Antidipolarization fronts observed by ARTEMIS

Abstract Near‐Earth reconnection on closed plasma sheet field lines is thought to generate plasmoids. A plasmoid is usually described as a plasma sheet expansion into the lobe, encompassed by closed magnetic loops or the helical fields of a flux rope (in this paper we do not distinguish plasmoids from flux ropes; rather we use the term plasmoid generically). Recently, sharp, highly asymmetric north‐then‐south bipolar variations (with a larger southward portion) in the magnetic field B Z component have been noted in midtail ( X GSM ~ −60 R E ) plasmoids. These variations do not fit the classical plasmoid model but are mirror images of earthward moving dipolarization fronts (DFs), which show asymmetric south‐then‐north B Z bipolar variations with a larger northward portion. Using case and statistical studies from 3 years of Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) data (at X GSM ~ −60 R E ), we show that magnetic and particle properties of these typically tailward moving fronts, which we refer to as “antidipolarization fronts” (ADFs), are very similar to those of classical, typically earthward moving DFs, except for their B Z polarity and flow direction. First, like DFs and plasmoids, ADFs are associated with auroral electrojet enhancements. Second, like DFs, ADFs exhibit a sharp density decrease, plasma pressure increase, magnetic pressure increase, and particle heating immediately following the sharp B Z change. Third, particle spectra indicate that, as with DFs, there are two distinctly different magnetically separated populations ahead of and behind ADFs. The energy spectrograms of plasmoids, however, indicate a single hot population at the plasmoid center. We conclude that midtail ADFs are likely products of fast reconnection, observed on the tailward side of the reconnection site, just as DFs are products of fast reconnection seen on the earthward side. ADFs are observed at ARTEMIS much less frequently (~10%) than typical plasmoids but twice as frequently as DFs at the same distance. We suggest that ADFs are protoplasmoids that emerge from near‐Earth reconnection and evolve quickly into plasmoids as they propagate down the tail.