Cluster observations of the local energy transfer at the dayside magnetopause under steady interplanetary magnetic field conditions

We use the multi-spacecraft mission Cluster to make observational estimates of the local energy transfer across the dayside high-latitude magnetopause during an interval of continuous reconnection. The instantaneous power input is estimated from jxB observations from the FGM instrument and CIS magnetosheath velocity observations and is compared to E·j measurements with the electric field deduced from the EFW instrument. The good agreement between these two independent measurements validates the quality of the technique. The energy flux is estimated during ten complete magnetopause crossings under steady southward and dawnward interplanetary magnetic field conditions (IMF). In most cases we find that there is no or little flow of energy across the magnetopause boundary as expected on the dayside magnetopause where the magnetic tension is mainly perpendicular to the solar wind flow. However, at some crossings when Cluster observes high-speed plasma jets energy it is found to be transferred from the magnetic field to the particles as a result of the magnetic reconnection process. Furthermore, it is possible to make estimations of the inflow reconnection velocity from the magnitude of the local energy estimate. This was found to be in reasonable agreement with the results from standard techniques. However, the inflow velocity as well as the energy transfer rate varies considerably during the course of the ten crossings. This may support the previous theory that the reconnection rate is continuously active but its rate is modulated.