Multi-Scale Observations of Magnetic Reconnection at the Subsolar Magnetopause

Magnetic reconnection is a fundamental process occurring in astrophysical plasma that is responsible for the conversion of electromagnetic energy into energy of charged particles, leading to the acceleration of fast plasma jets, plasma heating and supra-thermal particle acceleration. Reconnection is inherently a multi-scale process where the breaking of the frozen-in condition at electron scales leads to the formation of jets at ion scales that eventually affect at MHD/fluid scales plasma regions far away from the reconnection site. Despite of the importance of the scale coupling, however, the basic physics has been poorly understood from an experimental point of view due to the lack of multi-scale multi-point measurements. Here we present novel Cluster spacecraft observations at the subsolar magnetopause, where two spacecraft are separated by ~30 km while being apart ~10000 km from the others, corresponding to sub-ion and MHD scales, respectively. This configuration allows to resolve, although only partially, the scale coupling during reconnection. Two-point measurements at sub-ion scales allow to identify a very thin rotational discontinuity (~ 10 km) and to study the electron dynamics therein while single point measurements at MHD scales simultaneously provide a monitor of the large-scale jet dynamics on both sides of the reconnection site. We discuss how our observations could be used to test and/or provide input to current models and numerical simulations. We also discuss the role of Cluster multi-scale measurements in terms of preparation for the science of NASA/MMS and of future multi-scale multi-point missions.