Earth's magnetotail contains magnetic energy derived from the kinetic energy of the solar wind. Conversion of that energy back to particle energy ultimately powers Earth's auroras, heats the magnetospheric plasma, and energizes the Van Allen radiation belts. Where and how such electromagnetic energy conversion occurs has been unclear. Using a conjunction between eight spacecraft, we show that this conversion takes place within fronts of recently reconnected magnetic flux, predominantly at 1- to 10-electron inertial length scale, intense electrical current sheets (tens to hundreds of nanoamperes per square meter). Launched continually during intervals of geomagnetic activity, these reconnection outflow flux fronts convert ~10 to 100 gigawatts per square Earth radius of power, consistent with local magnetic flux transport, and a few times 10(15) joules of magnetic energy, consistent with global magnetotail flux reduction.

[1] We study the force balance surrounding the arrival of dipolarization fronts within bursty bulk flows near substorm onset by comparing curvature force densities and total pressure gradient force densities ahead of and behind the fronts using three inner Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes separated along the Xgsm and the Zgsm directions. Curvature force density estimates are obtained by field line modeling utilizing the Z separation of the probes and the self-similar structure of the front over short distances. A dipolarization front is a boundary between the energetic particle population in the flow burst magnetic flux bundle and the ambient colder plasma ahead of the front. Force density imbalance is found ahead of and behind the front. Ahead of the front, decrease in tailward pressure gradient force results in earthward flow acceleration. Behind the dipolarization front, even though the radius of field line curvature increases, the curvature force density increases even further, mostly due to the increase in the magnetic field magnitude. Thus, plasma acceleration at and immediately after the dipolarization front can be explained by the resultant increased curvature force density.

Dipolarization fronts (DFs), earthward-propagating structures in the Earth's magnetotail current sheet with sharp enhancements of the northward magnetic field B z , are typically preceded by minor decreases in B z . Other characteristic DF precursor signatures, including earthward flows and plasma density/pressure enhancements, have been explained in the context of ion acceleration and reflection at dipolarization fronts. In the same context here we simulate the spatial distribution of plasma pressure earthward of a convex DF. The resultant pressure distribution, which shows clear dawn-dusk asymmetries with greater enhancements at the DF duskside, agrees with statistical observations. The simulation further reveals that the reflected ions can carry a secondary current earthward of the advancing DF, which explains the characteristic signature of the B z dip immediately ahead of the DF.