Reconnection Rates at the Earth's Magnetopause and in the Magnetosheath

摘要

Reconnection electric fields and normalized reconnection rates (RRs) were determined with the four Magnetospheric Multiscale (MMS) spacecraft for 14 reconnection events on the dayside of the Earth's magnetosphere. Half of the events occurred at the magnetopause (MP) and the other half in the magnetosheath (MS). The RRs were determined by measurements of the reconnection electric field within electron diffusion regions. Since the reconnection electric field component is much smaller than the other components, special care had to be taken to eliminate contamination by the larger components. Normalized RRs between 0.02 and 0.48 were determined with average values in the range of theoretical predictions for steady-state reconnection (0.1-0.2). Using measurements from individual MMS spacecraft, 20 normalized RRs were determined for the seven MP events with a mean of 0.14 and a standard deviation of 0.09. For the 27 RRs that were determined for the seven MS events, the normalized RRs had a mean of 0.16 and a standard deviation of 0.12. These results show variations of over an order of magnitude between the fastest and slowest RRs. However, when summed over all spacecraft, calculations suggest that the RR per event is relatively constant, as predicted theoretically. Searches for dependence of the normalized RR on local or solar-wind parameters were inconclusive. However, we do find that for the MS events, the reconnection electric field showed a direct dependence on solar-wind dynamic pressure. Magnetic reconnection (MR) is a fundamental process that occurs naturally throughout the universe in magnetized plasmas in which energy is transformed and redistributed throughout a small region known as the diffusion region where plasma is decoupled from the magnetic field. This process is the main driver of energy and momentum from the solar wind into the magnetosphere. The reconnection rate (RR) is one of the most important properties of MR and can be defined as the rate at which magnetic flux is transferred from the inflow to the outflow of the diffusion region and is determined by the electric field component that is perpendicular to both the inflowing and outflowing plasma velocities. This component is called the reconnection electric field. In this study, we measure the reconnection electric field within the diffusion region to determine the RR and compare it to measured magnetic and plasma parameters in the solar wind. The average reconnection rate (RR) for 14 events is 0.15 with significant variability within relatively small spatial and temporal rangesBy analyzing 47 individual normalized RRs, we find no correlation with external parametersWe find that the reconnection electric field is positively correlated with solar-wind dynamic pressure in the magnetosheath