Stormtime Energetics: Energy Transport Across the Magnetopause in a Global MHD Simulation

Abstract

Coupling between the solar wind and magnetosphere can be expressed in terms of energy transfer through the separating boundary known as the magnetopause. Geospace simulation is performed using the Space Weather Modeling Framework (SWMF) of a multi-ICME impact event on February 18–20, 2014 in order to study the energy transfer through the magnetopause during storm conditions. The magnetopause boundary is identified using a modified plasma β and fully closed field line criteria to a downstream distance of −20Re. Observations from Geotail, Themis, and Cluster are used as well as the Shue 1998 model to verify the simulation field data results and magnetopause boundary location. Once the boundary is identified, energy transfer is calculated in terms of total energy flux K, Poynting flux S, and hydrodynamic flux H. Surface motion effects are considered and the regional distribution of energy transfer on the magnetopause surface is explored in terms of dayside X>0, flank X<0, and tail cross section X=Xmin regions. It is found that total integrated energy flux over the boundary is nearly balanced between injection and escape, and flank contributions dominate the Poynting flux injection. Poynting flux dominates net energy input, while hydrodynamic flux dominates energy output. Surface fluctuations contribute significantly to net energy transfer and comparison with the Shue model reveals varying levels of cylindrical asymmetry in the magnetopause flank throughout the event. Finally existing energy coupling proxies such as the Akasofu ϵ parameter and Newell coupling function are compared with the energy transfer results.

A global view of Pc3 wave activity in near-Earth space: Results from hybrid-Vlasov simulations

“…The large magnetic flux pile up in Run OBL_B10, due to the large IMF strength, caused β* to drop to rather low values even in the magnetosheath, which prevented us from selecting a higher threshold. In other runs, the magnetopause position is relatively insensitive to the exact β* value selected within the range proposed by Brenner et al (2021). The goodness of the magnetopause detection method is further demonstrated in the Results section, in particular in Figure 7.…”

“…The magnetopause is then defined as an isocontour of β*, as this parameter displays a clear gradient at the transition between magnetosphere and magnetosheath. Brenner et al (2021) use β* = 0.7 and indicate that values within the range 0.1-1.5 yield similar results in their global MHD run. This method also has the advantage of being computationally inexpensive, compared to methods based on field line or streamline tracing.…”

“…The equatorial 2D setup also renders the identification of the magnetopause position challenging. Here we used a contour of the β* parameter (Brenner et al, 2021) which provided an identification method that could be applied in an identical manner to all runs to facilitate their comparison. We find however that the magnetopause stand-off distances identified with the selected β* value are much lower than typically observed at Earth.…”

“…More recently, Brenner et al (2021) suggested a new identification method employing the β p parameter, a modified plasma β including the contribution of the dynamic pressure P dyn , defined as:…”

“…For our Vlasiator runs, we calculated the average β* over the 150 s interval under study, < β* > 150 s , in order to get an average magnetopause position and remove local irregularities of the magnetopause surface, likely caused by foreshock processes, especially in those runs with radial IMF. Based on the visual inspection of the < β* > 150 s profiles, we selected a threshold towards the lower end of the range in Brenner et al (2021), < β* > 150 s 0.28, which gives a good approximation of the magnetopause position in all runs under study. The large magnetic flux pile up in Run OBL_B10, due to the large IMF strength, caused β* to drop to rather low values even in the magnetosheath, which prevented us from selecting a higher threshold.…”