Magnetic Field Annihilation in a Magnetotail Electron Diffusion Region
with Electron-scale Magnetic Island
Abstract
We present observations in Earth’s magnetotail by the Magnetospheric
Multiscale spacecraft that are consistent with magnetic field
annihilation, rather than magnetic topology change, causing fast
magnetic-to-electron energy conversion in an electron-scale current
sheet. Multi-spacecraft analysis for the magnetic field reconstruction
shows that an electron-scale magnetic island was embedded in the
observed electron diffusion region (EDR), suggesting an elongated shape
of the EDR. Evidence for the annihilation was revealed in the form of
the island growing at a rate much lower than expected for the standard
collisionless reconnection, which indicates that magnetic flux injected
into the EDR was not ejected from the X-point or accumulated in the
island, but was dissipated in the EDR. This energy conversion process is
in contrast to that in the standard EDR of a reconnecting current sheet
where the energy of antiparallel magnetic fields is mostly converted to
electron bulk-flow energy. Fully kinetic simulation also demonstrates
that an elongated EDR is subject to the formation of electron-scale
magnetic islands in which fast but transient annihilation can occur.
Consistent with the observations and simulation, theoretical analysis
shows that fast magnetic diffusion can occur in an elongated EDR in the
presence of nongyrotropic electron effects. We suggest that the
annihilation in elongated EDRs may contribute to the dissipation of
magnetic energy in a turbulent collisionless plasma.