The Role of Mesoscale Plasma Sheet Dynamics in Ring Current Formation

K A Sorathia,J.G Lyon,J Garretson,V.G Merkin,A.Y Ukhorskiy,M Gkioulidou,F.R Toffoletto,D L Turner,A Michael

doi:10.3389/fspas.2021.761875

Abstract

During geomagnetically active periods ions are transported from the magnetotail into the inner magnetosphere and accelerated to energies of tens to hundreds of keV. These energetic ions, of mixed composition with the most important species being H+ and O+, become the dominant source of plasma pressure in the inner magnetosphere. Ion transport and acceleration can occur at different spatial and temporal scales ranging from global quasi-steady convection to localized impulsive injection events and may depend on the ion gyroradius. In this study we ascertain the relative importance of mesoscale flow structures and the effects of ion non-adiabaticity on the produced ring current. For this we use: global magnetohydrodynamic (MHD) simulations to generate self-consistent electromagnetic fields under typical driving conditions which exhibit bursty bulk flows (BBFs); and injected test particles, initialized to match the plasma moments of the MHD simulation, and subsequently evolved according to the kinetic equations of motion. We show that the BBFs produced by our simulation reproduce thermodynamic and magnetic statistics from in situ measurements and are numerically robust. Mining the simulation data we create a data set, over a billion points, connecting particle transport to characteristics of the MHD flow. From this we show that mesoscale bubbles, localized depleted entropy regions, and particle gradient drifts are critical for ion transport. Finally we show, using identical particle ensembles with varying mass, that O+ non-adiabaticity creates qualitative differences in energization and spatial distribution while H+ non-adiabaticity has non-negligible implications for loss timescales.

Highlights

The modern understanding of transport processes, and their multiscale nature, through the magnetotail transition region has advanced considerably in the decades following the early work that first recognized their importance (e.g., Mauk and McIlwain, 1974)
Test particles are weighted based on the density, temperature, and flux tube volume in the MHD simulation at the location and time of their injection
Prior to our discussion of particle transport, we show that the statistical properties of the bursty bulk flows (BBFs), those related to the quantities used in particle weighting, produced by our model are in agreement with observations (Section 3.1)

Summary

Introduction

The modern understanding of transport processes, and their multiscale nature, through the magnetotail transition region has advanced considerably in the decades following the early work that first recognized their importance (e.g., Mauk and McIlwain, 1974). It is known that much of the plasma transport in the transition region occurs by means of transient ( ∼ 10 minute), fast ( ∼ 400 km/s) bursty bulk flows (BBFs; Baumjohann et al, 1990; Angelopoulos et al, 1992, Angelopoulos et al, 1994) These flow bursts have typical cross-tail sizes of 1–3RE (e.g., Nakamura et al, 2004; Liu et al, 2013), which we will refer to here as mesoscale to distinguish them from both global and kinetic scales in the Mesoscale Plasma Sheet Dynamics magnetosphere. This interpretation was supported by regional modeling, i.e., localized subdomains of the global magnetosphere, which highlighted the role of reconnection, and the resultant reduction in flux tube volume, in creating bubbles (e.g., Birn et al, 2009, 2011; Liu et al, 2014) and the role of buoyancy (e.g., Yang et al, 2010, Yang et al, 2015; Sadeghzadeh et al, 2021) in the subsequent transport of these bubbles into the inner magnetosphere.

Methods

Results

Conclusion