Observational Evidence of Magnetic Reconnection in the Terrestrial Bow Shock Transition Region

We report evidence of magnetic reconnection in the transition region of the Earth's bow shock when the angle between the shock normal and the immediate upstream magnetic field is 65 degrees. An ion-skin-depth-scale current sheet exhibits the Hall current and field pattern, electron outflow jet, and enhanced energy conversion rate through the nonideal electric field, all consistent with a reconnection diffusion region close to the X-line. In the diffusion region, electrons are modulated by electromagnetic waves. An ion exhaust with energized field-aligned ions and electron parallel heating are observed in the same shock transition region. The energized ions are more separated from the inflowing ions in velocity above the current sheet than below, possibly due to the shear flow between the two inflow regions. The observation suggests that magnetic reconnection may contribute to shock energy dissipation. Plain Language Summary Collisionless shock and magnetic reconnection are two fundamental plasma processes where significant energy conversion between electromagnetic fields and particles occurs. Knowledge is still lacking for whether reconnection occurs at the shock transition region and whether the reconnection property at the shock is different from that which occurs elsewhere. In this letter, we report the existence of reconnection at the Earth's bow shock. Many features are consistent with those in standard reconnection, such as the electron flow and field structures in the diffusion region, and the feature of ion and electron heating in the exhaust. The results suggest reconnection to be one energy dissipation mechanism at the shock, which encourages further investigation.