Andris Vaivads

Electron acceleration signatures in the magnetotail associated with substorms

[1]xa0We present Cluster multisatellite observations of accelerated electrons in the near-Earth magnetotail associated with substorms. We found that the hardest electron energy spectra appear in the earliest stage of substorm expansion in the near-Earth tail region and that they gradually become softer during the events. Enhancement of the high-energy electron flux occurs generally associated with the bulk acceleration of ions (fast flow) and electrons. It is also shown that the high-energy electrons sometimes show preferential perpendicular acceleration associated with the temporal enhancement of the normal component of the magnetic field, and then the anisotropic distribution quickly becomes isotropic. During the dipolarization interval, in which no convection signature is observed, perpendicular flux drops to less than the initial value, and the parallel flux is more than the perpendicular flux. The results suggest that the electron acceleration mechanism is mostly consistent with adiabatic betatron acceleration, while Fermi acceleration is not clear in the high-energy part. The effect of the pitch angle scattering is also important. The dispersive signature of the high-energy electron flux indicates fast dawnward drift loss, namely, the three-dimensional effect of the limited plasma acceleration region.

Instability of Agyrotropic Electron Beams near the Electron Diffusion Region

During a magnetopause crossing the Magnetospheric Multiscale spacecraft encountered an electron diffusion region (EDR) of asymmetric reconnection. The EDR is characterized by agyrotropic beam and crescent electron distributions perpendicular to the magnetic field. Intense upper-hybrid (UH) waves are found at the boundary between the EDR and magnetosheath inflow region. The UH waves are generated by the agyrotropic electron beams. The UH waves are sufficiently large to contribute to electron diffusion and scattering, and are a potential source of radio emission near the EDR. These results provide observational evidence of wave-particle interactions at an EDR, and suggest that waves play an important role in determining the electron dynamics.

Generation of ion acoustic waves by fan instability.

In this paper we consider a fan instability as a possible source of oblique ion acoustic waves generated by ion beams above the auroral acceleration region. Our model includes a homogeneous plasma with protons as the only ion component. The ion distribution has a suprathermal tail along the ambient magnetic field. A beam with a positive derivative and a tail without a positive derivative in the ion distribution are considered. In the first case, the beam of the ion distribution generates electrostatic ion cyclotron waves and oblique ion acoustic waves. In the second case, the ion cyclotron waves are damped, but ion acoustic waves are still generated by the fan instability. The fan instability mechanism generates waves even when the positive derivative in the ion distribution has disappeared. The paper includes an analytical description and numerical calculations of the fan instability. Particle and wave data from the Viking satellite indicate that the fan instability is important above the auroral acceleration region.

The Cross‐Scale Mission

Collisionless space plasmas exhibit complex behavior on many scales. Fortunately, one can identify a small number of processes and phenomena, essentially shocks, reconnection and turbulence that play a predominant role in the dynamics of a plasma. These processes act to transfer energy between locations, scales and modes, a transfer characterized by variability and three‐dimensional structure on at least three scales: electron kinetic, ion kinetic and fluid scale. The nonlinear interaction between physical processes at these scales is the key to understanding these phenomena. Current and upcoming multi‐spacecraft missions such as Cluster, THEMIS, and MMS only study three‐dimensional variations on one scale at any given time, but one needs to measure the three scales simultaneously to understand the energy transfer processes and the coupling and interaction between the different scales. A mission called Cross‐Scale would comprise three nested groups, each consisting of up to four spacecraft. Each group wou...