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Register a new userAssessing the time dependence of reconnection with Poyntings theorem: MMS observations
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We investigate the time dependence of electromagnetic-field-to-plasma energy conversion in the electron diffusion region of asymmetric magnetic reconnection. To do so, we consider the terms in Poyntings theorem. In a steady state there is a perfect balance between the divergence of the electromagnetic energy flux $ abla cdot vec{S}$ and the conversion between electromagnetic field and particle energy $vec{J} cdot vec{E}$. This energy balance is demonstrated with a particle-in-cell simulation of reconnection. We also evaluate each of the terms in Poyntings theorem during an observation of a magnetopause reconnection region by Magnetospheric Multiscale (MMS). We take the equivalence of both sides of Poyntings theorem as an indication that the errors associated with the approximation of each term with MMS data are small. We find that, for this event, balance between $vec{J}cdotvec{E}=- ablacdotvec{S}$ is only achieved for a small fraction of the energy conversion region at/near the X-point. Magnetic energy was rapidly accumulating on either side of the current sheet at roughly three times the predicted energy conversion rate. Furthermore, we find that while $vec{J}cdotvec{E}>0$ and $ ablacdotvec{S}<0$ are observed, as is expected for reconnection, the energy accumulation is driven by the overcompensation for $vec{J}cdotvec{E}$ by $- ablacdotvec{S}>vec{J}cdotvec{E}$. We note that due to the assumptions necessary to do this calculation, the accurate evaluation of $ ablacdotvec{S}$ may not be possible for every MMS-observed reconnection event; but if possible, this is a simple approach to determine if reconnection is or is not in a steady-state.
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Magnetic reconnection (MR) and the associated concurrently occurring waves have been extensively studied at large-scale plasma boundaries, in quasi-symmetric and asymmetric configurations in the terrestrial magnetotail and at the magnetopause. Recent high-resolution observations by MMS (Magnetospheric Multiscale) spacecraft indicate that MR can occur also in the magnetosheath where the conditions are highly turbulent when the upstream shock geometry is quasi-parallel. The strong turbulent motions make the boundary conditions for evolving MR complicated. In this paper it is demonstrated that the wave observations in localized regions of MR can serve as an additional diagnostic tool reinforcing our capacity for identifying MR events in turbulent plasmas. It is shown that in a close resemblance with MR at large-scale boundaries, turbulent reconnection associated whistler waves occur at separatrix/outflow regions and at the outer boundary of the electron diffusion region, while lower hybrid drift waves are associated with density gradients during the crossing of the current sheet. The lower hybrid drift instability can make the density inhomogeneities rippled. The identification of MR associated waves in the magnetosheath represents also an important milestone for developing a better understanding of energy redistribution and dissipation in turbulent plasmas.
We identify a dayside electron diffusion region (EDR) encountered by the Magnetospheric Multiscale (MMS) mission and estimate the terms in generalized Ohms law that controlled energy conversion near the X-point. MMS crossed the moderate-shear (130 degrees) magnetopause southward of the exact X-point. MMS likely entered the magnetopause far from the X-point, outside the EDR, as the size of the reconnection layer was less than but comparable to the magnetosheath proton gyro-radius, and also as anisotropic gyrotropic outflow crescent electron distributions were observed. MMS then approached the X-point, where all four spacecraft simultaneously observed signatures of the EDR, e.g., an intense out-of-plane electron current, moderate electron agyrotropy, intense electron anisotropy, non-ideal electric fields, non-ideal energy conversion, etc. We find that the electric field associated with the non-ideal energy conversion is (a) well described by the sum of the electron inertial and pressure divergence terms in generalized Ohms law though (b) the pressure divergence term dominates the inertial term by roughly a factor of 5:1, (c) both the gyrotropic and agyrotropic pressure forces contribute to energy conversion at the X-point, and (d) both out-of-the-reconnection-plane gradients (d/dM) and in-plane (d/dL,N) in the pressure tensor contribute to energy conversion near the X-point. This indicates that this EDR had some electron-scale structure in the out-of-plane direction during the time when (and at the location where) the reconnection site was observed.
Studies of shocks have long suggested that a shock can undergo cyclically self-reformation in a time scale of ion cyclotron period. This process has been proposed as a primary mechanism for energy dissipation and energetic particle acceleration at shocks. Unambiguous observational evidence, however, has remained elusive. Here, we report direct observations for the self-reformation process of a collisionless, high Mach number, quasi-perpendicular shock using MMS measurements. We find that reflected ions by the old shock ramp form a clear phase-space vortex, which gives rise to a new ramp. The new ramp observed by MMS2 has not yet developed to a mature stage during the self-reformation, and is not strong enough to reflect incident ions. Consequently, these ions are only slightly slowed down and show a flat velocity profile from the new ramp all the way to the old one. The present results provide direct evidence for shock self-reformation, and also shed light on energy dissipation and energetic particle acceleration at collisionless shocks throughout the universe.
The properties of the turbulence which develops in the outflows of magnetic reconnection have been investigated using self-consistent plasma simulations, in three dimensions. As commonly observed in space plasmas, magnetic reconnection is characterized by the presence of turbulence. Here we provide a direct comparison of our simulations with reported observations of reconnection events in the magnetotail investigating the properties of the electromagnetic field and the energy conversion mechanisms. In particular, simulations show the development of a turbulent cascade consistent with spacecraft observations, statistics of the the dissipation mechanisms in the turbulent outflows similar to the one observed in reconnection jets in the magnetotail, and that the properties of turbulence vary as a function of the distance from the reconnecting X-line.
We investigate the accuracy with which the reconnection electric field $E_M$ can be determined from in-situ plasma data. We study the magnetotail electron diffusion region observed by NASAs Magnetospheric Multiscale (MMS) on 2017-07-11 at 22:34 UT and focus on the very large errors in $E_M$ that result from errors in an $LMN$ boundary-normal coordinate system. We determine several $LMN$ coordinates for this MMS event using several different methods. We use these $M$ axes to estimate $E_M$. We find some consensus that the reconnection rate was roughly $E_M$=3.2 mV/m $pm$ 0.06 mV/m, which corresponds to a normalized reconnection rate of $0.18pm0.035$. Minimum variance analysis of the electron velocity (MVA-$v_e$), MVA of $E$, minimization of Faraday residue, and an adjusted version of the maximum directional derivative of the magnetic field (MDD-$B$) technique all produce {reasonably} similar coordinate axes. We use virtual MMS data from a particle-in-cell simulation of this event to estimate the errors in the coordinate axes and reconnection rate associated with MVA-$v_e$ and MDD-$B$. The $L$ and $M$ directions are most reliably determined by MVA-$v_e$ when the spacecraft observes a clear electron jet reversal. When the magnetic field data has errors as small as 0.5% of the background field strength, the $M$ direction obtained by MDD-$B$ technique may be off by as much as 35$^circ$. The normal direction is most accurately obtained by MDD-$B$. Overall, we find that these techniques were able to identify $E_M$ from the virtual data within error bars $geq$20%.
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