SM53C-01 INVITED
Magnetic Reconnection in the Solar Wind: An Overview
We have recently demonstrated that local, quasi-stationary reconnection occurs relatively frequently in the solar wind over a large range (0.3 to 5.4 AU) of heliocentric distances. Direct evidence for such reconnection is found in the observation of Petschek-type exhausts, i.e., exhausts of jetting plasma bounded by Alfven or slow mode waves, emanating from reconnection sites. The exhausts are identified as intervals of roughly Alfvenic accelerated plasma flow confined to magnetic field reversal regions that usually take the form of bifurcated current sheets. The exhausts are observed almost exclusively in either the low-speed wind or in association with ICMEs in plasma predominantly having low proton beta. Field shears across the exhausts range from about 60 to 180 degrees, indicating the presence of significant guide fields in many of these events. The exhausts are embedded within the solar wind flow and typically are convected past a spacecraft on time scales ranging up to several tens of minutes, although considerably broader events have been observed on occasion. Multi-spacecraft observations have provided convincing evidence that the exhausts commonly result from prolonged reconnection at extended and continuous reconnection sites (X-lines). A relatively minor fraction of the exhausts occur at the heliospheric current sheet, HCS, but observations at the HCS are particularly useful for demonstrating magnetic field topology changes associated with reconnection and the effects of particle and plasma interpenetration within the exhausts. Although the exhausts are characterized by bulk plasma acceleration, we have not yet found any evidence for additional particle acceleration in these events.
SM53C-02 INVITED
Anti-parallel and Component Reconnection at the Magnetopause
Reconnection at the magnetopause is clearly the dominant mechanism by which magnetic fields in different regions change topology to create open magnetic field lines that allow energy and momentum to flow into the magnetosphere. Observations and data analysis methods have reached the maturity to address one of the major outstanding questions about magnetic reconnection: The location of the reconnection site. There are two scenarios discussed in the literature, a) anti-parallel reconnection where shear angles between the magnetospheric field and the IMF are near 180 degrees, and b) component reconnection where shear angles are as low as 50 degrees. One popular component reconnection model is the tilted neutral line model. Both reconnection scenarios have a profound impact on the location of the X-line and plasma transfer into the magnetosphere. We have analyzed 3D plasma measurements observed by the Polar satellite in the northern hemisphere cusp region during southward IMF conditions. These 3D plasma measurements are used to estimate the distance to the reconnection line by using the low-velocity cutoff technique for precipitating and mirrored magnetosheath populations in the cusp. The calculated distances are subsequently traced back along geomagnetic field lines to the expected reconnection sites at the magnetopause. The Polar survey of northern cusp passes reveal that both reconnection scenarios occur at the magnetopause. The IMF clock angle appears to be the dominant parameter in causing either the anti-parallel or the tilted X-line reconnection scenario.
SM53C-03 INVITED
Does the Hall effect solve the flux pileup saturation problem?
Previous analytic work on the theory of resistive Hall MHD magnetic reconnection suggests that Hall electric fields can mitigate the well known flux pileup saturation problem [Dorelli, J. C., Phys. Plasmas, 10, 3309, 2003.]. Specifically, the decoupling of electron and ion bulk velocities within the ion inertial region causes flux pileup to saturate (in the limit of vanishing resistivity) before the reconnection rate begins to drop; the reconnection rate becomes insensitive to the resistivity, but it depends strongly on the ion inertial length. The physics of pileup saturation in the Hall limit is straightforward: strong electron flows (associated with the out- of-plane quadrupole magnetic field structure) drive fast reconnection within the ion inertial region without piling up magnetic energy upstream of the current sheet. As the resistivity is decreased, the electron flow (and out-of-plane magnetic field) increases to accommodate the externally imposed reconnection rate. Recently, Craig and Watson [Phys. Plasmas, 12, 012306, 2005.] have argued that the increase of the out-of-plane field poses a new saturation problem, since the field cannot increase without limit. In this presentation, we demonstrate that this new saturation effect is an artifact of the analytic pileup solutions, in which the out-of-plane field blows up at infinity. In reality, the out-of-plane field vanishes at infinity (in the ideal MHD region of the solution), so that the analytic solutions are only relevant near the X line. We use resistive Hall MHD simulations to demonstrate that the amplitude of the out-of-plane field remains bounded (though the first derivative increases) as the resitivity is decreased. We conclude that the Hall effect mitigates the flux pileup saturation problem, allowing fast reconnection to occur in macroscopic thin current sheets.
SM53C-04 INVITED
Surprising New Results on the Structure of the Diffusion Region
The prevailing understanding of collisionless reconnection is based on the notion that the diffusion region has a multi-scale structure consisting of an outer ion-scale region and an inner electron-scale region that is presumed to remain microscopic (electron scale) in both the inflow and outflow directions. As a consequence, it has been argued that the reconnection rate is controlled by the ions and is insensitive to details of the electron physics. In contrast to this conventional picture, new results from large-scale fully kinetic simulations indicate that the electron diffusion region expands in time to form an elongated current layer with a width on the order of the electron meandering orbit but a length that can exceed tens of ion inertial lengths. As a result, the electron layer forms a bottleneck limiting the reconnection rate. Furthermore, this extended electron current sheet periodically becomes unstable to the formation of secondary islands leading to a reconnection process that is inherently unsteady. Although scaling these results to realistic parameter regimes remains a significant challenge, it already seems clear that the length of the electron diffusion region will be much larger than previously thought, with a length at the magnetopause of ~ 500 km rather than the expected ~ 8 km. These results have major implications for the study of reconnection and require a thorough re-examination of the previous theories. Similarly, the diagnostics for identification of the diffusion region and interpretation of observational data need to be reconsidered for both existing missions such as Cluster as well as upcoming missions such as the Magnetospheric Multiscale mission.
SM53C-05
Cluster measurements of the divergence of the electron pressure tensor and J×B: Relative contributions to Ohm's law
On the 17th August 2003 Cluster observed a flow reversal in the tail plasma sheet, consistent with the passage of a reconnection X-line. Henderson et al. (2006) have investigated the divergence of the observed electron pressure tensor, derived solely from measurements measured from the PEACE electron spectrometer instruments on each of the 4 Cluster spacecraft, during a current sheet crossing in this period. This calculation is possible when the spacecraft are in burst mode and carefully calibrated, full 3D particle distributions can be used to determine the electron moments. The Curlometer technique, applied to data from the Cluster magnetometers, was used to assess J×B, and thus the relative contributions of the electron pressure divergence and Hall terms in the generalised Ohm's law were determined. An anti-correlation between these two terms in the component normal to the current sheet was reported. In this paper we report an extension to Henderson et al.'s (2006) case study, in which we investigate the generality of those results by investigating further current sheet crossings which occurred on the 17th August 2003, as well as a number of additional periods of burst mode data available from the 2003 tail season. Data from this period are particularly useful for this study as the inter- spacecraft separations are at their smallest for the Cluster mission. We confirm the generality of the observed anti-correlation between the Hall and electron pressure tensor divergence terms in Ohm's law, and consider the theoretical background in an attempt to explain the relative magnitude of their contributions to the total electric field. Henderson et al., "Cluster PEACE observations of electron pressure tensor divergence in the magnetotail", Geophys, Res, Lett. (2006), 33, L22106
SM53C-06
Cluster observation of magnetic islands and energetic electrons
Magnetic reconnection is widely accepted to be a mechanism for electron acceleration, but exactly how electrons are accelerated during reconnection remains a long-standing question. A series of magnetic islands is observed in the magnetotail current sheet during active reconnection by multiple spacecraft. Electrons are hot within islands. The islands move away from the main reconnection sites with a velocity ~ 500 km/s based on multi-spacecraft timing analysis. The electric current distribution within an island varies significantly over a fraction of an ion inertial length in the out-of-plane direction, necessitating a 3D description. Associated with each island, is a burst of energetic electrons with energies ~35 to 120 keV. High time-resolution data reveal that energetic electron fluxes peak at sites of compressed density within magnetic islands. Within the islands, the density of O+ is higher than that of H+. The O+ ions are unmagnetized, and their density exhibits similar compression as the electron density, indicating that the density compression is not due to magnetic trapping by contracting islands, but most likely, due to the continuous injection of the ion and electron jets from the two reconnection sites bounding an island. The observation establishes a link between energetic electrons and magnetic islands, and provides supporting evidence for multiple reconnection sites.