Particle tracing modeling of ion fluxes at geosynchronous orbit

Highlights

• Coupled MHD/particle tracing model is used to calculate fluxes in magnetosphere.

• Modeling setup used is able to capture substorm injections with high resolution.

• Modeling setup could be used to improve coupling with ring current models.

Abstract

The first results of a coupled MHD/particle tracing method to evaluate particle fluxes in the inner magnetosphere are presented. This setup is capable of capturing the earthward particle acceleration process resulting from dipolarization events in the tail region of the magnetosphere. On the period of study, the MHD code was able to capture a dipolarization event and the particle tracing algorithm was able to capture the results of these disturbances and calculate proton fluxes in the night side geosynchronous orbit region. The simulation captured dispersionless injections as well as the energy dispersion signatures that are frequently observed by satellites at geosynchronous orbit. Currently, ring current models rely on Maxwellian-type distributions based on either empirical flux values or sparse satellite data for their boundary conditions close to geosynchronous orbit. Despite some differences in intensity and timing, the setup presented here is able to capture substorm injections, which represents an improvement regarding a reverse way of coupling these ring current models with MHD codes through the use of boundary conditions.

Introduction

The inner magnetosphere often presents complex and highly dynamical behavior. It is a system consisting of several plasma populations with different characteristics all coupled together. Therefore, it is a valuable modeling effort to try to use different simulation methods when investigating the behavior of these different populations. There have been many different approaches, including kinetic and MHD, used over the years to create models of the various space plasma populations such as the ring current (Chen et al., 1994, Jordanova et al., 1997), the magnetosphere (Powell et al., 1999), plasmasphere (Goldstein et al., 2005), radiation belts (Subbotin and Shprits, 2009), and population coupled models (Wiltberger et al., 2004, Pembroke et al., 2012, Welling et al., 2015). This is the context in which the Space Hazards Induced near Earth by Large Dynamic Storms (SHIELDS) project is inserted. One of the main objectives of SHIELDS is to couple different models of the space environment, that are extensively used in a stand-alone manner to model different parts of the magnetosphere, to produce a better global understanding of the whole plasma system near the Earth. Different plasma populations in this system, as well as distinct processes and interactions relevant to the system, operate in largely different spatio-temporal and energy scales, thus different modeling approaches are necessary to represent these different populations and processes. For instance, approaches such as Magneto-Hydro-Dynamics (MHD), Particle-In-Cell (PIC) and particle tracing may be used to treat the magnetosphere as a fluid, to model interactions between particles and waves and to investigate the behavior of the radiation belt population during storms, respectively. If all these processes and populations, which are highly dependent on each other, could be simulated concurrently and in a coupled manner, then a more realistic model of the whole magnetospheric environment would be achieved leading to a more accurate description of the whole near-Earth environment. In the context of this effort to connect all the diverse space physics models, the results presented here are addressed.

One of the coupling efforts undertaken by this project is the one between the nightside magnetospheric environment, especially in the tail region, and the ring current population. The former can be most successfully simulated using the MHD approach, while the latter focuses on the energy-dependent drift of particles. There have been recent work involving the coupling between models for these two populations with some success (De Zeeuw et al., 2004, Pembroke et al., 2012). Most of the plasma pressure in the inner magnetospheric region is sustained by the ring current, so the changes in its intensity and morphology have a great impact on the magnetic field topology in the region. One of the instances where these changes occur is during geomagnetic substorms which happen often and can affect the coupling between these two modeling concepts. In this study, the BATS-R-US MHD model was used to simulate the magnetosphere and the RAM-SCB model was used to simulate the ring current. Details about these models are presented in Section 2.

Substorms are usually associated with the transport and energization of particles from the plasmasheet region into the inner magnetosphere. A typical signature of a substorm is the rapid increase of particle and energy fluxes observed in the night sector by satellites at geosynchronous orbit (Nakamura et al., 2002). These increases happen for ions and electrons with energies of tens to hundreds of keV. This phenomenon happens concurrently with magnetic dipolarization events (Sergeev et al., 2009). These consist of a sudden change in the configuration of the tail magnetic field lines from a stretched one to a more dipolar one, which is measured in-situ as a fast increase in $B_Z$ within the central plasmasheet. This process is also described as producing flow bursts (Zhou et al., 2010) called “dipolarizing flux bundles” (Liu et al., 2014) (DBFs) or simply “bubbles” (Pontius and Wolf, 1990). The nature of these bursts is thought to be highly structured in terms of space and time (Sergeev et al., 2009). The occurrence of flow bursts is also associated with an increase in the cross-tail electric field and with the manifestation of the magnetic reconnection phenomena (Birn et al., 2004).

There have been many efforts of modeling substorm injections using a particle approach. These efforts involve modeling the electric and magnetic fields either using a prescribed but realistic electric pulse based on in situ data, usually during a specific event (Li et al., 1998, Sarris et al., 2002, Liu et al., 2009, Gabrielse et al., 2012), or using electric and magnetic fields from an MHD simulation (Birn et al., 1997, Birn et al., 2004, Birn et al., 2014). Efforts using the prescribed fields have been successful in reproducing the spatial and temporal characteristics of dispersionless injection results from in situ data using realistic transient fields and propagation velocities. Studies using MHD fields have revealed more detailed features of injections, such as the dominance of perpendicular vs. parallel anisotropies with respect to the distance down the tail and distance to the equatorial plane, specific energy flux spatial distributions and the identification of the boundary layer. However, these studies have not used real solar wind parameters and therefore do not make assertions about the temporal features of the injections.

MHD models are usually able to describe macroscopic consequences of reconnection, such as dipolarization and flow bursts. However, since reconnection in these models is typically initiated by ad-hoc anomalous resistivity or numerical resistivity, the onset, temporal evolution, location, and spatial properties of reconnection events need not be physical (Kuznetsova et al., 2007). Flows, associated with dipolarization events, can often reach the inner magnetosphere and ring current region and significantly affect the pressure balance in that region. It is therefore important to have a more accurate quantification of this process. Another limitation of the MHD model is that it does not provide details on the particle populations associated with flow bursts and dipolarization events. To overcome this limitation, test particle tracing techniques have been successfully employed to provide such additional information (Birn et al., 2015a, Birn et al., 2015b), particularly on the suprathermal populations of 10s–100s of keV.

The details of the particle tracing implementation used in the current study are presented in Section 2. The analysis of proton fluxes at the nightside inner magnetosphere during a period of intense substorm activity is presented in Section 3. The analysis is done along the night side geosynchronous orbit and at a specific satellite location over time. This approach is intended to be used as a way of coupling the tail magnetosphere simulations with ring current simulations. By having higher resolution flux data as a function of energy, space and time along the boundary of the ring current model it is possible to improve its accuracy and consequently improve the accuracy of the entire suite of coupled models.

In this study, we investigate the first step of the coupling between the MHD model and the ring current model done with the backwards particle tracing approach focusing on the proton flux response to substorm activity at the night side geosynchronous orbit. The fluxes are compared to in-situ measurements done by the LANL-GEO spacecraft during a period of four hours on 18 July 2013, which showed strong activity. In Section 2, we describe the models used and the methodology applied in this approach. In Section 3, we present observations of geomagnetic indices of the period of study, the results from the MHD simulation as well as the results from the particle tracing simulations. And finally in Section 4, we present our conclusions including a broader discussion of the results.