36 RE, and not seen by MMS at ~17 RE, indicating a highly localized source region. The authors evaluated potential adiabatic (betatron and Fermi) and non-adiabatic (turbulence and wave–particle interactions) mechanisms.

Adiabatic explanations would require multi-step processes (e.g., trapping and large magnetic field increases or strong field line shortening) plus efficient, prompt scattering, while known wave modes have not been shown to directly accelerate electrons to relativistic energies in the magnetotail. With no direct in-situ observation at the acceleration site, the mechanism remains unresolved.