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Abstract The magnetospheric substorm is a key mode of flux and energy transport throughout the magnetosphere associated with distinct and repeatable magnetotail dynamical processes and plasma injections. The substorm growth phase is characterized by current sheet thinning and magnetic field reconfiguration around the equatorial plane. The global characteristics of current sheet thinning are important for understanding of magnetotail state right before the onset of magnetic reconnection and of the key substorm expansion phase. In this paper, we investigate this thinning at different radial distances using plasma sheet (PS) energetic (>50 keV) electrons that reach from the equator to low altitudes during their fast (∼1 s) travel along magnetic field lines. We perform a multi‐case study and a statistical analysis of 34 events with near‐equatorial observations of the current sheet thinning by equatorial missions and concurrent, latitudinal crossings of the ionospheric projection of the magnetotail by the low‐altitude Electron Losses and Fields Investigation (ELFIN) CubeSats at approximately the same local time sector. Energetic electron fluxes thus collected by ELFIN provide near‐instantaneous (<5 min duration) radial snapshots of magnetotail fluxes. Main findings of this study confirm the previously proposed concepts with low‐altitude energetic electron measurements: (a) Energy distributions of low‐altitude fluxes are quantitatively close to the near‐equatorial distributions, which justifies the investigation of the magnetotail current sheet reconfiguration using low‐altitude measurements. (b) The magnetic field reconfiguration during the current sheet thinning (which lasts ≥ an hour) results in a rapid shrinking of the low‐altitude projection of the entire PS (from near‐Earth, ∼10RE, to the lunar orbit ∼60RE) to 1–2° of magnetic latitude in the ionosphere. (c) The current sheet dipolarization, common during the substorm onset, is associated with a very quick (∼10 min) change of the tail magnetic field configuration to its dipolar state, as implied by a poleward expansion of the PSPS at low altitudes.
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Omnidirectional Energetic Electron Fluxes From 150 to 20,000 km: An ELFIN‐Based Model
Abstract The strong variations of energetic electron fluxes in the Earth's inner magnetosphere are notoriously hard to forecast. Developing accurate empirical models of electron fluxes from low to high altitudes at all latitudes is therefore useful to improve our understanding of flux variations and to assess radiation hazards for spacecraft systems. In the present work, energy‐ and pitch‐angle‐resolved precipitating, trapped, and backscattered electron fluxes measured at low altitude by Electron Loss and Fields Investigation (ELFIN) CubeSats are used to infer omnidirectional fluxes at altitudes below and above the spacecraft, from 150 to 20,000 km, making use of adiabatic transport theory and quasi‐linear diffusion theory. The inferred fluxes are fitted as a function of selected parameters using a stepwise multivariate optimization procedure, providing an analytical model of omnidirectional electron flux along each geomagnetic field line, based on measurements from only one spacecraft in low Earth orbit. The modeled electron fluxes are provided as a function of ‐shell, altitude, energy, and two different indices of past substorm activity, computed over the preceding 4 hr or 3 days, potentially allowing to disentangle impulsive processes (such as rapid injections) from cumulative processes (such as inward radial diffusion and wave‐driven energization). The model is validated through comparisons with equatorial measurements from the Van Allen Probes, demonstrating the broad applicability of the present method. The model indicates that both impulsive and time‐integrated substorm activity partly control electron fluxes in the outer radiation belt and in the plasma sheet.
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- Award ID(s):
- 2329897
- PAR ID:
- 10546903
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 129
- Issue:
- 10
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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