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

Abstract

We investigate waves close to the lower‐hybrid frequency in 12 magnetotail reconnection electron diffusion region (EDR) events with guide field levels of near‐zero to 30%. In about half of the events, the wave vector has a small component along the current sheet normal, consistent with known lower‐hybrid drift wave properties, but the perpendicular magnetic field fluctuations can be comparable or greater than the parallel component, a feature unique to the waves inside and adjacent to EDRs. Another new wave property is that the wave vector has a significant component along the current sheet normal in some events and completely along the normal for one event. In 1/4 of the events, the ∇·Pe $\nabla \cdot {\boldsymbol{P}}_{e}$ term has a significant contribution to the wave electric field, possibly a feature of lower‐hybrid waves more likely to exist in the diffusion region than further away from the X‐line. Electron temperature variations are correlated with the wave potential, due to wave electric field acceleration and crossings at the corrugated separatrix region with different amounts of mixing between reconnection inflowing and outflowing populations. The latter also leads to the anti‐correlation between parallel and perpendicular temperature components. Using four‐spacecraft measurements, the magnetic field line twisting is demonstrated by the correlated fluctuations in ∇×VE×B|| ${\left(\nabla \times {\boldsymbol{V}}_{E\times B}\right)}_{\vert \vert }$ and (∇×boldB)|| ${(\nabla \times \mathbf{B})}_{\vert \vert }$. The lower‐hybrid wave in the EDR of weak guide field reconnection may be generated near separatrices and penetrate to the mid‐plane or locally generated, and the latter possibility is beyond the prediction of previous reconnection simulations.

Magnetospheric Multiscale Mission Observations of Lower-hybrid Drift Waves in Terrestrial Magnetotail Reconnection with Moderate Guide Field and Asymmetric Plasma Density

“…In the magnetotail, reconnection is generally symmetric and antiparallel (Eastwood et al 2010a). Meanwhile, an increasing number of reconnection events with the guide field were also detected in the magnetotail (Chen et al 2021;Tang et al 2022;Wang et al 2022).…”

“…Magnetic reconnection, responsible for many explosive energy release processes in laboratory and astrophysical plasmas (Chen & Shibata 2000;Ji et al 2004;Angelopoulos et al 2008;Fu et al 2013;Cao et al 2013;, efficiently converts magnetic energy into plasma energy as magnetic field lines break and reconnect (Birn et al 2001;Yamada et al 2010;Fu et al 2019aFu et al , 2019bFujimoto & Cao 2021). In the terrestrial magnetosphere, it is often observed in the magnetotail (e.g., Fu et al 2017;Wang et al 2022), at the magnetopause (e.g., Fuselier et al 2017;Cao et al 2017;Graham et al 2017;Webster et al 2018;Dong et al 2021), in the magnetosheath (e.g., Phan et al 2018;Stawarz et al 2019), and in the transition region of bow shocks (Chen et al 2019b;Gingell et al 2020). At the magnetopause, reconnection is generally asymmetric and not antiparallel (Peng et al 2017;Webster et al 2018), where plasma density, plasma temperature, and magnetic field magnitude are different on the two sides of the reconnection current sheet, and the guide field exists.…”

“…The LHDWs, developing at a frequency near the lowerhybrid frequency, were often observed in the current sheet (Norgren et al 2012;Liu et al 2018a;Graham et al 2019;Cozzani et al 2021;Wang et al 2022). It contributes to the strong electric field (Graham et al 2016), anomalous diffusion, and resistivity (Bale et al 2002;Vaivads et al 2004;Graham et al 2017;Tang et al 2020), and electron heating Wang et al 2022), and potentially affects the EDR structure (Cozzani et al 2021). The LHDWs are driven by the lower-hybrid drift instability (LHDI) whose free energy is provided by inhomogeneities in plasma density and magnetic field (Norgren et al 2012).…”

“…Besides, the LHDWs can be also induced by the modified two-stream instability (Graham et al 2017(Graham et al , 2019. The short-wavelength mode with k ⊥ ρ e ∼ 1 often develops at the current sheet boundary, while the longwavelength mode with k ⊥ (ρ i ρ e ) 0.5 ∼ 1 develops in the current sheet center (Norgren et al 2012;Yoo et al 2020;Wang et al 2022).…”

“…In magnetotail reconnection without the guide field, electromagnetic and electrostatic LHDWs were observed in the current center and separatrices (Zhou et al 2009), respectively. Recently, some new properties of LHDWs were also reported in magnetotail reconnection with a weak or intermediate guide field (weaker than 30% of the reconnecting component; Yu et al 2019;Cozzani et al 2021;Wang et al 2022). Owing to the presence of the guide field, the LHDWs can propagate mainly along the outflow direction .…”