Abstract
NASA’s Magnetospheric Multi-Scale (MMS) mission is designed to explore the proton- and electron-gyroscale kinetics of plasma turbulence where the bulk of particle acceleration and heating takes place. Understanding the nature of cross-scale structures ubiquitous as magnetic cavities is important to assess the energy partition, cascade and conversion in the plasma universe. Here, we present theoretical insight into magnetic cavities by deriving a self-consistent, kinetic theory of these coherent structures. By taking advantage of the multipoint measurements from the MMS constellation, we demonstrate that our kinetic model can utilize magnetic cavity observations by one MMS spacecraft to predict measurements from a second/third spacecraft. The methodology of “observe and predict” validates the theory we have derived, and confirms that nested magnetic cavities are self-organized plasma structures supported by trapped proton and electron populations in analogous to the classical theta-pinches in laboratory plasmas.
Highlights
A classical example of these nonlinear structures is the magnetic cavities, referred to as magnetic holes, that have been reported to travel in the solar wind for at least several astronomical units[9]. Such magnetic cavities, characterized by quasi-symmetric depressions of magnetic field strength accompanied by plasma density and pressure enhancements, have been intensively investigated in the observations of space and astrophysical plasma environments, including the solar wind[9,10,11,12], heliosheath[13], terrestrial magnetosheath[14,15,16], magnetotail[17,18,19,20,21,22,23,24], planetary[25,26], and even cometary environments[27,28]
The availability of the high-resolution observations from the Magnetospheric Multi-Scale (MMS) constellation[31] further enables identification of electron-scale kinetic cavities[32], which in specific occasions are found to be embedded within proton-scale cavities[16]
In deriving a kinetic equilibrium state consistent with observations, proton and electron distributions must be taken as functions of the invariants of particle motion[46,48,49] so as to satisfy the Vlasov equation
Summary
Our kinetic model detailed in the “Methods” section is used to reconstruct profiles of nested magnetic cavities observed by the MMS constellation on 23 October 2015. During this time interval, the MMS spacecraft were separated from one another by approximately 10 km. The strong anisotropy within the electron-scale cavity has been attributed to two magnetic mirrors bracketing the cavity in the field-aligned direction[16], between which electrons with near-90° pitch angles (outside the “local loss cones”) are trapped by magnetic mirror force This is not the only interpretation though; we will show that the observed pitch angle spectrum can be reproduced in our cylindrical model without invoking adjacent magnetic mirrors.
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