Determining Dayside Cold Plasma Dynamics during Geomagnetic Storms with ULF waves

Magnetic field eigenfrequencies determine where in the Earth's magnetosphere and radiation belts ULF wave power will be distributed. Recent work has shown that the eigenfrequencies of magnetic field lines decrease during geomagnetic storms. This decrease can be caused by the magnetic field weakening in magnitude or the plasma mass density distribution increasing. Currently, studies investigating the plasma mass density changes have produced conflicting results about whether it increases or decreases.

In this work, we have applied the cross-phase technique to magnetometer data during 132 geomagnetic storms and then used a superposed epoch analysis to understand the average variation in the eigenfrequencies. In conjunction with a Tsyganenko magnetic field model, we have then solved the MHD wave equation to calculate the plasma mass density change for the average storm. This has been repeated for six latitudes.

This work shows that the eigenfrequencies decrease for L > 4 by as much as 50%, while they increase for L < 4. The corresponding plasma mass density at the equator is shown to increase for L > 4 by more than a factor of 2, while it decreases for L < 4. At L > 4, we suggest that geomagnetic storms increase the plasma mass density through the enhanced ring current, while the decrease below L = 4 is due to plasmapause erosion. These results show that ULF waves can be used to understand magnetospheric behaviour during extreme space weather events.