Temporal, spatial, and velocity‐space variations of electron phase space density are measured observationally and compared for the first time using the four magnetospheric multiscale (MMS) spacecraft ...at Earth's magnetopause. Equipped with these unprecedented spatiotemporal measurements offered by the MMS tetrahedron, we compute each term of the electron Vlasov equation that governs the evolution of collisionless plasmas found throughout the universe. We demonstrate how to use single spacecraft measurements to improve the resolution of the electron pressure gradient that supports nonideal parallel electric fields, and we develop a model to intuit the types of kinetic velocity‐space signatures that are observed in the Vlasov equation terms. Furthermore, we discuss how the gradient in velocity‐space sheds light on plasma energy conversion mechanisms and wave‐particle interactions that occur in fundamental physical processes such as magnetic reconnection and turbulence.
Plain Language Summary
Measuring spatial and temporal variations of space plasmas usually requires choosing between the following two approaches: (a) measure how the quantity of interest changes in time as the plasma flows past a single spacecraft, or (b) compare measurements of the quantity gathered from multiple, spatially separated spacecraft. The first approach requires measurements at two different times from the same spatial location, while the second requires simultaneous measurements taken from multiple spatial points. There are advantages and disadvantages to each of these existing approaches. While single‐spacecraft measurements may be gathered at high time resolution, a known limitation of single‐point data sets is the inability to distinguish between spatial and temporal variations: both a thin, slow‐moving structure and a thick, fast‐moving one could produce the same measured time series of a quantity when sampled at only a single spatial location. In many situations, multipoint measurements, such as those provided by NASA's Magnetospheric Multiscale (MMS) four‐spacecraft mission enable us to overcome that limitation; however, oftentimes electron‐scale structures of interest are even smaller than the close inter‐spacecraft separation of MMS, which means typical techniques for estimating spatial gradients from the four spacecraft also become inaccurate for those events. In this paper, we present a new approach for quantifying variations in a collisionless plasma that only requires information about the plasma particles and fields taken at a single point in time and space.
Key Points
Temporal, spatial, and velocity‐space derivatives of electron phase space density are computed and compared for the first time using the Magnetospheric Multiscale spacecraft
Single‐spacecraft measurements show how parallel electric fields are balanced by electron pressure gradients at Earth's magnetopause
A simplified form of the electron distribution function provides intuition for interpreting nongyrotropic velocity‐space structures
We will present results of 2 1/2‐dimensional particle‐in‐cell simulations where the impact of ion cyclotron waves on auroral kilometric radiation (AKR) has been studied. A horseshoe electron ...distribution function in a strong magnetic field is unstable to modes with frequencies slightly below the electron cyclotron frequency and can generate AKR. In the presence of an electron beam superimposed on the horseshoe distribution, the system is unstable to ion cyclotron waves with phase velocity parallel to the magnetic field equal to the electron beam velocity. Ion cyclotron waves are often observed near the source region of AKR and have been predicted to have an influence on AKR. We have found that the parallel electric field in the ion cyclotron wave modulates the energy density of the electrons at the ion cyclotron frequency. Forced oscillation of the parallel electric field with the ion cyclotron frequency has been added in the simulations to see the influence on AKR. The growth rate of AKR decreases significantly when the ion cyclotron wave has the parallel phase velocity in the same direction as the electron beam. We have found that the intensity of the AKR as well as the electron energy density is also modulated by the ion cyclotron wave. We will discuss how the modulation of the electron energy affects the growth of AKR and how this modulation may generate negative frequency drifting fine structure of AKR.
Lower-hybrid-drift waves driving vortical flows have recently been discovered in the electron current layer during magnetic reconnection in the terrestrial magnetotail. Yet, spacecraft measurements ...cannot address how pervasive the waves are. In this work, we perform three-dimensional particle-in-cell simulations of guide field reconnection to demonstrate that electron vortices driven by the lower-hybrid-drift instability (LHDI) are excited immediately downstream from the electron jet reversal in 3-D channels of enhanced electron outflow. The resulting fluctuations generate a series of alternating vortices, producing magnetic field perturbations opposing and enhancing the local guide field and causing kinking of the enhanced electron outflow and patches of increased current. Our results demonstrate for the first time that LHDI exists in the electron current layer and enhanced outflow channels, providing a conceptual breakthrough on the LHDI in reconnection.
Lower-hybrid-drift waves driving vortical flows have recently been discovered in the electron current layer during magnetic reconnection in the terrestrial magnetotail. Yet, spacecraft measurements ...cannot address how pervasive the waves are. In this work, we perform three-dimensional particle-in-cell simulations of guide field reconnection to demonstrate that electron vortices driven by the lower-hybrid-drift instability (LHDI) are excited immediately downstream from the electron jet reversal in 3-D channels of enhanced electron outflow. The resulting fluctuations generate a series of alternating vortices, producing magnetic field perturbations opposing and enhancing the local guide field and causing kinking of the enhanced electron outflow and patches of increased current. Our results demonstrate for the first time that LHDI exists in the electron current layer and enhanced outflow channels, providing a conceptual breakthrough on the LHDI in reconnection.
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