Energetic Electron Acceleration in Unconfined Reconnection Jets

Magnetic reconnection in astronomical objects such as solar corona and the Earth's magnetotail theoretically produces a fast jet toward the object (known as a confined jet as it connects to the object through magnetic field lines) and a fast jet departing the object (known as an unconfined jet as it propagates freely in space). So far, energetic electron acceleration has been observed in the confined jet but never in the unconfined jet, arousing a controversy about whether or not reconnection jets can intrinsically accelerate electrons. By analyzing spacecraft measurements in the magnetotail, here we report three events showing strong electron energization in unconfined reconnection jets. Such energization, occurring in the growing phase of the jet, is quasi-adiabatic; it leads to 30 times of flux enhancements and it is probably caused by the compression of the magnetic field (betatron effect) as well as the shrinking of magnetic field lines (Fermi effect). We quantitatively reproduce this energization process using a 2.5D particle-in-cell simulation. This finding implies that electron acceleration can happen in the solar wind and magnetosheath, where reconnection jets are usually unconfined.