Energy conversion at the terrestrial bow shock

At Earth’s bow shock, the supersonic solar wind is slowed down and deflected around the magnetosphere. To many this is "just a bow shock", a simple and quite passive element of solar-terrestrial physics. However, it has recently been realized that the bow shock plays a significantly more important role with currents on the bow shock connecting through the magnetosheath to the magnetospheric current systems. The bow shock current cannot close locally, since the magnetic field compression in the magnetosheath cannot be maintained globally. The bow shock current is inevitably a generator current extracting mechanical energy from the supersonic solar wind, and feeding it to other processes such as acceleration of the magnetosheath flow, local particle acceleration at the bow shock and dissipation in the distant ionosphere. Here we use data from the first dayside season of the Magnetospheric Multiscale (MMS) mission to investigate the generator properties of the terrestrial bow shock. Typically, the main shock ramp shows clear generator properties, but for some of the more turbulent bow shocks, generator properties may also be observed slightly downstream the ramp. This may be due to effects from shock motions and shock nonstationaity and reformation. Moreover, sometimes a weaker load can be seen in the upstream foot region due to local particle acceleration. We also find that the generator capacity of the bow shock decreases with decreasing bow shock angle as well as with increasing upstream plasma beta and solar Mach number. A better understanding of the energy conversion properties of the terrestrial bow shock will be useful also for the understanding of other astrophysical shock currents. The currents must close somewhere and deposit energy somewhere.