Wave Activity in a Dynamically Evolving Reconnection Separatrix

Abstract

Magnetic reconnection's overall structure consists of a region of slow advective inflow and rapid outflow energized by the release of magnetic tension. Inflowing electrons are initially frozen-in to the magnetic field, confined to a narrow flux tube which expands into the electron diffusion region (EDR), causing acceleration of field-aligned electron beams. The nature of the parallel electric field required to accelerate inflowing electrons remains unclear, and its study is made difficult by the strong waves frequently observed at the separatrix which divides the inflow and outflow. In this region, a combination of magnetic shear, pressure gradient, and counterstreaming plasma flows give rise to numerous plasma instabilities.In the most straightforward case, electron gyro-radii of inflow and outflow populations are expected to overlap at the separatrix, leading to a narrow region unstable to counterstreaming modes, growing waves and electron phase-space holes which are frequently seen in simulations and in situ observations by spacecraft

Lower‐Hybrid Wave Structures and Interactions With Electrons Observed in Magnetotail Reconnection Diffusion Regions

“…In the zero guide field simulation, the long‐wavelength mode wave develops at the X‐line, where electrons perform the demagnetized meandering motion not following the E × B drift (Wang, Chen, Ng, et al., 2021). In observations, deviations between the electron bulk velocity and the $$\boldsymbol{E}\times \boldsymbol{B}$$ drift are observed, but usually the $$\boldsymbol{E}\times \boldsymbol{B}$$ drift is able to account for most of the electron bulk velocity, including the fluctuations in the wave (e.g., Graham et al., 2019, 2017; Holmes et al., 2021).…”

“…The lower‐hybrid wave that develops in regions with a strong magnetic field and low plasma $$\beta $$ is typically the short‐wavelength mode, in which the wave number $$k{\rho }_{e}$$ is a fraction of unity. Examples include waves at the boundary of the current sheet (e.g., Le Contel et al., 2017; Norgren et al., 2012; Zhou et al., 2009), in the separatrix region of magnetic reconnection far away from the electron diffusion region (EDR) (the satellite does not encounter the EDR during the current sheet crossing) at the magnetotail (e.g., Holmes et al., 2021), at dayside magnetopause on the magnetospheric side (Graham et al., 2019), and at dayside magnetopause on the magnetosheath side (Tang et al., 2020). The wave propagates mainly in the L‐M plane roughly perpendicular to the magnetic field, where L is along the reversing magnetic field direction, N is along the current sheet normal, and M is along the current direction to form the right‐hand orthogonal LMN coordinate system.…”

“…Such a picture was discussed based on observations at the dayside magnetopause (e.g., Ergun et al., 2019) and magnetotail (Chen et al., 2020). The field line twisting was later examined in the simulation by plotting the perpendicular magnetic field patterns and tracing field lines (Wang, Chen, Ng, et al., 2021), as well as in magnetotail observations by reconstructed magnetic field structures (Holmes et al., 2021).…”

“…In addition to the lower-hybrid drift instability, the relative drift between two ion populations (Graham et al, 2017) or between ions and electrons (Graham et al, 2019) without the effect of the pressure gradient can also induce lower-hybrid waves.The lower-hybrid wave that develops in regions with a strong magnetic field and low plasma 𝐴𝐴 𝐴𝐴 is typically the short-wavelength mode, in which the wave number 𝐴𝐴 𝐴𝐴𝐴𝐴𝑒𝑒 is a fraction of unity. Examples include waves at the boundary of the current sheet (e.g., Le Contel et al, 2017;Norgren et al, 2012;Zhou et al, 2009), in the separatrix region of magnetic reconnection far away from the electron diffusion region (EDR) (the satellite does not encounter the EDR during the current sheet crossing) at the magnetotail (e.g., Holmes et al, 2021), at dayside magnetopause on the magnetospheric side (Graham et al, 2019), and at dayside magnetopause on the magnetosheath side (Tang et al, 2020). The wave propagates mainly in the L-M plane roughly perpendicular to the…”

Electron magnetohydrodynamics Grad–Shafranov reconstruction of the magnetic reconnection electron diffusion region

“…These include the EDR in reconnection with the guide field (Zhou et al, 2019a), containing a secondary island (Denton et al, 2021) and wavy CSs associated with lower hybrid waves (Chen et al, 2020;Cozzani et al, 2021). Furthermore, the electron physics in the magnetic separatrix region, where complex wave-particle interactions take place due to the mixing of cold inflow and jetting outflow electrons, was also resolved by MMS (Nakamura et al, 2016;Norgren et al, 2020;Holmes et al, 2021).…”