By Tom Elsden (University of Glasgow)
Field line resonances (FLRs) are the manifestation of a magnetohydrodynamic (MHD) wave coupling process where energy is transferred from a global to local field-aligned wave. The ‘resonance’ comes from a frequency matching between these waves and being a resonant process, can result in a significant accumulation of energy on a given field line. These waves play an important role in magnetospheric wave-particle interactions, the generation of aurora and can further be used as a seismological tool to remote sense the magnetosphere from ground-based observations.
The location where FLRs occur is intrinsically linked to the current state of the magnetosphere, with the magnetic field structure, plasma density and solar wind driving conditions all being key factors. Given the drastic effect of a geomagnetic storm on the morphology of the magnetosphere, we considered how such changes impact where FLRs form between storm and non-storm times.
We used ground magnetometer data to determine how the fundamental Alfven frequency of field lines varies over the course of a storm on average. These frequencies were then used to infer a plasma density profile to be used in a numerical MHD model to investigate where the FLRs would form under broadband solar wind driving conditions.
Figure 1 shows results from these simulations, displaying the field-aligned current density as an indication of FLRs, mapped from the ionosphere to the equatorial plane to display the radial structure. The left panel is from a simulation modelling the initial phase of a storm, with the right panel modelling the main phase. We show that for an average storm, the FLR moves radially inward by ~1.7RE (compare vertical line locations). This is caused by a decrease in the fundamental Alfven frequency as well as an increase in the global (fast) wave frequency which drives the FLRs.
The important aspect of the results is the overall trend of more Earthward FLR formation during storms. Particularly if extrapolated to more severe storms, this could have an impact on storm-time wave-particle interactions in the radiation belts.
Figure 1 Caption: Colour contours of field-aligned current density from near the ionospheric end of field lines, mapped to the equatorial plane. Left: simulation modelling storm initial phase. Right: simulation modelling storm main phase. Vertical lines indicate field line resonance locations.
Please see paper for full details: Elsden, T., Yeoman, T. K., Wharton, S. J., Rae, I. J., Sandhu, J. K., Walach, M.-T., et al. (2022). Modeling the varying location of field line resonances during geomagnetic storms. Journal of Geophysical Research: Space Physics, 127, e2021JA029804. https://doi.org/10.1029/2021JA029804