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

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. Key Points: The wedge‐like ion structures in the nightside inner magnetosphere are reproduced with a simple modelThe ions within the wedge originate from transient injections associated with substorm activitiesThe wedges are formed by the slow yet deep penetration of injected ions with specific magnetic moments [ABSTRACT FROM AUTHOR]