Magnetic reconnection is a fundamental physical process in plasmas whereby stored 40 magnetic energy is converted into heat and kinetic energy of charged particles. 41Reconnection occurs in many astrophysical plasma environments and in laboratory 42 plasmas. Using very high time resolution measurements, NASA's Magnetospheric 43 2 Multiscale Mission (MMS) has found direct evidence for electron demagnetization and 44 acceleration at sites along the sunward boundary of Earth's magnetosphere where the 45 interplanetary magnetic field reconnects with the terrestrial magnetic field. We have (i) 46 observed the conversion of magnetic energy to particle energy, (ii) measured the electric 47 field and current, which together cause the dissipation of magnetic energy, and (iii) 48identified the electron population that carries the current as a result of demagnetization 49 and acceleration within the reconnection diffusion/dissipation region. 50 51 Introduction 52

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This paper extends to all interplanetary magnetic field (IMF) orientations the qualitative convection pattern presented by Burch et al. (this issue), containing viscous, merging, and lobe cells driven, respectively, by diffusion or other quasi‐viscous processes, merging of interplanetary fields with closed dayside field lines, and merging of interplanetary fields with open tail lobe field lines. The model is based on the antiparallel merging hypothesis of Crooker (1979a) with the addition of small but finite cells driven by quasi‐viscous processes on the dawn and dusk edges of the polar cap. The data and model presented by Burch et al. pertained to southward IMF conditions. This paper generalizes that model and proposes a qualitative dependence of the three types of convection cells on the x, y, and z components of the IMF. For example, the lobe cell should be enhanced in the northern hemisphere if Bx < 0 and in the southern for Bx > 0, and, for a given Bx and Bz, should be larger as |By| increases. For northward IMF, the merging cell disappears, leaving only the lobe cell in a smaller polar cap. If the y component of the IMF (By) is small, we infer the four‐cell pattern of Burke et al. (1979), with two counter‐rotating lobe cells having sunward flow in the central polar cap and tailward flow on the flanks. If By is large, the antiparallel merging model predicts a single lobe cell filling the polar cap, whose direction of rotation depends on the sign of By and is opposite in opposite hemispheres. We argue that this vortex is unstable to reconnection in the magnetotail, leading to two (or more) vortices in each polar cap, each with the same sense of rotation, but again differing between the hemispheres. Each vortex has a region of closed field lines in the sunward‐flowing section and open field lines in the antisunward‐flowing section. This model encompasses several features of the theta arc phenomenon, and makes several predictions with respect to symmetries and antisymmetries between the two polar caps.

We have computed the convection potential drop across the polar cap from data obtained on high‐inclination low‐altitude satellites (AE‐C, AE‐D, S3‐3) and correlated these potential measurements with various combinations of parameters measured simultaneously in the upstream solar wind. These combinations of solar wind parameters consist of predictions based on magnetic merging theory and suggestions based on earlier empirical work. We find that the bulk of the potential drop, and its variation with interplanetary magnetic field (IMF) parameters, are successfully predicted by merging theory (to the accuracy with which they can presently be measured), but that a significant ‘background’ potential drop (∼35 kV) does not depend on IMF parameters and may thus be attributed to an unknown process other than merging. Our results indicate that small values of the IMF are amplified by a factor of 5–10 at the dayside magnetopause as a combined effect of bow shock compression and the Zwan‐Wolf depletion layer effect; correlations between IMF parameters and the polar cap potential drop are dramatically improved when this amplification is taken into account. The potential drop is better correlated with IMF parameters than with geomagnetic activity indices, presumably because the latter are affected by nonlinear reponses of the magnetosphere to the polar cap input.

Two mechanisms have been proposed for solar wind particle injection at the dayside magnetospheric cusps: magnetic merging and cross-field diffusion. These two mechanisms are experimentally distinguishable in that they produce different latitudinal distributions of particles penetrating to the low-altitude cusp. An examination of proton and electron measurements obtained by the AE-C satellite in the lowaltitude dayside cusp reveals evidence of both types of injection processes. A majority of the injection events, especially the more intense fluxes, are best explained by a merging injection model in which cusp particles are confined to the poleward side of the last closed field line and have a characteristic energy that decreases with increasing latitudinal distance from the last closed field line. Less frequent and less intense injection events are better explained in terms of a diffusive injection of cusp particles onto closed dayside field lines with a characteristic energy that increases with increasing latitudinal distance from the last closed field line. Although diffusion appears to be quantitatively less important than merging in terms of the instantaneous particle injection rate, cross-field diffusion nevertheless appears to proceed at an unexpectedly fast rate, possibly exceeding the Bohm diffusion limit.