Theory and Modelling of Large Scale Plasmapause Surface Waves

Tom Elsden (University of St. Andrews)

Wednesday 3rd December 15:00-16:00
Maths 311B

Abstract

This talk will be in the realm of MHD waves in Earth's magnetosphere. MHD waves manifest in the magnetosphere as so called ultra-low frequency (ULF) waves, with periods in the range from a few seconds to a few minutes. They represent large scale plasma/magnetic field oscillations, often on the scale size of the entire magnetosphere, or in the case of Alfven waves, the length of the Earth's dipolar field lines. The oscillations of the field lines can make it to the ground (through the ionosphere), and have been measured for about ~150 years as close to sinusoidal wave forms by ground magnetometers. Over many decades, a picture has been formed of how MHD waves are driven and propagate through the inhomogeneous and constantly evolving magnetospheric medium, however, the 3D non-uniformity still makes for many outstanding problems today. ULF waves actively contribute to the topic of space weather, in particular by their ability to accelerate particles (through wave-particle interactions) to very high energies, helping to form the radiation belts (also known as the Van Allen belts) around the Earth. Hence, there is extremely active interest in understanding the spatial and temporal structure of ULF waves.

 
Specifically, this talk will focus on some new theory as to how an MHD surface wave can form on a sharp density gradient in the magnetosphere known as the plasmapause. The plasmapause is the 'boundary' between plasma which co-rotates with the Earth, and the less dense plasma further out, whose movement is driven by large scale magnetic reconnection on the day and night side of the Earth (known as the Dungey cycle). We show analytically how such a surface wave may exist on this density gradient in a simplified MHD model, and then test this analytical theory with MHD simulations. The results show that such a large scale surface wave should be able to exist on the plasmapause, and has potential implications for the driving of radiation belt particles in this region of the magnetosphere.
 
This is based on the following recently published paper in JGR Space Physics: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025JA033830 

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