R. Mistry

Ion reflection and acceleration near magnetotail dipolarization fronts associated with magnetic reconnection

Dipolarization fronts (DFs) are often associated with the leading edge of earthward bursty bulk flows in the magnetotail plasma sheet. Here multispacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations are used to show that a spatially limited region of counterpropagating ion beams, whose existence is not evident in either the plasma moments or the electric field, is observed on the low-density side of DFs. The THEMIS magnetic field data are used to establish appropriate comparison cuts through a particle-in-cell simulation of reconnection, and very good agreement is found between the observed and simulated ion distributions on both sides of the DF. Self-consistent back tracing shows that the ion beams originate from the thermal component of the preexisting high-density plasma into which the DF is propagating; they do not originate from the inflow region in the traditional sense. Forward tracing shows that some of these ions can subsequently overtake the DF and pass back into the high-density preexisting plasma sheet with an order-of-magnitude increase in energy; this process is distinct from other ion reflection processes that occur directly at the DF. The interaction of the reconnection jet with the preexisting plasma sheet therefore occurs over a macroscopic region, rather than simply being limited to the thin DF interface. A more general consequence of this study is the conclusion that reconnection jets are not simply fed by plasma inflow across the separatrices but are also fed by plasma from the region into which the jet is propagating; the implications of this finding are discussed.

Statistical properties of solar wind reconnection exhausts

The solar wind provides an excellent opportunity to study the exhausts that form as a result of symmetric guide field reconnection, where spacecraft rapidly cross the exhausts far downstream of the X-line. We study the statistical properties of solar wind exhausts through a superposed epoch analysis of 188 events observed at 1 AU using the Wind spacecraft. These events span a range of guide fields of 0 to 10 times the reconnecting magnetic field, and inflow region plasma beta of 0.1 to 6.6. This analysis reveals that the out-of-plane magnetic field is enhanced within solar wind exhausts. Furthermore, the amount by which the plasma density and ion temperature increase from inflow region to exhaust region is found to be a function of the inflow region plasma beta and reconnection guide field, which explains the lack of these enhancements in a subset of previous observations. This dependence is consistent with the scaling of ion heating with inflow region Alfven speed, which is measured to be consistent with previous observations in the solar wind and at the magnetopause.

Detection of small-scale folds at a solar wind reconnection exhaust

Observations of reconnection in the solar wind over the last few years appear to indicate that the majority of large-scale reconnecting current sheets are roughly planar, and that reconnection itself is quasi-steady. Most studies of solar wind exhausts have used spacecraft with large separations and relatively low time cadence ion measurements. Here we present multipoint Cluster observations of a reconnection exhaust and the associated current sheet at ACE and Wind, enabling it to be studied on multiple length scales and at high time resolution. While analysis shows that on large scales the current sheet is planar, detailed measurements using the four closely spaced Cluster spacecraft show that the trailing edge of the reconnection jet is nonplanar with folds orthogonal to the reconnection plane, with length scales of approximately 230 ion inertial lengths. Our findings thus suggest that while solar wind current sheets undergoing reconnection may be planar on large scales, they may also exhibit complex smaller-scale structure. Such structure is difficult to observe and has rarely been detected because exhausts are rapidly convected past the spacecraft in a single cut; there is therefore a limited set of spacecraft trajectories through the exhaust which would allow the nonplanar features to be intercepted. We consider how such nonplanar reconnection current sheets can form and the processes which may have generated the 3-D structure that was observed.

Separating drivers of Saturnian magnetopause motion

Surface waves on Saturns magnetopause and oscillations of the magnetopause at a period associated with that of planetary rotation have previously been detected. How the amplitudes of these two key perturbations to the magnetopause position compare, however, is unclear. We construct a one-dimensional magnetopause model that considers both types of boundary dynamics and compare it to six sets of magnetopause crossings observed by the Cassini magnetometer instrument, in order to estimate and compare properties of the two phenomena. We identify their relative amplitudes to be approximately one in all cases, suggesting that magnetopause oscillations notably affect the magnetopause position, even when surface waves are present. Furthermore, using this technique, we estimate the surface wave period for crossings on 14 July 2007 as 47 min and periods for all other sets at values between 2.4 h and 4.9 h. These periods are in agreement with previous studies which employed minimum variance analysis of magnetometer data.

Guide Field Reconnection: Exhaust Structure and Heating

Abstract Magnetospheric Multiscale observations are used to probe the structure and temperature profile of a guide field reconnection exhaust ~100 ion inertial lengths downstream from the X‐line in the Earths magnetosheath. Asymmetric Hall electric and magnetic field signatures were detected, together with a density cavity confined near 1 edge of the exhaust and containing electron flow toward the X‐line. Electron holes were also detected both on the cavity edge and at the Hall magnetic field reversal. Predominantly parallel ion and electron heating was observed in the main exhaust, but within the cavity, electron cooling and enhanced parallel ion heating were found. This is explained in terms of the parallel electric field, which inhibits electron mixing within the cavity on newly reconnected field lines but accelerates ions. Consequently, guide field reconnection causes inhomogeneous changes in ion and electron temperature across the exhaust.