The THEMIS plasma instrument is designed to measure the ion and electron distribution functions over the energy range from a few eV up to 30 keV for electrons and 25 keV for ions. The instrument consists of a pair of "top hat" electrostatic analyzers with common 180°×6°fields-of-view that sweep out 4π steradians each 3 s spin period. Particles are detected by microchannel plate detectors and binned into six distributions whose energy, angle, and time resolution depend upon instrument mode. On-board moments are calculated, and processing includes corrections for spacecraft potential. This paper focuses on the ground and in-flight calibrations of the 10 sensors on five spacecraft. Cross-calibrations were facilitated by having all the plasma measurements available with the same resolution and format, along with spacecraft potential and magnetic field measurements in the same data set. Lessons learned from this effort should be useful for future multi-satellite missions.

The near‐Earth neutral line (NENL) model of magnetospheric substorms is reviewed. The observed phenomenology of substorms is discussed including the role of coupling with the solar wind and interplanetary magnetic field, the growth phase sequence, the expansion phase (and onset), and the recovery phase. New observations and modeling results are put into the context of the prior model framework. Significant issues and concerns about the shortcomings of the NENL model are addressed. Such issues as ionosphere‐tail coupling, large‐scale mapping, onset triggering, and observational timing are discussed. It is concluded that the NENL model is evolving and being improved so as to include new observations and theoretical insights. More work is clearly required in order to incorporate fully the complete set of ionospheric, near‐tail, midtail, and deep tail features of substorms. Nonetheless, the NENL model still seems to provide the best available framework for ordering the complex, global manifestations of substorms.

[1] We report THEMIS observations of a dipolarization front, a sharp, large-amplitude increase in the Z-component of the magnetic field. The front was detected in the central plasma sheet sequentially at X = À20.1 R E (THEMIS P1 probe), at X = À16.7 R E (P2), and at X = À11.0 R E (P3/P4 pair), suggesting its earthward propagation as a coherent structure over a distance more than 10 R E at a velocity of 300 km/s. The front thickness was found to be as small as the ion inertial length. Comparison with simulations allows us to interpret the front as the leading edge of a plasma fast flow formed by a burst of magnetic reconnection in the midtail.

Magnetospheric substorms explosively release solar wind energy previously stored in Earth's magnetotail, encompassing the entire magnetosphere and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing across the near-Earth magnetotail, at approximately 10 R(E) (R(E): Earth radius, or 6374 kilometers), or by the process of magnetic reconnection typically seen farther out in the magnetotail, at approximately 20 to 30 R(E). We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm onset. Reconnection was observed at 20 R(E), at least 1.5 minutes before auroral intensification, at least 2 minutes before substorm expansion, and about 3 minutes before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.

[1] Using Time History of Events and Macroscale Interactions during Substorms observations from four tail seasons, we study the three-dimensional structure of the dipolarization front current sheet (DFCS), which demarcates the magnetic boundary of a dipolarizing flux bundle (DFB, the strong magnetic field region led by a dipolarization front) in Earth's magnetotail. An equatorial cross section of the DFCS is convex; a meridional cross section is consistent with a dipolarized field line. The equatorial flow pattern in the ambient plasma ahead of the DFCS exhibits diversions of opposite sense on its evening and morning sides. The magnetic field perturbations are consistent with local field-aligned current generation of region-2 sense ahead of the front and region-1 sense at the front. The median thickness of the DFCS increases from 800 to 2000 km with increasing distance from the neutral sheet, indicating bundle compression near the neutral sheet. On a meridional cross section, DFCS's linear current density (1.2-1.8 nA/m) peaks~AE0.55 l from the neutral sheet (where l is the ambient cross-tail current sheet half-thickness, l~1.5 R E in our database). This peak, reminiscent of active-time cross-tail current sheet bifurcation noted in past studies, suggests that the intense but thin DFCS (10 to 20 nA/m 2 ) may be produced by redistribution (diversion) of the extended but weaker cross-tail current (~1 nA/m 2 ). Near the neutral sheet, the average DFCS current over the dipolarization front (DF) thickness is perpendicular to both the magnetic field interior to the DFB and the average field direction over the DF thickness. Away from the neutral sheet, the average current becomes progressively parallel to the internal field and the average field direction. The average current directions are indicative of region-1-sense field-aligned current on the DF. As few as approximately three DFBs can carry sufficient total current that, if redirected into the auroral ionosphere, can account for the substorm current wedge's peak current for a sizable substorm (~1 MA). A collapsing DFB could thus be an elemental substorm current wedge, or "wedgelet," that can divert a sizable portion of the cross-tail current into the auroral ionosphere.Citation: Liu, J., V. Angelopoulos, A. Runov, and X.-Z. Zhou (2013), On the current sheets surrounding dipolarizing flux bundles in the magnetotail: The case for wedgelets,

[1] A critical, long-standing problem in substorm research is identification of the sequence of events leading to substorm auroral onset. Based on event and statistical analysis of THEMIS all-sky imager data, we show that there is a distinct and repeatable sequence of events leading to onset, the sequence having similarities to and important differences from previous ideas. The sequence is initiated by a poleward boundary intensification (PBI) and followed by a north-south (N-S) arc moving equatorward toward the onset latitude. Because of the linkage of fast magnetotail flows to PBIs and to N-S auroras, the results indicate that onset is preceded by enhanced earthward plasma flows associated with enhanced reconnection near the pre-existing open-closed field line boundary. The flows carry new plasma from the open field line region to the plasma sheet. The auroral observations indicate that Earthward-transport of the new plasma leads to a near-Earth instability and auroral breakup ∼5.5 min after PBI formation. Our observations also indicate the importance of region 2 magnetosphere-ionosphere electrodynamic coupling, which may play an important role in the motion of pre-onset auroral forms and determining the local times of onsets. Furthermore, we find motion of the pre-onset auroral forms around the Harang reversal and along the growth phase arc, reflecting a well-developed region 2 current system within the duskside convection cell, and also a high probability of diffuse-appearing aurora occurrence near the onset latitude, indicating high plasma pressure along these inner plasma sheet field lines, which would drive large region 2 currents.

[1] We discuss results of a superposed epoch analysis of dipolarization fronts, rapid (dt < 30 s), high-amplitude (dB z > 10 nT) increases in the northward magnetic field component, observed during six Time History of Events and Macroscale Interactions during Substorms (THEMIS) conjunction events. All six fronts propagated earthward; time delays at multiple probes were used to determine their propagation velocity. We define typical magnetic and electric field and plasma parameter variations during dipolarization front crossings and estimate their characteristic gradient scales. The study reveals (1) a rapid 50% decrease in plasma density and ion pressure, (2) a factor of 2-3 increase in high-energy (30-200 keV) electron flux and electron temperature, and (3) transient enhancements of ∼5 mV/m in duskward and earthward electric field components. Gradient scales of magnetic field, plasma density, and particle flux were found to be comparable to the ion thermal gyroradius. Current densities associated with the B z increase are, on average, 20 nA/m 2 , 5-7 times larger than the current density in the cross-tail current sheet. Because j · E > 0, the dipolarization fronts are kinetic-scale dissipative regions with Joule heating rates of 10% of the total bursty bulk flow energy.

, can drive storm activity, but several outstanding questions remain concerning dropouts and the precise channels to which outer belt electrons are lost during these events. By analysing data collected at multiple altitudes by the THEMIS, GOES, and NOAA-POES spacecraft, we show that the sudden electron depletion observed during a recent storm's main phase is primarily a result of outward transport rather than loss to the atmosphere.Trapped radiation belt electrons undergo three characteristic types of motion: gyro-motion around magnetic field lines due to any velocity component perpendicular to the field, bounce-motion along field lines between magnetic mirror points due to any velocity component parallel to the field, and drift-motion around the Earth resulting from magnetic gradient and curvature drifts. Associated with each of these oscillatory motions are adiabatic invariants, which are conserved so long as electric and/or magnetic fields do not change on scales similar to those of the associated motions. The first invariant conserves the magnetic moment of the particle and is proportional to the perpendicular momentum squared divided by the local magnetic field strength; the second and third invariants conserve the integral of parallel momentum over one full bounce period and the magnetic flux through a particle's drift orbit, respectively. Magnetospheric changes on timescales much longer than electron drift periods are considered fully adiabatic, that is, they are fully reversible. Initially, it was thought that the observed flux dropouts were fully adiabatic changes in the system. Essentially, electrons moved radially outward (inward) during a storm's main (recovery) phase to conserve their third invariant as Earth's magnetic field was altered by the field produced by an enhanced (weakening) ring current 7,8 , which is a magnetospheric current system resulting from charge-dependent particle drift. As electrons moved radially outward (inward) in the field, their fluxes decreased (increased) for fixed energy as the first adiabatic invariant was also conserved. It was later shown that although this 'Dst effect' (after the disturbance storm time (Dst) geomagnetic index (Kp) used to indicate storm activity) does play a role in the flux dynamics, many flux dropouts do not return to the pre-storm flux level