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~À 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~À 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.

Two fundamentally different drivers of dipolarizations at Saturn

“…Considered a boundary between reconnection outflow and the ambient plasma, TDFB is typically accompanied by quick density drop and particle heating, while a CRDD does not correspond to a sharp boundary of plasma populations. The morphological difference is similar to the comparison of antidipolarization and plasmoids presented in Li et al [].…”

Near‐Earth Reconnection Ejecta at Lunar Distances

“…These characteristics are similar to those of DFBs observed at R > 15 R E by THEMIS (Runov et al, ). It is notable that the positive B z excursion preceding the fronts, which is clearly visible in the ADF statistics (Li et al, ) and in the example shown in Figure , is not evident in the superposed B z in Figure b. This is likely because of our choice of t 0 : the time delay between the B z >0 interval and t 0 varies from event to event and does not leave a signature in the superposed epoch plot.…”

“…In contrast to the earthward DFB‐associated flows in the near‐Earth plasma sheet (see Figure 2 in Runov et al, ), the tailward flows on average begin more than 100 s ahead of the B z front and continue more than 300 s behind the front. It was shown that the tailward plasma flow speed starts to increase, on average, 2 to 3 min prior to the ADF detection time (Li, Liu, et al, , Zhou et al, ). This precursor flow may be explained by a compression of ambient plasma ahead of the ADF or by reflection of ambient ions from the ADF (Zhou et al, ).…”

“…Tailward flows with embedded small north, then strong south magnetic variations are considered counterparts of the earthward DFBs and interpreted as tailward outflows from near‐Earth reconnection. The sharp negative B z variation also referred to as “antidipolarization fronts” (ADFs; Li et al, ; Zhou et al, ) or “reconnection fronts” (Angelopoulos et al, ). It was shown that these fronts are followed by minute‐long enhancements in | B z | ( B z <0) and E y >0, which are tailward counterpart of the DFBs.…”

“…It was shown that these fronts are followed by minute‐long enhancements in | B z | ( B z <0) and E y >0, which are tailward counterpart of the DFBs. The magnetic flux transfer rate during these enhancements typically exceeds 2 mV/m (Li et al, ), which has been previously used as the threshold value for earthward rapid flux transport (RFT; Schödel et al, ). Therefore, we refer these structures to as tailward RFTs.…”