Open Magnetic Flux and Magnetic Reconnection During Steady Magnetic Convection Intervals.
The Imager for Magnetopause to Aurora Global Exploration (IMAGE) spacecraft was launched in 2000 with several imaging instruments onboard. The Far UltraViolet (FUV) experiment imaged the N2 LBH (Wideband Imaging Camera - WIC-), OI 135.6 nm (Spectrographic Imager -SI13-) and Doppler-shifted Lyman alpha auroral emission (SI12). The Doppler-shifted Lyman-alpha emission allows to monitor the auroral oval both on the day and night sides. Remote sensing of the polar aurora is completed by ground based data of the Super Dual Auroral Radar Network (SuperDARN) that monitors the ionospheric convection flow pattern in the polar region. In the present study, SI12 images are used to estimate the open/closed (o/c) field line boundary location, and monitor its movement. The SuperDARN data are used to compute the electric field of the polar cap at the location of the o/c boundary. The total electric field is then computed along the boundary accounting for its movement applying Faraday's law, so that the dayside and nightside reconnection voltages can be retrieved. We apply this method to the study of several intervals of steady magnetic convection (SMC). SMC events are intervals of enhanced convection without classical substorm signatures. During these intervals, it is expected that the amount of open magnetic flux remains fairly constant, and it has been suggested that the rate of opening (at the magnetopause) and closure (in the magnetotail) of magnetic flux balance each other. These rates can be expressed as voltages with a positive sign for the opening and a negative sign for closure. The net reconnection voltage then represents the net rate of accumulation of open flux by the magnetosphere. We find that, during SMC intervals, the open magnetic flux varies only slowly, and sometimes remains stationary during several hours. As a consequence, the net voltage often remains close to zero during SMC intervals. Occasionally, we find that an increase in the opening voltage is followed by a similar intensification of the closure voltage after downtail convection of the newly created open flux. The convection time can be roughly estimated and ranges between 20 and 40 minutes, i.e. the typical order of magnitude of the convection time in the magnetosphere.
Solar Wind Transport Into Magnetosphere Caused by Magnetic Reconnection During Southward and Northward IMF
Reconnection is considered as the dominant mechanism of the solar wind transport into magnetosphere. Here, two cases under southward and northward IMF respectively are analyzed, with the results as follows: Firstly, by analyzing measurements from Cluster, an event of magnetopause crossing has been investigated. At the latitude of about 40º and MLT of 13:20 during southward IMF, a transition layer was observed, with the magnetospheric field configuration and cold dense plasma features of magnetosheath. The particle energy- time spectrograms inside the layer were similar to but still a little different from those in magnetosheath, obviously indicating solar wind entry into magnetosphere. The direction and magnitude of the accelerated ion flow implied that reconnection might possibly cause such a solar wind entry phenomenon. The bipolar signature of the normal magnetic component BN in magnetopause coordinates further supported happening of reconnection there. Solar wind plasma flowed toward magnetopause and entered magnetosphere along the reconnected flux tube. The magnetospheric branch of the reconnected flux tube was still inside the magnetosphere after reconnection and supplied the path for solar wind entry into the dayside magnetosphere. Secondly, an event of Cluster-Double Star conjunction observations of magnetic reconnection at high latitude magnetopause nightside of the cusp and solar wind transport into magnetosphere caused by such a reconnection process has been investigated. During northward IMF, Cluster/SC1 observed accelerated flows and ion heating associated with magnetic reconnection at high latitude magnetopause nightside of southern cusp. And Double Star observed cold dense solar wind plasma transported into dayside magnetosphere. The analysis on such conjunction observations shows that: during northward IMF, magnetic reconnection occurs at high latitude nightside of southern cusp, accompanied by accelerated flows that are observed by Cluster/SC1; the direction of the accelerated flows, with its sunward component Vx, dawnward component Vy, northward component Vz, is quite consistent with the theoretical anticipation under the condition of northward IMF with dawnward component By; reconnection can heat plasma more in parallel direction than in perpendicular direction, to a level of about 4 keV; with reconnection taking place at high latitude magnetopause nightside of the southern cusp, TC-1 observed cold and dense plasma transported into magnetosphere; by reconnection at high latitude magnetopause nightside of both cusps, solar wind flux tube can be captured by magnetosphere and pulled into dayside magnetosphere. The case analysis gave more detail and observational evidence of the solar wind transport into magnetosphere by reconnection under southward or northward IMF.
Three Dimensional Dynamics of Magnetic Reconnection in Large-Scale Pair Plasmas
Using the largest three dimensional particle-in-cell simulations to date, collisionless magnetic reconnection in large-scale electron-positron plasmas without a guide field is shown to involve complex interaction of tearing and kink modes. The reconnection onset is patchy and occurs at multiple sites which self-organize to form a single, large diffusion region. The diffusion region further expands in both outflow direction and current direction and become unstable to secondary kinking and formation of "plasmoid-rope" structures. The secondary kink leads to folding of the reconnection current layer, while plasmoid ropes at times follow the folding of the current layer. The interplay among these secondary instabilities plays a key role in controlling the time dependent reconnection rate in large-scale systems. These dynamics found in collisionless reconnection are compared with those in the collisional regime.
Electron-scale Structures in Collisionless Magnetic Reconnection with Multiple Reconnection Sites
The electron-magnetohydrodynamics (EMHD) model is used to study electron scale structures during the early phase of collisionless reconnection. In this model the electron inertia leads to the breakdown of frozen-in condition and the electron dynamics is responsible for the evolution of reconnection. The simulations show significant differences in the spatial structures when reconnection takes place at a single or multiple reconnection sites. The main differences in the structures and their scale lengths in the two cases arise due to the important roles of electron flows in the two cases. In the case of multiple reconnection sites, interaction of flows generated at the neighbouring sites leads to nested structure of quadrupoles of the out-of-plane magnetic field. The length of the reconnecting current sheet is also strongly modified due to the electron flows. Initially the current sheet length is determined essentially by the wavelength of the maximally growing mode, and subsequently it reduces due to the secondary instabilites in the nonlinear phase. The reduced length scales linearly with the initial width of the current sheet. The out-of-plane current sheet becomes highly structured, with bifurcated, triple peak, or filamentry features. These structures are compared with Cluster observations. The implications of these electron scale structures for the NASA/MMS mission, with the capability to resolve the short scale structures ~ de, will be presented.
Reconnection at the magnetopause of Saturn: perspectives from FTE and magnetosphere size
Reconnection events at the magnetopause of Saturn have been identified in the Cassini observations, but we do not know how important this reconnection is for the Saturnian system. At Earth, the magnetopause reconnection removes magnetic flux from the dayside magnetosphere and transports it to the magnetotail, playing an important role in the magnetosphere dynamics. Flux transfer events (FTEs) are interconnected flux tubes seen at the terrestrial magnetopause that contribute to the transfer of magnetic flux from the dayside magnetopause to the geomagnetic tail. At Earth these events occur principally when the IMF is southward and their occurrence rate decreases with increasing solar wind Mach number. Because they have a size and flux content that can be estimated these events can provide a lower limit of the magnetic flux transferred from the dayside to the tail in a planetary magnetosphere. At Saturn, we have found no well-formed FTE events near the subsolar magnetopause in the Cassini MAG data, which may imply reconnection not important at Saturn. We can further examine this by studying the size of the magnetosphere during northward and southward IMF. The stand-off distance of the terrestrial magnetopause is smaller during southward IMF than that during northward IMF, because southward IMF leads to magnetopause reconnection which removes magnetic flux from the subsolar region. The size and shape of Saturn's magnetopause is well modeled by Arridge et al. (2006). We use their model to estimate the magnetopause stand-off distance of the Cassini observed magnetopause crossing near noon, and compare the distributions of stand-off distances during northward and southward IMF.
Fast Reconnection Rates Based on Group Velocity Cones: Whistler Regime and Pair Plasmas
Based on the group velocity vector of the whistler mode, we predict the range of whistler-regime reconnection rate depending on the half width (w) of the current sheet (CS. During the reconnection process electromagnetic perturbations (EMPs) are generated in the localized diffusion region (DR, which acts like an antenna and radiates whistler waves for certain range of CS widths. The reconnection structure (exhaust) is approximately the radiation pattern of the DR antenna and it is determined by the group velocity directions. Since the whistler waves originate from the electromagnetic perturbations (EMPs) localized in the DR, we calculate R over a range of the discrete values of the perpendicular wave number (kƒÎ) contained in the Fourier spectrum of the EMPs. We have used such calculations to determine the reconnection rates <R> averaged over the wave number spectrum of a Gaussian shaped EMP as a function of the CS width. We find that <R> has a fairly constant value at <R> ∼ 0.23 for CS widths in the range 0.4 < w/di ∼ 1 and for w < 0.3di it decreases with decreasing w and it attains a value <R> ∼ 0.06 in an extremely thin CS with w ∼ 0.05di, where di is the ion skin depth. We compare the values of <R> and R with those found from simulations and experiments, and find them in good agreement. We also report the properties of the whistler waves radiated from the DR into the exhaust region. We also demonstrate that our theoretical method developed for whistler regime reconnection could be easily adopted to predict fast reconnection rates in pair plasmas, which support inertial Alfven waves.
Empirical survey of the magnetosheath
Magnetic field and plasma moments obtained from thousands of spacecraft passes through the magnetosheath are assimilated to build synoptic maps of characteristic plasma parameters, including the Alfven speed and Mach number. The observations are also matched to convected solar wind observations, to determine how variations in the solar wind conditions change the macroscopic behavior of the magnetosheath. In addition, comparisons are made with theoretical expectations for magnetohydrodynamic jump conditions across the bow shock. Lastly, the statistical magnetosheath parameters provide insight as to where steady and unsteady reconnection can occur at the magnetopause.