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.

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.

As a direct result of magnetic reconnection, plasma sheet fast flows act as primary transporter of mass, flux, and energy in the Earth's magnetotail. During the last decades, these flows were mainly studied within X GSM >−60 R E , as observations near or beyond lunar orbit were limited. By using 5 years (2011–2015) of ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moons Interaction with the Sun) data, we statistically investigate earthward and tailward flows at around 60 R E downtail. A significant fraction of fast flows is directed earthward, comprising 43% ( v x >400 km/s) to 56% ( v x >100 km/s) of all observed flows. This suggests that near‐Earth and midtail reconnection are equally probable of occurring on either side of the ARTEMIS downtail distance. For fast convective flows ( v ⊥ x >400 km/s), this fraction of earthward flows is reduced to about 29%, which is in line with reconnection as source of these flows and a downtail decreasing Alfvén velocity. More than 60% of tailward convective flows occur in the dusk sector (as opposed to the dawn sector), while earthward convective flows are nearly symmetrically distributed between the two sectors for low AL (>−400 nT) and asymmetrically distributed toward the dusk sector for high AL (<−400 nT). This indicates that the dawn‐dusk asymmetry is more pronounced closer to Earth and moves farther downtail during high geomagnetic activity. This is consistent with similar observations pointing to the asymmetric nature of tail reconnection as the origin of the dawn‐dusk asymmetry of flows and other related observables. We infer that near‐Earth reconnection is preferentially located at dusk, whereas midtail reconnection ( X >− 60 R E ) is likely symmetric across the tail during weak substorms and asymmetric toward the dusk sector for strong substorms, as the dawn‐dusk asymmetric nature of reconnection onset in the near‐Earth region progresses downtail.

The reconstruction problem for steady symmetrical two-dimensional magnetic reconnection is addressed in the frame of a two-fluid approximation with neglected ion current. This approach yields Poisson's equation for the magnetic potential of the in-plane magnetic field, where the right-hand side contains the out-of-plane electron current density with the reversed sign. In the simplest case of uniform electron temperature and number density and neglecting the electron inertia, Poisson's equation turns to the Grad-Shafranov one. With boundary conditions fixed at any unclosed curve (the satellite trajectory), both equations result in an ill-posed problem. Since the magnetic configuration in the reconnection region is highly stretched, one can make use of the boundary layer approximation; hence, the problem becomes well-posed. The described approach is generalized for the case of nonuniform electron temperature and number density. The benchmark reconstruction of the PIC simulations data has shown that the main contribution for inaccuracy arises from replacing Poisson's equation by the equation of Grad-Shafranov. Under this substitution, the reachable cross-size of the reconstructed region is shrinking down to fractions of the proton inertial length. Artificial smoothing, demanded by solving the ill-posed problem, and boundary layer approximation represent two alternative methods of problem regularization. In terms of the reconstruction error, they perform nearly the same; the second method benefits from the comparative simplicity and less restrictions imposed on the boundary shape.

[1] On 21 October 2010, ARTEMIS spacecraft P2, located at about À57 R E GSM in the Earth's magnetotail, observed a series of flux ropes during the course of a moderate substorm. Subsequently, ARTEMIS spacecraft P1, located about 20 R E farther downtail and farther into the lobe than P2, observed a series of TCRs, consistent with the flux ropes observed by P2. The dual-spacecraft configuration allows simultaneous examination of these phenomena, which are interpreted as an O-line, followed by a series of flux ropes/TCRs. An inter-spacecraft time of flight analysis, assuming tailward propagation of cross-tail aligned ropes, suggests propagation speeds of up to $2000 km/s. A principal axis investigation, however, indicates that the flux ropes were tilted between 41 and 45 in the GSM x-y-plane with respect to the noon-midnight meridional plane. Taking this into account, the tailward propagation speed of the different flux ropes is determined to be between 900 and 1400 km/s. The same timing analysis also reveals that the flux rope velocity increased progressively from one flux rope to the next. A clear correlation between the magnetic field and plasma flow components inside the flux ropes was observed. As possible mechanisms leading to the formation of tilted flux ropes we suggest (a) a progressive spreading of the reconnection line along the east-west direction, leading to a boomerang-like shape and (b) a tilting of flux ropes during their formation by non-uniform reconnection with open field lines at the ends of the flux ropes. The progressive increase in the propagation velocity from the first to the last flux rope may be evidence of impulsive reconnection: initially deep inside the plasma sheet the reconnection rate is slow but as reconnection proceeds at the plasma sheet boundary and possibly lobes, the reconnection rate increases.

[1] We present a method to determine the location of the reconnection site and the amount of reconnected magnetic flux out of an analytical time-dependent reconnection model and apply this method to disturbances observed on 2 February 2008 at about 0200 and 0815 UT by THEMIS B (P1). During these events, P1 detected two tailward propagating traveling compression regions, associated with typical variations in B z and B x . We find the reconnection site to be located at about À16 R E for the event at 0200 UT and À17.5 R E for the event at 0815 UT. These locations are consistent with simple timing considerations with respect to disturbances detected by the inner THEMIS spacecraft. The amount of reconnected flux in our 2-D model can be found to be in the order of 10 8 nT m for both events. The calculations for the reconnection site's location are done by using two approaches, i.e., by using the B z and the B x signals, yielding consistent results. The reconnected flux can be determined using B z and v z . Also, these results are in good agreement. A comparison between the disturbances detected by P1 and the modeled variations shows that our model describes disturbances in the magnetic field and the background plasma very well.

Fast magnetic reconnection is an explosive plasma process, bringing the topological reconfiguration of magnetic fields, plasma heating, and acceleration in laboratory and space plasmas (e.g., Gonzalez & Parker, 2016;Yamada et al., 2010). In general, this is a time-dependent multi-scale three-dimensional process (e.g., Bhattacharjee, 2004;Dorfman et al., 2013;Frank, 1999;Xiao et al., 2006), but sometimes reconnection may demonstrate a symmetric configuration and be quasi-stationary. Particularly, at the day side of the Earth's magnetopause, quasi-stationary reconnection has been detected in-situ on several occasions (e.g., Gosling et al., 1982;Phan et al., 2004;Retinò et al., 2005) as well as anti-parallel reconnection (Cassak & Fuselier, 2016, and references therein). The latter is more common in the Earth's magnetotail (Paschmann et al., 2013). It is also important that in many cases reconnection can be studied analytically in the frame of two-dimensional models for the considerable length of the reconnection X-line. Configurations with short X-lines demonstrate spreading in the X-line direction (see, e.g., Li et al., 2020, and references therein) in course of time. At last, both at the dayside magnetopause and in the magnetotail (e.g., Cassak &

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