Analyzing Substorm Dynamics and Magnetotail Plasma Energetics With The WINDMI Model

Abstract:

The different substorm phases, growth, onset, dipolarization and subsequently recovery, have significant impacts on electron acceleration and precipitation, particle wave interactions, different types of aurora, ULF wave properties, and other associated phenomena. One way to determine the phases of a substorm is through statistical analysis of the Auroral Electrojet Indices (AE/Al) and the SuperMAG SML indices data. Rules for identifying the phases are formulated based on the measured levels of magnetic fields, as well as their time rate of change. The former is an indicator of the magnetic field energy, while the latter is an indicator of the effect of the induction electric fields in accelerating particles, as well as triggering waves.

The WINDMI model uses solar wind and IMF measurements from the ACE spacecraft as input into a system of 8 nonlinear ordinary differential equations. The state variables of the differential equations represent the energy stored in the geomagnetic tail, central plasma sheet, ring current and field aligned currents. The output from the model is the ground based geomagnetic westward auroral electrojet (AL) index, and the Dst index. We can constrain the WINDMI model to trigger substorm events. By forcing the model to be consistent with satellite electric and magnetic field observations, we are able to track the magnetotail energy dynamics, the field aligned current contributions, energy injections into the ring current, and ensure that they are within allowable limits.

In this work, we analyze a set of substorms identified by superMAG, using the WINDMI model, and compare the behavior of the model during different phases of substorms with the rules identified by several authors [Partamies et al., 2013, DOI-10.5194/angeo-31-349-2013 ; Forsyth et al., 2015, DOI-10.1002/2015JA021343]. We show results of tuning the WINDMI model in order to be consistent with the energy levels in different parts of the nightside magnetosphere, while simultaneously producing the growth, onset, expansion, and recovery phases inferred from the SML/AL measurements.