Cold Ionospheric Ions in the Magnetic Reconnection Outflow Region

Magnetosheathplasma usually determines properties of asymmetric magnetic reconnectionat the subsolar region of Earth's magnetopause. However, cold plasma that originated from the ionosphere can also reach the magnetopauseand modify the kinetic physics of asymmetric reconnection. We present a magnetopausecrossing with high-density (10–60 cm−3) cold ionsand ongoing reconnection from the observation of the Magnetospheric Multiscale (MMS) spacecraft. The magnetopausecrossing is estimated to be 300 ioninertial lengths south of the X line. Two distinct ionpopulations are observed on the magnetosheathedge of the ionjet. One population with high parallel velocities (200–300 km/s) is identified to be cold ion beams, and the other population is the magnetosheath ions. In the deHoffman-Teller frame, the field-aligned magnetosheath ionsare Alfvenic and move toward the jet region, while the field-aligned cold ion beamsmove toward the magnetosheathboundary layer, with much lower speeds. These cold ion beamsare suggested to be from the cold ionsentering the jet close to the X line. This is the first observation of the cold ionospheric ionsin the reconnection outflow region, including the reconnection jet and the magnetosheathboundary layer.