Electron jet of asymmetric reconnection

Abstract

We present Magnetospheric Multiscale observations of an electron‐scale current sheet and electron outflow jet for asymmetric reconnection with guide field at the subsolar magnetopause. The electron jet observed within the reconnection region has an electron Mach number of 0.35 and is associated with electron agyrotropy. The jet is unstable to an electrostatic instability which generates intense waves with E∥ amplitudes reaching up to 300 mV m−1 and potentials up to 20% of the electron thermal energy. We see evidence of interaction between the waves and the electron beam, leading to quick thermalization of the beam and stabilization of the instability. The wave phase speed is comparable to the ion thermal speed, suggesting that the instability is of Buneman type, and therefore introduces electron‐ion drag and leads to braking of the electron flow. Our observations demonstrate that electrostatic turbulence plays an important role in the electron‐scale physics of asymmetric reconnection.

Lower hybrid waves in the ion diffusion and magnetospheric inflow regions

“…The furthest point reached by the magnetosheath ions into the magnetosphere is indicated by the blue dashed line in Figure , and termed the ion edge. Figure d shows that these ions have pitch angles θ centered around 90°, consistent with the spacecraft crossing the magnetopause close to the X line [ Khotyaintsev et al , ]. Further from the X line field‐aligned ions are expected to be seen first by the spacecraft [ Khotyaintsev et al , ].…”

“…To calculate the phase velocity v ph of the lower hybrid waves, we fit the potential calculated by integrating δ E (for f > 10 Hz), . The phase speed v ph and propagation direction are found by finding the best fit of ϕ E to ϕ B , where v ph is a free parameter [ Norgren et al , ; Graham et al , ; Khotyaintsev et al , ; Innocenti et al , ]. Figures f–i show ϕ B and the best fit of ϕ E to ϕ B for the lower hybrid waves observed by each spacecraft in the diffusion region and near the ion edge.…”

Evidence of Magnetic Nulls in Electron Diffusion Region

“…This means that most of the reconnection events (10 events) can be described using the two-dimensional model, although they are essentially three-dimensional. Such back-and-forth motion could be caused by time-varying solar wind conditions, large-amplitude magnetic waves, outflow obstructions, etc., but not caused by lower-hybrid waves (Ergun et al, 2017;Khotyaintsev et al, 2016), which exhibit fluctuations only in the electric field components. Here ξ < 40% is an empirical threshold given by Fu et al (2015) for quantifying the accuracy of FOTE.…”

Crescent‐Shaped Electron Distributions at the Nonreconnecting Magnetopause: Magnetospheric Multiscale Observations

“…These gyrotropic distributions may not survive either when a plasma/magnetic field boundary has a width comparable to the particle's gyroradius ( = mv ⟂ qB ) due to finite gyroradius effect or when the particles become decoupled from the magnetic field. However, the recently launched Magnetospheric Multiscale (MMS) mission , measuring charged particle orders of magnitude faster, provides a good opportunity to explore electron-scale physics (e.g., Chen et al, 2016;Lavraud et al, 2016;Phan et al, 2018;Khotyaintsev et al, 2016). On the other hand, due to electron's small mass, such agyrotropic distributions of electrons are difficult to investigate in space because in situ measurements RESEARCH LETTER 10.1029 in previous space missions are too slow to electron scales.…”