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Register a new userElectron Heating by Debye-Scale Turbulence in Guide-Field Reconnection
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We report electrostatic Debye-scale turbulence developing within the diffusion region of asymmetric magnetopause reconnection with moderate guide field using observations by the Magnetospheric Multiscale (MMS) mission. We show that Buneman waves and beam modes cause efficient and fast thermalization of the reconnection electron jet by irreversible phase mixing, during which the jet kinetic energy is transferred into thermal energy. Our results show that the reconnection diffusion region in the presence of a moderate guide field is highly turbulent, and that electrostatic turbulence plays an important role in electron heating.
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We analyze the development and influence of turbulence in three-dimensional particle-in-cell simulations of guide-field magnetic reconnection at the magnetopause with parameters based on observations of an electron diffusion region by the Magnetospheric Multiscale (MMS) mission. Along the separatrices the turbulence is a variant of the lower hybrid drift instability (LHDI) that produces electric field fluctuations with amplitudes much greater than the reconnection electric field. The turbulence controls the scale length of the density and current profiles while enabling significant transport across the magnetopause despite the electrons remaining frozen-in to the magnetic field. Near the X-line the electrons are not frozen-in and the turbulence, which differs from the LHDI, makes a significant net contribution to the generalized Ohms law through an anomalous viscosity. The characteristics of the turbulence and associated particle transport are consistent with fluctuation amplitudes in the MMS observations. However, for this event the simulations suggest that the MMS spacecraft were not close enough to the core of the electron diffusion region to identify the region where anomalous viscosity is important.
The plasma in low-luminosity accretion flows, such as the one around the black hole at the center of M87 or Sgr A* at our Galactic Center, is expected to be collisioness and two-temperature, with protons hotter than electrons. Here, particle heating is expected to be controlled by magnetic reconnection in the transrelativistic regime $sigma_{w}sim 0.1$-$1$, where the magnetization $sigma_{w}$ is the ratio of magnetic energy density to plasma enthalpy density. By means of large-scale 2D particle-in-cell simulations, we explore for a fiducial $sigma_w=0.1$ how the dissipated magnetic energy gets partitioned between electrons and protons, as a function of $beta_{rm i}$ (the ratio of proton thermal pressure to magnetic pressure) and of the strength of a guide field $B_{rm g}$ perpendicular to the reversing field $B_0$. At low $beta_{rm i};(lesssim 0.1)$, we find that the fraction of initial magnetic energy per particle converted into electron irreversible heat is nearly independent of $B_{rm g}/B_0$, whereas protons get heated much less with increasing $B_{rm g}/B_0$. As a result, for large $B_{rm g} /B_{0}$, electrons receive the overwhelming majority of irreversible particle heating (${sim}93%$ for $B_{rm g} /B_{0}=6$). This is significantly different than the antiparallel case $B_{rm g}/B_0=0$, in which electron irreversible heating accounts for only ${sim}18%$ of the total particle heating. At $beta_{rm i} sim 2$, when both species start already relativistically hot (for our fiducial $sigma_w=0.1$), electrons and protons each receive ${sim}50%$ of the irreversible particle heating, regardless of the guide field strength. Our results provide important insights into the plasma physics of electron and proton heating in hot accretion flows around supermassive black holes.
Particle-in-Cell simulations of collisionless magnetic reconnection with a guide field reveal for the first time the three dimensional features of the low density regions along the magnetic reconnection separatrices, the so-called cavities. It is found that structures with further lower density develop within the cavities. Because their appearance is similar to the rib shape, these formations are here called low density ribs. Their location remains approximately fixed in time and their density progressively decreases, as electron currents along the cavities evacuate them. They develop along the magnetic field lines and are supported by a strong perpendicular electric field that oscillates in space. In addition, bipolar parallel electric field structures form as isolated spheres between the cavities and the outflow plasma, along the direction of the low density ribs and of magnetic field lines.
Using kinetic particle-in-cell (PIC) simulations, we simulate reconnection conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta (ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic shear (strong guide field). Changing the simulation domain size, we find that the ion response varies greatly. For reconnecting regions with scales comparable to the ion Larmor radius, the ions do not respond to the reconnection dynamics leading to electron-only reconnection with very large quasi-steady reconnection rates. The transition to more traditional ion-coupled reconnection is gradual as the reconnection domain size increases, with the ions becoming frozen-in in the exhaust when the magnetic island width in the normal direction reaches many ion inertial lengths. During this transition, the quasi-steady reconnection rate decreases until the ions are fully coupled, ultimately reaching an asymptotic value. The scaling of the ion outflow velocity with exhaust width during this electron-only to ion-coupled transition is found to be consistent with a theoretical model of a newly reconnected field line. In order to have a fully frozen-in ion exhaust with ion flows comparable to the reconnection Alfven speed, an exhaust width of at least several ion inertial lengths is needed. In turbulent systems with reconnection occurring between magnetic bubbles associated with fluctuations, using geometric arguments we estimate that fully ion-coupled reconnection requires magnetic bubble length scales of at least several tens of ion inertial lengths.
We present analysis of more than one hundred large-amplitude bipolar electrostatic structures in a quasi-perpendicular supercritical Earths bow shock crossing, measured by the Magnetospheric Multiscale spacecraft. The occurrence of the bipolar structures is shown to be tightly correlated with magnetic field gradients in the shock transition region. The bipolar structures have negative electrostatic potentials and spatial scales of a few Debye lengths. The bipolar structures propagate highly oblique to the shock normal with velocities (in the plasma rest frame) of the order of the ion-acoustic velocity. We argue that the bipolar structures are ion phase space holes produced by the two-stream instability between incoming and reflected ions. This is the first identification of the ion two-stream instability in collisionless shocks. The implications for electron acceleration are discussed.
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