Ion Dynamics during Substorm Events Modeled with Coupled Global MHD and Kinetic Models

Abstract:

We have studied ion dynamics during a substorm by using a coupled fluid-kinetic approach. The UCLA global magnetospheric model was applied first and its results in a region encompassing a magnetotail reconnection site and earthward propagating dipolarization fronts was selected as input state for a full kinetic simulation based on the iPic3D code [1]. The coupling is one-way: the MHD result is used to create a full kinetic initial state by using the approach described by [2] and to force the boundary conditions. The kinetic results are not fed back into the MHD run. This approach previously has been shown [3] to provide correctly the large scale picture for a kinetic approach when the duration of the kinetic run is not so long as to alter significantly the macroscopic state captured by MHD.

Here we focus especially on the ions. The electrons were described in [3]. Three aspects are analyzed. First, the ions are accelerated during the event and we track the localization of the energy exchanged, separating the contributions to the directed energy and those to the thermal energy. Second, we consider the ion motion to identify the regions where the finite Larmor radius effects violate the drift approximation and the frozen-in condition, thereby identifying the ion diffusion region. Our approach follows individual electrons and ions fully kinetically and no approximation is made in the particle orbit, so our code is equipped to study accurately the regions where the drift approximation is valid. Finally, we consider the ion distribution as a possible source of instabilities, focusing especially on temperature anisotropy instabilities and on the firehose instability.

The novelty of the approach is this kinetic study is done for a specific substorm by using the global state of the magnetosphere as provided by a global MHD simulation. This differs sharply from previous approaches based on analytical approximations such as the Harris or the (quasi)-parabolic equilibria.

[1] S.Markidis, G. Lapenta, Rizwan-uddin, Math. Computers Simulation 80.7 (2010): 1509-1519.

[2] G. Baumann, Å. Nordlund, Ap J. Lett 759.1 (2012): L9.

[3] M. Ashour-Abdalla, G. Lapenta, R.J. Walker, M. El Alaoui, H. Liang, Multiscale Study of Electron Energization during Unsteady Reconnection Events, J. Geophys. Res., submitted.