Observational Verification of the Electron Vlasov Equation with MMS
Presentation Type
Talk
Presenter Format
Virtual Meeting Talk
Topic
Fundamental Processes in Comparative Magnetospheres
Start Date
13-5-2022 2:45 PM
Abstract
The Fast Plasma Investigation (FPI) onboard NASA’s Magnetospheric Multiscale (MMS) four-spacecraft mission constitutes an historic advance in plasma physics particle detection capabilities. As a result of the unprecedented temporal, spatial, and velocity-space resolution offered by the suite of dual electron spectrometers (DES), MMS is capable of directly measuring the velocity-space structure of terms in the electron Vlasov equation as demonstrated by Shuster et al. [2021] for electron-scale current layers observed near Earth’s dayside reconnecting magnetopause. In this work, we present MMS observations of all three terms in the electron Vlasov equation: the temporal derivative term ∂fe/∂t, the spatial gradient term v⋅∇fe, and the velocity-space gradient term (F/me)⋅∇vfe. We discuss various applications, implications, and limitations of our methodology. The immediate advantage of multipoint measurements is the ability to distinguish between temporal and spatial variations in the plasma; both a thin, slow-moving structure and a thick, fast-moving one would produce the same measured time series of a quantity when sampled at only a single spatial location. FPI offers a novel way to infer spatiotemporal variations of the plasma from a single spacecraft: careful measurement of how the shape of the plasma’s distribution function fe(v) changes via the velocity-space gradient term ∇vfe yields qualitatively similar velocity-space structures as those captured by ∂fe/∂t and the four-spacecraft measurement of v⋅∇fe. Making use of ∂fe/∂t for steady-state structures, we obtain a higher spatially-resolved (~1 to 3 km) single-spacecraft version of the parallel component of the electron pressure divergence (∇⋅Pe)// that exhibits agreement with the parallel electric field measured by the electric field double probes (EDP), whereas the standard four-spacecraft method for computing ∇⋅Pe cannot capture spatial variations smaller than the spacecraft separation (~10 km). Just as v⋅∇fe offers a kinetic perspective into the origins of ∇⋅Pe, the (F/me)⋅∇vfe term offers an analogous velocity-space perspective into the energy conversion term J⋅E’ whenever J can be approximated by Je = −eneUe, which is often the case for electron-scale structures. Thus, these results are immediately relevant to the study of fundamentally kinetic energy conversion processes, including electron diffusion regions (EDRs) fueling magnetic reconnection, kinetic-scale turbulence, and wave-particle interactions such as those believed to be consistent with Landau damping as inferred from recently reported velocity-space signatures using MMS data from the turbulent magnetosheath.
Observational Verification of the Electron Vlasov Equation with MMS
The Fast Plasma Investigation (FPI) onboard NASA’s Magnetospheric Multiscale (MMS) four-spacecraft mission constitutes an historic advance in plasma physics particle detection capabilities. As a result of the unprecedented temporal, spatial, and velocity-space resolution offered by the suite of dual electron spectrometers (DES), MMS is capable of directly measuring the velocity-space structure of terms in the electron Vlasov equation as demonstrated by Shuster et al. [2021] for electron-scale current layers observed near Earth’s dayside reconnecting magnetopause. In this work, we present MMS observations of all three terms in the electron Vlasov equation: the temporal derivative term ∂fe/∂t, the spatial gradient term v⋅∇fe, and the velocity-space gradient term (F/me)⋅∇vfe. We discuss various applications, implications, and limitations of our methodology. The immediate advantage of multipoint measurements is the ability to distinguish between temporal and spatial variations in the plasma; both a thin, slow-moving structure and a thick, fast-moving one would produce the same measured time series of a quantity when sampled at only a single spatial location. FPI offers a novel way to infer spatiotemporal variations of the plasma from a single spacecraft: careful measurement of how the shape of the plasma’s distribution function fe(v) changes via the velocity-space gradient term ∇vfe yields qualitatively similar velocity-space structures as those captured by ∂fe/∂t and the four-spacecraft measurement of v⋅∇fe. Making use of ∂fe/∂t for steady-state structures, we obtain a higher spatially-resolved (~1 to 3 km) single-spacecraft version of the parallel component of the electron pressure divergence (∇⋅Pe)// that exhibits agreement with the parallel electric field measured by the electric field double probes (EDP), whereas the standard four-spacecraft method for computing ∇⋅Pe cannot capture spatial variations smaller than the spacecraft separation (~10 km). Just as v⋅∇fe offers a kinetic perspective into the origins of ∇⋅Pe, the (F/me)⋅∇vfe term offers an analogous velocity-space perspective into the energy conversion term J⋅E’ whenever J can be approximated by Je = −eneUe, which is often the case for electron-scale structures. Thus, these results are immediately relevant to the study of fundamentally kinetic energy conversion processes, including electron diffusion regions (EDRs) fueling magnetic reconnection, kinetic-scale turbulence, and wave-particle interactions such as those believed to be consistent with Landau damping as inferred from recently reported velocity-space signatures using MMS data from the turbulent magnetosheath.
