Chorus (whistler mode) and electron cyclotron harmonic (ECH) waves play the most important role in hot electron acceleration and scattering to the ionosphere. These in turn dictate the electron seed…

This is a $601,257.94 grant award from the National Aeronautics and Space Administration (NASA) to the University of California, Los Angeles (UCLA) through its Office of Research Administration division. The grant supports research to understand the physical mechanisms driving modulations of electromagnetic waves, known as chorus and electron cyclotron harmonic (ECH) waves, in the Earth's magnetosphere and their impact on magnetosphere-ionosphere coupling. The research aims to analyze electron distributions and wave characteristics using data from NASA's THEMIS and Van Allen Probes missions to resolve the underlying mechanisms behind observed wave modulations. This work is directly relevant to NASA's Heliophysics Decadal Survey goals of understanding the dynamics and coupling of the Earth's magnetosphere, ionosphere, and atmosphere. The contract has no set-aside designation and is being performed at UCLA's facilities in Los Angeles.

Chorus (whistler mode) and electron cyclotron harmonic (ECH) waves play the most important role in hot electron acceleration and scattering to the ionosphere. These in turn dictate the electron seed population in the outer radiation belt and alter the ionospheric conductivity spatial profile and thus affect global magnetospheric convection, contributing to magnetosphere-ionosphere coupling. Although both acceleration and scattering are local processes, a good correlation between electron fluxes and wave intensities measured at different locations has often been captured, even at significantly separated spacecraft. A noteworthy example is the pulsating aurorae (quasi-periodic electron precipitation) over a large spatial domain. The likely candidate for connecting these processes at disparate site is large-scale, ultra-low frequency (ULF) waves, which can control other plasma waves effectively (turn-on/ off and modulate their amplitudes). Understanding this ULF control mechanism of whistler and ECH waves is thus critical to modeling the electron dynamics in the earth's magnetosphere. Despite the large number of chorus modulation events being reported, the fundamental physical mechanism responsible for the wave modulation remains poorly understood. Moreover, previous studies investigated plasma wave modulation at a single location rather than explore a common non-local, ULF wave driver. In this project, we explore events with simultaneous modulation of chorus and ECH waves (correlation or anti-correlation between chorus and ECH). To explain and model wave modulation, we will concentrate on properties of the electron distribution modulated by ULF waves and effects of this modulation on whistler/ECH wave generation and damping. Because observed electron distributions accompanying in-situ waves are often in the marginal stable state of interacting with chorus/ECH, during such modulation events we can directly investigate the source distribution responsible for the wave generation. This is especially important for systems where the modulation of a background, stable electron population is the only way to drive whistler/ECH waves and provide further electron acceleration and scattering. Using the large database from themis, van allen probes measurements, existing hot plasma dispersion solvers, and our experience in modeling electron distributions modified by ULF waves, we will systematically evaluate the characteristics of electron distributions during the course of ULF wave perturbations and the parameters controlling the generation/damping of ECH and chorus waves to explain the observations. Our main objective is to understand the physical mechanism for ECH and chorus wave modulations by ULF pulsations. Towards that goal we will: 1. Resolve the mechanism of ECH and chorus wave modulation 2. Explore the occurrence rate of the modulation events with both ECH and chorus waves and those with only ECH waves 3. Determine the critical parameters controlling the ECH and chorus waves generation/damping by modulated electron population to answer these questions, we will: (1) analyze and model electron distributions and wave characteristics collected by themis spacecraft and van allen probes for selected events; (2) examine the distribution characteristics of chorus/ECH modulation events (multi-event basis for correlation events; statistics for anti-correlation events and ECH-only modulations); and (3) perform a parametric study on collected statistics of wave characteristics using existing numerical models. Our proposal is directly related to the heliophysics decadal survey goal 2: "determine the dynamics and coupling of earth's magnetosphere, ionosphere, and atmosphere and their response to solar and terrestrial inputs", and goal 4: "discover and characterize fundamental processes that occur both within the heliosphere and throughout the universe".