Fingerprints of collisionless reconnection at the separator, I, Ambipolar-Hall signatures

Plasma, electric, and magnetic field data on the Polar spacecraft have been analyzed for the 29 May 1996 magnetopause traversal searching for evidence of in situ reconnection and traversal of the separator. In this paper we confine our analysis to model-free observations and intrasensor coherence of detection of the environs of the separator. (1) We illustrate the first documented penetration of the separator of collisionless magnetic reconnection in temporal proximity to successful Walen tests with opposite slopes. (2) We present the first direct measurements of E-parallel to at the magnetopause. (3) We make the first empirical argument that E-parallel to derives from the electron pressure gradient force. (4) We document the first detection of the electron pressure ridge astride the magnetic depression that extends from the separator. (5) We provide the first empirical detection of the reconnection rate at the magnetopause with the locally sub-Alfvenic ion inflow, M(i)(A)similar or equal to0.1, and trans-Alfvenic exhaust at high electron pressure of M(i)(A)similar or equal to1.1-5. (6) We exhibit the first empirical detection of supra-Alfvenic electron flows parallel to B in excess of 5 in narrow sheets. (7) We illustrate the detection of heat flux sheets indicative of separatrices near, but not always in superposition, with the supra-Alfvenic parallel electron bulk flows. (8) We present the first evidence that pressure gradient scales are short enough to explain the electron fluid's measured cross-field drifts not explained by E x B drift but predicted by the measured size of E-parallel to. (9) We illustrate that the size of the observed E-parallel to is well organized with the limit implied by Vasyliunas's analysis of the generalized Ohm's law of scale length rho(s)=beta(1/2)c/omegapi, indicative of the intermediate scale of the diffusion region. (10) We document the first detection of departure from electron gyrotropy not only at the separator crossing but also in its vicinity, an effect presaged by Vasyliunas [1975]. (11) We make the first reports of very large values of electron beta(e)similar or equal to680 localized at the separator, which imply that the electron thermal gyroradius exceeds the electron inertial length by more than an order of magnitude there. This clearly delineates that the environs of the reversed field region in this data contain non-MHD scales. The ambipolar association and the measured E-parallel to data imply the presence of the nonideal rho(s) scale in these layers surrounding the null point. The high beta(e) signals the possible demagnetization of the thermal electrons in any structures with spatial scales of the electron skin depth, which is theoretically anticipated to surround the magnetic null line of the separator proper. This possibility is supported by the large number of temporally unaliased spectra at high beta(e) that are inconsistent with gyrotropy.