Stormtime Ultra-low-frequency (ULF) Waves in the Inner Magnetosphere: Mode Structure and Particle Environment

Various plasma waves are excited in near-Earth space during geomagnetic storms, and the waves affect the behavior of particles carrying the ring current and the strength of the flux of energetic electrons in the radiation belts. Ultra-low-frequency (ULF) waves with frequencies below 100 mHz play an essential role in this regard, and understanding the spatial and temporal occurrence patterns and the excitation mechanism of the waves is a crucial element in magnetospheric research.

The investigation focuses on a recently reported new type of compressional ULF waves in the inner magnetosphere with a symmetric (fundamental) mode structure about the magnetic equator. Antisymmetric (second harmonic) waves are commonly detected and well-documented in the literature. The team aims to determine the relevance of drift compressional instability (DCI) to stormtime magnetospheric ULF waves.

The investigation has broader impacts regarding wave-particle interactions occurring on other planets and in laboratory plasmas. In addition, the investigation will contribute to building improved space weather models that are valuable beyond the context of basic research.

The goal of the investigation is to observationally determine whether the physical properties of symmetric ULF waves and the particle environment associated with the symmetric waves are consistent with DCI. To this end, we will use data from the Van Allen Probes (RBSP), which include electric and magnetic fields, plasma, and energetic particles. With the RBSP mission providing measurements in the inner magnetosphere ( < 6) over a 7-year period (2012–2019), the data are ideal for determining the mode structure of ULF waves, their spatial and temporal distributions, and the properties of the background particles.

We will determine the mode structure by paying attention to the relation between the spacecraft latitude and the location of the equatorial node/antinode of ULF waves. The relevance of DCI to observed symmetric waves will be discussed by evaluating the theoretical instability condition using observationally determined energy and radial distance dependence of the phase space density of ions in the ring current energy range (1-300 keV).

We will discuss the statistically determined spatial occurrence pattern of the waves in relation to the drift of ions injected from a nightside source region.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.