Modeling the Varying Location of Field Line Resonances During Geomagnetic Storms

Abstract

Previous observational studies have shown that the natural Alfvén frequencies of geomagnetic field lines vary significantly over the course of a geomagnetic storm, decreasing by up to 50% from their quiet time values outside the plasmasphere. This was recently demonstrated statistically using ground magnetometer observations across 132 geomagnetic storm events (Wharton et al., 2020). This then brings into question where field line resonances (FLRs) will form in storm‐time conditions relative to quiet times. With storm‐time radiation belt dynamics depending heavily upon wave‐particle interactions, understanding how FLR locations change over the course of a storm will have important implications for this area. Using 3D magnetohydrodynamic (MHD) simulations, we investigate how changes in the Alfvén frequency continuum of the Earth's dayside magnetosphere over the course of a geomagnetic storm affect the fast‐Alfvén wave coupling. By setting the model Alfvén frequencies consistent with the observations, and permitting a modest change in the plasmapause/magnetopause locations consistent with storm‐time behavior, we show that FLR locations can change substantially during storms. The combined effects of higher fast waveguide frequencies and lower Alfvén frequencies during storm main phases, act together to move the FLR locations radially inwards compared to quiet times. FLRs outside of the plasmasphere are moved radially inward by 1.7 Earth radii for the cases considered.

Polarization of Magnetospheric ULF Waves Excited by an Interplanetary Shock on 27 February 2014

“…(2018) illustrated how a magnetospheric plasma plume alters the spatial distribution of the amplitude and polarization of externally driven fast mode waves and FLRs. Related numerical studies emphasized that one needs to be careful in distinguishing between toroidal and poloidal waves when the magnetospheric mass density varies with local time (Elsden et al., 2022; Wright & Elsden, 2020; Wright et al., 2018).…”

On the Role of ULF Waves in the Spatial and Temporal Periodicity of Energetic Electron Precipitation

“…The subject ULF waves are typically generated at the magnetopause by solar wind transients and propagate toward the inner magnetosphere (e.g., O. V. Agapitov et al, 2009;Hartinger et al, 2013Hartinger et al, , 2014Hwang & Sibeck, 2016). Both the ULF waves (Di Matteo et al, 2022;Elsden et al, 2022;Klimushkin et al, 2019;Wright & Elsden, 2020) and the correlated whistler-mode waves (Zhang et al, 2020) have been observed to extend over a large L-shell and MLT range. However, the effi- ciency of electron precipitation by such ULF-driven whistler-mode waves (e.g., the precipitating electron energy range, flux magnitude and net contribution to the ionospheric energy input) cannot be reliably determined from equatorial spacecraft measurements alone.…”

On the role of ULF waves in the spatial and temporal periodicity of energetic electron precipitation

“…The subject ULF waves are typically generated at the magnetopause by solar wind transients and propagate towards the inner magnetosphere (e.g., O. V. Agapitov et al, 2009;Hartinger et al, 2013Hartinger et al, , 2014Hwang & Sibeck, 2016). Both the ULF waves (Klimushkin et al, 2019;Wright & Elsden, 2020;Elsden et al, 2022;Di Matteo et al, 2022) and the correlated whistler-mode waves (Zhang et al, 2020) have been observed to extend over a large Lshell and MLT range. However, the efficiency of electron precipitation by such ULF-driven whistler-mode waves (e.g., the precipitating electron energy range, flux magnitude and net contribution to the ionospheric energy input) cannot be reliably determined from equatorial spacecraft measurements alone.…”