Do Statistical Models Capture the Dynamics of the Magnetopause During Sudden Magnetospheric Compressions?

Abstract

Under periods of strong solar wind driving, the magnetopause can become compressed, playing a significant role in draining electrons from the outer radiation belt. Also termed "magnetopause shadowing," this loss process has traditionally been attributed to a combination of magnetospheric compression and outward radial diffusion of electrons. However, the drift paths of relativistic electrons and the location of the magnetopause are usually calculated from statistical models and, as such, may not represent the time-varying nature of this highly dynamic process. In this study, we construct a database ∼20,000 spacecraft crossings of the dayside magnetopause to quantify the accuracy of the commonly used Shue et al. (1998, https://doi.org/10.1029/98JA01103) model. We find that, for the majority of events (74%), the magnetopause model can be used to estimate magnetopause location to within ±1 R E . However, if the magnetopause is compressed below 8 R E , the observed magnetopause is greater than 1 R E inside of the model location on average. The observed magnetopause is also significantly displaced from the model location during storm sudden commencements, when measurements are on average 6% closer to the radiation belts, with a maximum of 42%. We find that the magnetopause is rarely close enough to the outer radiation belt to cause direct magnetopause shadowing, and hence rapid outward radial transport of electrons is also required. We conclude that statistical magnetopause parameterizations may not be appropriate during dynamic compressions. We suggest that statistical models should only be used during quiescent solar wind conditions and supplemented by magnetopause observations wherever possible.

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

“…In Figure 5, times when the spacecraft locator (Staples et al, 2020) predicted magnetopause crossings are marked with red vertical lines (Figure 2). The local B magnitude average near the identified magnetopause crossings gives an indication that the magnetopause location is well reproduced with the simulation.…”

Interplanetary Shock‐Induced Magnetopause Motion: Comparison Between Theory and Global Magnetohydrodynamic Simulations

“…These empirical studies revealed additional influences from the Interplanetary Magnetic Field (IMF) orientation, which modulates magnetic reconnection and the Dungey (1961) cycle, solar wind magnetic pressure and dipole tilt (Lin et al., 2010), IMF cone angle (Merka et al., 2003), and ionospheric conductivity and solar wind velocity (Němeček et al., 2016). These best‐fit models are, however, static and can deviate when compared to specific observations (Samsonov et al., 2020), particularly during extreme solar wind conditions with discrepancies of 1 observed when located less than 8 upstream (Staples et al., 2020).…”

Constraining the Location of the Outer Boundary of Earth's Outer Radiation Belt

“…The dawn-dusk asymmetries observed might be explained by similar dawn-dusk asymmetries in the magnetosphere (Haaland et al, 2017;Staples et al, 2020;Walsh et al, 2014). As we have excluded the lower energy particles from this portion of analysis, we do not expect this asymmetry to be primarily due to E × B drift, since the curvature and gradient drifts are energy dependent and hence will dominate over the electric field drift (though some recent works have shown that the electric field may still contribute: Sillanpää et al, 2017;Califf et al, 2017).…”

“…This causes the electrons to follow the field and drift further out because of the gradient drift experienced. The sense of the dawn-dusk asymmetry suggests it is not simply the result of the algorithm identifying the magnetopause rather than the OBORB, the magnetopause can be compressed to below 8 R E , but this happens much more frequently at dawn than dusk (Staples et al, 2020). Whilst there may be some contamination of the data due to sampling the magnetopause or solar wind, we infer that this is negligible, since electron populations (and hence their distribution functions) are very different.…”