S.-S. Li

Electromagnetic Energy Conversion at Reconnection Fronts

Observing Earths Magnetotail Magnetic reconnection is a process that converts magnetic energy to kinetic energy, thermal energy, and particle acceleration. The process operates in Earths magnetotail, the narrow and elongated region of the magnetosphere that extends away from the Sun, and is believed to power Earths auroras and other space physics phenomena. Angelopoulos et al. (p. 1478) present an observational study of energy conversion in the magnetotail and the associated transport of magnetic flux during a geomagnetic substorm. Data from various satellites in Earth’s magnetotail clarify where and how electromagnetic energy conversion occurs. 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 1015 joules of magnetic energy, consistent with global magnetotail flux reduction.

On the origin of pressure and magnetic perturbations ahead of dipolarization fronts

Dipolarization fronts (DFs), earthward-propagating structures in the Earths magnetotail current sheet with sharp enhancements of the northward magnetic field Bz, are typically preceded by minor decreases in Bz. 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 Bz dip immediately ahead of the DF.

Antidipolarization fronts observed by ARTEMIS

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 BZ component have been noted in midtail (XGSM ~ −60 RE) 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 BZ bipolar variations with a larger northward portion. Using case and statistical studies from 3 years of Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moons Interaction with the Sun (ARTEMIS) data (at XGSM ~ −60 RE), 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 BZ 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 BZ 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.

Azimuthal extent and properties of midtail plasmoids from two-point ARTEMIS observations at the Earth-Moon Lagrange points

Although distant-tail plasmoids are perceived to extend across most of the magnetotail (~40 RE), recent studies in the near-Earth region (X > −30 RE) have revealed that near-Earth reconnection (where plasmoids originate) is likely localized and takes place preferentially on the duskside. This discrepancy in plasmoid azimuthal extent suggests that a plasmoid may grow as it moves from near-Earth to the distant tail. Comprehensive multipoint, midtail plasmoid observations can be used to test this hypothesis. Between October 2010 and July 2011 the ARTEMIS spacecraft (P1 and P2) at the Earth-Moon Lagrange points (midtail, X ~ −45 to −65 RE) provided simultaneous two-point observations across the magnetotail for 4 days every lunar month, with a large range of spacecraft separations (0.1 to 25 RE). We find that plasmoids near lunar orbit, like other near-Earth reconnection-related phenomena, occur preferentially on the duskside of the magnetotail. Two-point ARTEMIS observations reveal that the typical plasmoid azimuthal size in our data set is about 5 to 10 RE, much smaller than expected from previous distant-tail observations. Plasmoids with an azimuthal size greater than 9 RE also exist but only at geomagnetic activity levels higher (AEpeak > 400 nT) than typically found in our data set (median AEpeak ~ 230 nT for our plasmoid data set and median AE ~ 100 nT during the entire period of ARTEMIS magnetotail observations). We conclude that plasmoids during small to moderate substorms (AEpeak   400 nT), however, either grow beyond ~10 RE before they reach lunar distance or initially extend across a large portion of the magnetotail.