On the Formation of Wedge‐Like Ion Spectral Structures in the Nightside Inner Magnetosphere

Abstract

Recent observations in the nightside inner magnetosphere have identified a series of wedge-like spectral structures in the energy-time spectrograms of oxygen, helium, and hydrogen ion fluxes. Although the shapes and distributions of these structures have been characterized by case and statistical studies, their formation mechanism remains unclear. Here we utilize a particle tracing model to reproduce the wedge-like structures successively observed by the twin Van Allen Probes. The model suggests that these structures originate from intermittent substorm injection, and it is the accessibility region of these injected ions that determines their shapes. This mechanism is similar to the formation of another kind of structures, the inner magnetospheric nose-like structures, except that the wedge-like structures are separated from the tail population by the discontinuation of ion injections. This scenario is also supported by the distribution statistics of wedge-like structures, which provides new insights into the dynamics of the magnetotail-inner magnetosphere coupled system. Plain Language Summary In Earth's inner magnetosphere, multiple plasma populations interact with one another to render a highly coupled system, with its dynamic evolution also subject to magnetospheric convection and magnetotail particle injections. The interplay of these complicated processes is manifested in spacecraft observations by various spectral structures of particle fluxes, and the understanding of their generation provides unique insights into the underlying processes. Here we focus on a specific type of spectral structures in the nightside inner magnetosphere, named the wedge-like structures. These structures are characterized by enhanced fluxes of oxygen, helium, and hydrogen ions at increasing energies (from a few eV to several keV) with decreasing equatorial distance to the Earth. Based on a particle tracing model, we successfully reproduce the structure characteristics observed by NASA's (National Aeronautics and Space Administration) twin Van Allen Probes. The model suggests that these structures originate from transient injections associated with substorm activities, and their shapes are determined by the energy-dependent ion accessibility into the inner magnetosphere. This scenario of wedge-like structure formation is consistent with both case and statistical results, which sheds new light on our current understanding of the inner magnetospheric dynamics and its coupling process with the magnetotail plasma sheet.

Simultaneously Formed Wedge‐Like Structures of Different Ion Species Deep in the Inner Magnetosphere

“…This is probably the reason why the wedge‐like structures deep in the inner magnetosphere are rarely observed in the dayside, as shown in Figure 5a. In the scenario of Zhou et al (2020), the drift velocity of injected ions depends on particle energy and charge rather than particle mass. Therefore, the differences of O + and He + in comparison with H + in Figures 3–6 are not related to the formation mechanism of the wedge‐like structure.…”

“…This article is a companion to Zhou et al (2020), https://doi.org/10.1029/ 2020JA028420. energy for increasing latitude, (2) bridge-type with energy monotonically increasing with latitude but subsequently decreasing with latitude, and…”

Inner Magnetospheric Magnetic Dips and Energetic Protons Trapped Therein: Multi‐Spacecraft Observations and Simulations

“…Energetic particle injections are often accompanied by a dispersionless injection region bounded by the “injection boundary” (McIlwain, 1974), at which the particle fluxes across a wide energy range increase simultaneously. After leaving the injection region, the particles drift around the Earth with velocities depending on their energies and species, which leads to the dispersive injection signals (e.g., Belian et al., 1978; Liu et al., 2018; Mauk & Meng, 1983; Reeves et al., 1990; Zhou et al., 2020).…”

The Link Between Wedge‐Like and Nose‐Like Ion Spectral Structures in the Inner Magnetosphere

“…We next utilize the particle‐tracing model from Zhou et al. (2020) to examine whether the wedge‐like and nose‐like spectral structures can be reproduced along the Arase and Van Allen Probes trajectories. In this model, the ion motion at different energies is controlled by the competition between the magnetic field gradient‐curvature drift and the convection‐corotation electric field drift.…”