Structure and Dissipation Characteristics of an Electron Diffusion Region Observed by MMS During a Rapid, Normal‐Incidence Magnetopause Crossing

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Abstract
On 22 October 2016, the Magnetospheric Multiscale (MMS) spacecraft encountered the electron diffusion region (EDR) when the magnetosheath field was southward, and there were signatures of fast reconnection, including flow jets, Hall fields, and large power dissipation. One rapid, normal‐incidence crossing, during which the EDR structure was almost stationary in the boundary frame, provided an opportunity to observe the spatial structure for the zero guide field case of magnetic reconnection. The reconnection electric field was determined unambiguously to be 2–3 mV/m. There were clear signals of fluctuating parallel electric fields, up to 6 mV/m on the magnetosphere side of the diffusion region, associated with a Hall‐like parallel current feature on the electron scale. The width of the main EDR structure was determined to be ~2 km (1.8 d e ). Although the MMS spacecraft were in their closest tetrahedral separation of ~8 km, the divergences and curls for these thin current structures could therefore not be computed in the usual manner. A method is developed to determine these quantities on a much smaller scale and applied to compute the normal component of terms in the generalized Ohm's law for the positions of each individual spacecraft (not a barocentric average). Although the gradient pressure term has a qualitative dependence that follows the observed variation of E + V e × B , the quantitative magnitude of these terms differs by more than a factor of 2, which is shown to be greater than the respective errors. Thus, future research is required to find the manner in which Ohm's law is balanced.
Funding Information
  • National Aeronautics and Space Administration (NNG04EB99C)

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