Diagnosis of ULF Wave‐Particle Interactions With Megaelectron Volt Electrons: The Importance of Ultrahigh‐Resolution Energy Channels

Abstract

Electron flux measurements are an important diagnostic for interactions between ultralow-frequency (ULF) waves and relativistic (∼1 MeV) electrons. Since measurements are collected by particle detectors with finite energy channel width, they are affected by a phase mixing process that can obscure these interactions. We demonstrate that ultrahigh-resolution electron measurements from the Magnetic Electron Ion Spectrometer on the Van Allen Probes mission-obtained using a data product that improves the energy resolution by roughly an order of magnitude-are crucial for understanding ULF wave-particle interactions. In particular, the ultrahigh-resolution measurements reveal a range of complex dynamics that cannot be resolved by standard measurements. Furthermore, the standard measurements provide estimates for the ULF flux modulation amplitude, period, and phase that may not be representative of true flux modulations, potentially leading to ambiguous conclusions concerning electron dynamics. Plain Language Summary Plasma waves with periods of a few minutes interact with high-energyelectrons in the near-Earth space environment. Understanding how the waves affect the electrons is important, as these electrons can potentially damage orbiting satellites. However, these interactions are obscured by a process known as phase mixing where electrons with different energies behave differently yet are mixed together in measurements that have finite energy resolution. We show how a unique data set obtained from National Aeronautics and Space Administration's Van Allen Probes mission can dramatically increase the energy resolution, revealing a range of interactions that are obscured by measurements with lower energy resolution.

Alteration of Particle Drift Resonance Dynamics Near Poloidal Mode Field Line Resonance Structures

“…While such a signature should be observable in existing openly available electron flux data sets from the Van Allan Probes mission, comparing the leftand right-hand columns of Figure 7 makes clear the potential benefits of higher-resolution electron flux observations. Such ultrahigh-resolution observations from the magEIS and REPT instruments onboard the RBSP satellites have recently been reported in Hartinger et al (2018) and have revealed complex structures in the modulated electron flux following a southward-Kivelson type drift-resonance signature. Arguably, some features of these structures bare a resemblance to those shown in Figure 7g; however, these were taken during an event in which drift resonance with a toroidal mode Pc5 wave occurred, for which the ULF wave profile with L-shell is distinctly different than the poloidal mode.…”

“…This gives rise to a specific signature in satellite observations characterized by modulations in particle flux at the wave frequency, with a 180 • phase shift across the resonant energy (for which the unperturbed drift speed matches the azimuthal phase speed of the wave; Southwood & Kivelson, 1981). In cases of both electrons and ions, such signatures have been found in a number of event studies Dai et al, 2013;Hao et al, 2017;Hartinger et al, 2018;Mann et al, 2013;Zong et al, 2009Zong et al, , 2012. Further refinements to the theory and modeling incorporate the effects of day/night asymmetry, azimuthal localization and spectral bandwidth of the wave amplitude, and nonlinear effects on particle dynamics Degeling et al, 2007Degeling et al, , 2011Li et al, 2017Li et al, , 2018Zhou et al, 2016).…”

Simulations of Electron Flux Oscillations as Observed by MagEIS in Response to Broadband ULF Waves

“…In the following we explore similar flux oscillation amplitudes as observed through a novel MagEIS data product: ultrahigh energy resolution electron measurements. Claudepierre et al (2017) and Hartinger et al (2018) have recently demonstrated a technique to obtain ultrahigh energy resolution electron measurements, which are derived from the histogram data used for background removal of the main rate channels. It is a nonstandard data product that is possible due to the designed functionality of the MagEIS instruments.…”

“…The technique is based on the fact that, whereas the standard MagEIS energy channels are obtained from the full pulse-height spectrum from each MagEIS detector or pixel over a narrow pulse-height range centered on the pixel response (see Claudepierre et al, 2017), a subsampling of the full pulse-height spectrum is also retained, via onboard look-up tables, to produce the MagEIS histogram data product; these data can be used to obtain ultrahigh energy resolution fluxes when count rates are large enough. In the study of Hartinger et al (2018), these ultra-high-resolution measurements have revealed a range of complex dynamics that cannot be resolved by standard measurements. Furthermore, as Hartinger et al (2018) demonstrated using the histogram channels, the flux modulation amplitudes as observed using the standard-resolution MagEIS energy channels appear lower than the ultrahigh resolution channels, and can differ by as much as a factor of 2.…”

“…In the study of Hartinger et al (2018), these ultra-high-resolution measurements have revealed a range of complex dynamics that cannot be resolved by standard measurements. Furthermore, as Hartinger et al (2018) demonstrated using the histogram channels, the flux modulation amplitudes as observed using the standard-resolution MagEIS energy channels appear lower than the ultrahigh resolution channels, and can differ by as much as a factor of 2. In the following we explore the different flux oscillation amplitudes as they are observed via the ultrahigh resolution and the standard-resolution MagEIS data.…”