Search

This work was supported by Russian Science Foundation grant 14-17-00072. The CLUSTER-based tail magnetic flux computation for years 2001–2009 was supported by FP7 ECLAT project, they are now available via CSA. We are grateful to all teams for the opportunity to use their observational data. We thank NASA CDAWeb (http://cdaweb.gsfc.nasa.gov/istp_public/) for IMF, solar wind, and magnetic index data; the AMPERE project (http://ampere.jhuapl.edu) for AMPERE data; the THEMIS website (http://themis.ssl.berkeley.edu/) for THEMIS spacecraft data, Cluster Science Archive (http://www.cosmos.esa.int/web/csa) for Cluster data; the IMAGE-FUV data were supplied by the NASA Space Science Data Centre(NSSDC). The global MHD simulations were made possible due to NASA Community Coordinated Modeling Center effort (http://ccmc.gsfc.nasa.gov/). S.E.M. was supported by the Science and Technology Facilities Council (STFC), UK, grant ST/K001000/1. The work at the Birkeland Centre for Space Centre, University of Bergen, Norway, was supported by the Research Council of Norway/CoE under contract 223252/F50. We also thank Stepan Dubyagin for help in calculating THEMIS total pressure, Peter Boakes for data on magnetotail plasma domains, and Marianna Kholeva for help with the manuscript preparation.

The ultimate formation mechanism of the substorm current wedge (SCW) remains to-date unclear. In this study, we investigate its relationship to plasma flows at substorm onset and throughout the following expansion phase. We revisit the case of September 8, 2002, which has been defined as "the best textbook example for a localized substorm onset observation" because of its excellent coverage by both spacecraft in the magnetotail and ground-based observatories is revisited. We found that a dense sequence of arrival of nightside flux transfer events (which can be understood as the lobe magnetic signature due to a bursty bulk flow travelling earthward in the central plasmasheet) in the near-Earth tail leads to a modulation (and further step-like built-up) of the SCW intensity during the substorm expansion phase. In addition, we found that small SCWs are created also during the growth phase of the event in association with another less intense sequence of NFTEs. The differences between the sequence of NFTEs in the growth and expansion phase are discussed. We conclude that the envelope of the magnetic disturbances which we typically refer to as an intense magnetic substorm is the result of a group or sequence of more intense and more frequent NFTEs.

We present a case study of eight successive plasma sheet (PS) activations (usually referred to as bursty bulk flows or dipolarization fronts), associated with small individual B-ZGSM increases on 31 March 2009 (0200-0900 UT), observed by the Time History of Events and Macroscale Interactions During Substorms mission. This series of events happens during very quiet solar wind conditions, over a period of 7 h preceding a substorm onset at 1230 UT. The amplitude of the dipolarizations increases with time. The low-amplitude dipolarization fronts are associated with few (1 or 2) rapid flux transport events (RFT, E-h > 2 mV/m), whereas the large-amplitude ones encompass many more RFT events. All PS activations are associated with small and localized substorm current wedge (SCW)-like current system signatures, which seems to be the consequence of RFT arrival in the near tail. The associated ground magnetic perturbations affect a larger part of the contracted auroral oval when, in the magnetotail, more RFT are embedded in PS activations (> 5). Dipolarization fronts with very low amplitude, a type usually not included in statistical studies, are of particular interest because we found even those to be associated with clear small SCW-like current system and particle injections at geosynchronous orbit. This exceptional data set highlights the role of flow bursts in the magnetotail and leads to the conclusion that we may be observing the smallest form of a substorm or rather its smallest element. This study also highlights the gradual evolution of the ionospheric current disturbance as the plasma sheet is observed to heat up.

Unprecedented spacecraft and instrumental coverage and the isolated nature and distinct step-like development of a substorm on 17 March 2010 has allowed validation of the two-loop substorm current wedge model (SCW2L). We find a close spatiotemporal relationship of the SCW with many other essential signatures of substorm activity in the magnetotail and demonstrate its azimuthally localized structure and stepwise expansion in the magnetotail. We confirm that ground SCW diagnostics makes it possible to reconstruct and organize the azimuthal spatiotemporal substorm development pattern with accuracy better than 1 h magnetic local time (MLT) in the case of medium-scale substorm. The Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE)-based study of global field-aligned current distribution indicates that (a) the SCW-related field-aligned current system consists of simultaneously activated R1- and R2-type currents, (b) their net currents have a R1-sense, and (c) locations of net current peaks are consistent with the SCW edge locations inferred from midlatitude variations. Thanks to good azimuthal coverage of four GOES and three Time History of Events and Macroscale Interactions during Substorms spacecraft, we evaluated the intensities of the SCW R1- and R2-like current loops (using the SCW2L model) obtained from combined magnetospheric and ground midlatitude magnetic observations and found the net currents consistent (within a factor of 2) with the AMPERE-based estimate. We also ran an adaptive magnetospheric model and show that SCW2L model outperforms it in predicting the magnetic configuration changes during substorm dipolarizations.

On 23 March 2009 between 6:00 and 6:40 UT, three Time History of Events and Macroscale Interactions during Substorms probes (P3, P4, and P5) were at about −11.5 Earth radii (RE) and two (P1 and P2) were at −14 RE downtail. The inner probes (P3–P5) started to observe oscillatory flow braking with plasma sheet dipolarization due to flux pileup at about 6:04 UT. After 6:16 UT the flux pileup region (FPR) expanded tailward as the outer probes (P1 and P2) moved closer to the neutral sheet and began to observe oscillatory braking also. During the FPR tailward expansion, the flow oscillation period increased from about 3.5 min at P3–P5 to about 6.2 min at P1 and P2. Meanwhile, as observed by the all-sky camera at Rankin Inlet, auroral activity gradually moved northward indicating that the characteristics of oscillatory flows at the tailward retreating FPR may be crucial for understanding the magnetosphere-ionosphere coupling.

[2011] during the events to find THEMIS footprints. We next statistically comparethe period and damping factor of the plasma sheet oscillating flows with those of themagnetic pulsations at the conjugate ionospheric locations.Magnetotail observations were provided by the probes’ fluxgate magnetometers (FGM)[

Flapping waves are most noticeable large-scale perturbations of the magnetotail current sheet, whose nature is still under discussion. They represent rather slow (an order of magnitude less than typical Alfven speed) waves propagating from the center of the sheet to its flanks with a typical speed of 20–60 km/s, amplitude of 1–2 Re and quasiperiod of 2–10 min. The double-gradient MHD model, which was elaborated in Erkaev et al. (2007) predicts two (kink and sausage) modes of the flapping waves with differences in their geometry and propagation velocity, but the mode structure is hard to resolve observationally. We investigate the possibility of mode identification by observing the rotation of magnetic field and plasma velocity vectors from a single spacecraft. We test theoretical results by analyzing the flapping oscillations observed by Time History of Events and Macroscale Interactions during Substorms spacecraft and confirm that character of observed rotation is consistent with kink mode determination made by using multispacecraft methods. Also, we checked how the existence of some obstructive conditions, such as noise, combined modes, and multiple sources of the flapping oscillations, can affect on the possibility of the modes separation with suggested method.

It is well established that fast flows in the magnetotail plasma sheet which are separated from the ambient plasma by dipolarization fronts brake in the tail-dipole transition region. Flow/front braking is suggested to play an important role in generation of compressional waves in the inner magnetosphere and geomagnetic pulsations. Because of the paucity of multipoint observations in the tail-dipole transition region, however, details of wave generation during flow/front braking are unknown. Using comprehensive coverage of the near-Earth plasma sheet and geostationary orbit by six spacecraft, we explore the relationship between dipolarization fronts that propagated earthward at x=−11 to −9RE and stopped at x=−9 to −8RE and compressional oscillations observed at x≈−8RE. The oscillations, which were diamagnetic (i.e., exhibited antiphase variations in magnetic and plasma pressures), were observed about a minute prior to front detection. The amplitude of the magnetic oscillations at −8RE was ∼5 nT; the wavelength was ∼0.5RE. Enhancements of magnetic oscillations with different frequencies and amplitudes of 1 to 2 and 2 to 4 nT were detected at geosynchronous orbit and on the ground, respectively. Analysis of observations reveals that although the fast flow/front stopped a few RE beyond geosynchronous orbit, the plasma compression propagated farther inward and excited compressional diamagnetic oscillations in the tail-dipole transition region.

Recent quantitative testing of the classical (region 1 sense) substorm current wedge (SCI) model revealed systematic discrepancies between the observed and predicted amplitudes, which suggested us to include additional region 2 sense currents (R2 loop) earthward of the dipolarized region (SCW2L model). Here we discuss alternative circuit geometries of the 3-D substorm current system and interpret observations of the magnetic field dipolarizations made between 6.6RE and 11RE, to quantitatively investigate the SCW2L model parameters. During two cases of a dipole-like magnetotail configuration, the dipolarization/injection front fortuitously stopped at r ~ 9RE for the entire duration of ~ 30 min long SCW-related dipolarization within a unique, radially distributed multispacecraft constellation, which allowed us to determine the locations and total currents of both SCW2L loops. In addition, we analyzed the dipolarization amplitudes in events, simultaneously observed at 6.6RE, 11RE and at colatitudes under a wide range of magnetograph conditions. We infer that the ratio I2/I1 varies in the range 0.2 to 0.6 (median value 0.4) and that the equatorial part of the R2 current loop stays at r>6.6RE in the case of a dipole-like field geometry (BZ0>75 nT at 6.6RE prior to the onset), but it is located at r

[1] Global magnetohydrodynamic (MHD) modeling is a powerful tool in space physics research. There are several advanced and still developing global MHD codes that are widely used to simulate plasma processes in solar wind magnetosphere-ionosphere system. The verification of global simulation codes is an important but a difficult problem. We present an approach for systematic and quantitative testing of code performance based on statistical empirical dependencies of the key magnetospheric parameters obtained from observations. We demonstrate the applicability of the method by testing the Grand Unified Magnetosphere Ionosphere Coupling simulation (GUMICS-4) global MHD model. A large set of nearly stationary solutions (162 runs altogether) with different stationary interplanetary magnetic field (IMF) and solar wind inputs were generated for different dipole tilts and levels of solar EUV radiation. As key parameters, we use the large-scale characteristics of the magnetosphere, including the magnetopause size and shape, geometry of the tail neutral sheet, magnetotail plasma pressure, tail lobe magnetic field, and cross-polar cap electric potential. We found that the GUMICS-4 stationary solutions generally fit the statistical relations, however, with some discrepancies. Particularly, position of the subsolar magnetopause, neutral sheet shape and position, and the plasma sheet pressure during northward IMF agree well with statistical models. At the same time, the size of the tail magnetopause and the lobe magnetic field magnitude appear to be systematically lower compared to their empirical values. Furthermore, the ionospheric potential is smaller in magnitude compared to empirical relations. These results provide an important starting point in the further development of the GUMICS simulation.

[1] We study the ionospheric response to oscillatory braking of bursty bulk flow observed by THEMIS on 17 March 2008 between 10:22 and 10:36 UT. By calculating different current components generated in the plasma sheet and correlating the space and ground observations, we discriminate the ionospheric current relevant to the large-scale substorm wedge currents produced by the general reconfiguration of the magnetotail pressure gradient from the currents that appeared as a result of the flow oscillation. While the former currents are large and quasi-stable, the latter (oscillating) currents are substantially (2–3 times) weaker and flow in opposite directions during earthward and tailward flow bursts. The oscillating currents include the polarization current and the current generated by the oscillating part of the pressure gradient. The two oscillating currents appear to produce modulation of the ionospheric currents (with about 2.5 min period) that was seen as Pi2 pulsations in the ground magnetometer observations. Our estimates of the ionospheric conductance suggest that the damping of the plasma sheet flow oscillation is due to heating the ionosphere through Pedersen currents. We also found that the all-sky imager at Fort Yukon observed four auroral forms during the first two periods of the oscillatory flow braking: two auroral forms related to the earthward plasma sheet flows and the other two auroral forms related to the tailward rebounds of the earthward flow. The auroral forms evolve in accordance with the appearance and motion of the upward field-aligned current spot of the modulated part of the ionospheric field-aligned current.

Flow burst events with a flux transfer rate exceeding 2 mV/m and with a duration of less than 10 min observed by Geotail are compared with auroral signatures obtained from the Polar ultraviolet imager. It is shown that all the flow bursts correspond either to localized auroral intensifications associated with small poleward expansions and pseudobreakups or to an activation starting at the poleward edge of the expanded auroral oval that develop equatorward toward the foot point of the satellite, including auroral streamers. Earthward flow bursts related to pseudobreakups and small expansions are observed mainly in the region earthward of 15 RE, more inward than those flows related to high-latitude auroral activations and auroral streamers. Although most of these auroral activations precede the observations of the flow bursts by a few minutes, the activations that break up near the foot point of the satellite start typically within ±1 min of the onset of flow burst observation.

On the basis of several years of Geotail data we performed a comprehensive statistical analysis of rapid convective transport in the neartail and midtail central plasma sheet. We chose a new approach by using flux transport and not ion bulk velocity as the threshold parameter for the identification of rapid flows. This criterion for rapid convection is independent of the radial distance from the Earth. We found that the occurrence rate of earthward rapid flux transport events was constant at radial distances > 15 R E and that it started to drop only earthward of 15 R E . Tailward rapid flux transport events with B z 0 could be seen at all distances. Their occurrence rate has a minimum at radial distances between 20 and 30 R E and increases earthward and tailward of that region. They are likely to have a different nature and different origins in the near-Earth region and in the midtail beyond about 25-30 R E . In close analogy to bursty bulk flows we defined rapid convection events by using the flux transport criterion instead of a velocity criterion. We found that rapid convection events transport about the same amount of mass, energy, and magnetic flux and have about the same duration at all radial distances between 10 and 50 R E . We found that rapid convection was responsible for 30-50% of the observed total transport of mass, energy, and magnetic flux past Geotail at all observed distances in the central plasma sheet.

Multiple and sporadic time-of-flight velocity dispersed ion structures (TDIS) are systematically observed above the ionosphere at ∼3 Re altitude by Interball/Auroral spacecraft near the poleward edge of the auroral bulge. These events represent direct snapshots of the impulsive ion acceleration process in the equatorial plasma sheet which allow us to study the details of the connection between ionospheric and plasma sheet manifestations of the magnetospheric substorm. Two events are analyzed during which the spacecraft footpoints passed over the Scandinavian ground network. We found that the TDIS correlate with the intensifications of westward current and auroral activations at the poleward edge of the bulge, which confirms the association of these dispersed ion beams with the temporal evolution of impulsive reconnection in the tail. Furthermore, we present direct evidence of an active neutral line in the magnetotail during one of the events using plasma sheet measurements made concurrently by the Interball/Tail and Geotail spacecraft. The 2–3 min repetition period of these ∼1 min long activations indicates a fundamental time constant of the substorm instability. On the other hand, the estimated injection distances of the energy-dispersed ions were inferred to be smaller than the estimated position of the reconnection region in the tail. We also found that the TDIS ion beams are released within the closed plasma flux tubes deep inside the plasma sheet, and yet they are synchronized with auroral activations at the poleward boundary. These facts imply that the ion beams are formed in a spatially extended region of the plasma sheet rather than in the close vicinity of the neutral line. We argue that braking of the reconnection-induced fast flow bursts when they interact with the closed plasma flux tubes and the earthward propagating fast wave electric field generated in the braking region may be important in forming the observed multiple, sporadic, energy-dispersed ion beams.

We report on a new feature of auroral substorms, namely, the sporadic though recurrent injections of magnetospheric ions throughout the auroral bulge. These injections are interpreted as time of flight dispersed ion structures (TDIS). Our analysis builds on a combination of measurements from Interball-Auroral, from UV imagery onboard Polar, from ground magnetometers, and also from observations on Geotail and from geostationary spacecraft. Backward tracing of ion trajectories from Interball-Auroral orbit using realistic three-dimensional magnetic and electric field models indicates that the injection region can extend over a wide range of radial distances, from ∼7–40 RE in the nearly equatorial magnetosphere. Both hydrogen and oxygen ions are shown to be injected toward the Earth's upper ionosphere. At Interball altitudes we find that ion injections are associated with two types of low-frequency torsional oscillations of the magnetic field: (1) shear Alfven waves with a period of a few minutes with the highest amplitude near the bulge front and decreasing amplitude at lower latitudes and (2) higher-frequency shear Alfven waves of the PlB type, strictly restricted to the poleward boundary of the surge, with a characteristic period of ∼40 s. The systematic observation of sporadic TDIS during the auroral bulge expansion leads us to conclude that the same physical process is at work throughout the midtail. We also show that ion injections are detected well inside the bulge, which suggests that the injection fronts propagate from the outer to the inner magnetosphere over large distances. This topic is more extensively studied by Sergeev et al. [1999]. We also show that the poleward boundary of the surge is associated with a prominent outflow of ionospheric H+ and O+. These ions in the hundred of eV to the keV range are heated perpendicularly to the local magnetic field and subsequently transported into the magnetotail. The expanding auroral bulge thus forms a significant source of ionospheric ions for the midtail magnetosphere.

We introduce and test a new approach suitable to model the magnetotail configuration during individual events. This Hybrid Input Algorithm (HIA) uses, in addition to spacecraft magnetic measurements in the tail, other complementary information (in this version, the isotropic boundaries of energetic particles observed at low altitudes), therefore increasing the amount of input data for the modeling and making models more accurate for mapping purposes as well as for evaluation of the current sheet thickness and current density. We test the HIA on two previously well-studied and modeled events, compare different models as well as the model- based and observation-based estimates for the plasma pressure and current density, and discuss the uncertainty in the resulting mapping. We apply the HIA-based models to evaluate the location of substorm onset in the tail during Coordinated Data Analysis Workshop 6A (CDAW-6A) substorm event and the characteristics of the current sheet at 7-15 Rjust before the substorm onset obtained for several other events. We found moderate values for the maximal current densities in the thin current sheet region (10-35 nA/m 2) and the minimal current sheet half thickness between 0.1 and 0.7 R. The X scale of the thinning region was sire 5 R. By assembling the modeling results for many events we found that the maximum current densities in the tail current sheet could be effectively predicted by taking into account the observed positions of isotropic boundaries and dipole tilt angle. We also briefly discuss possible future extension of the Hybrid Input Algorithm.

We present a comprehensive study of a sequence of two substorms and multiple pseudobreakups using optical, magnetic and incoherent scatter radar measurements, energetic particles from two geosynchronous satellites and particle and field data from the Geotail spacecraft located at Xgsm ∼ −86 RE. Following conventional nomenclature, we classified as pseudobreakups those auroral breakups which did not exhibit significant poleward expansion (< 2° magnetic latitude). Auroral intensifications following substorm breakups were also observed, and were classified separately. Pseudobreakups were found not to differ from substorm breakups in longitudinal extent (from 1.3 to 6.1 hours of magnetic local time), or in duration (from 5 to 16 minutes). In general, the ionospheric currents producing ground magnetic disturbances were more intense during substorms than pseudobreakups. We found that pseudobreakups are associated with the same magnetospheric processes as substorm breakups which involve current wedge formation, midlatitude magnetic Pi2 pulsations and energetic particle injections at the geosynchronous altitude. Moreover, pseudobreakups are associated with magnetic reconnection in the near-Earth region, evidenced by the typical subsequent detection of a plasmoid at Geotail. This implies that the magnetotail volume influenced by a pseudobreakup is quite large in radial distance. We conclude that there is no definitive qualitative distinction between pseudobreakups and substorms but there is a continuum of states between the small pseudobreakups and large substorms.

We present ground-based, plasma sheet, and magnetosheath observations of two subsequent short-duration (10–20 min) increases of the postmidnight westward electrojet on November 28, 1995. Appearing as though small (150–200 nT) substorms, they were not accompanied by any substorm expansion onset signatures. Auroral breakup, worldwide Pi2 pulsations, and the corresponding plasma sheet activity, such as fast flows, current disruption, and plasmoid generation, were all observed only at the recovery of the second electrojet increase. These convection bays were associated with the equatorward expansion of the auroras and simultaneous magnetic variations in the polar cap and middle latitudes. Growth phase signatures of the lobe field increase and tailward stretching of magnetic field were also observed in the plasma sheet. Bursty bulk flows in the plasma sheet seem to be quenched at the onset of first convection bay and did not resume until the auroral breakup which concluded the second convection bay. A point of interest of this event was the “incomplete” convection/current system with a well-developed dawn vortex in the absence of well-defined dusk vortex; instead, a complicated transient activity dominated over the afternoon-dusk local time sector. We interpret this asymmetry either in terms of the magnetopause encounter with the edge of the solar wind driver, i.e., strong southward IMF, which hits only the dawn part of the magnetosphere, or with an extremely slant interplanetary discontinuity. This unique configuration was inferred from observations of uncorrelated strong southward Bz events by the Wind and IMP 8 spacecraft in the dusk and dawn magnetosheath, respectively, as well as from the directional analysis of the interplanetary discontinuities which form the edges of these structures. We suggest that interaction of the magnetosphere with very slant solar wind discontinuities may bring various specific features to magnetospheric and ionospheric dynamics that have not been reported.

The origin of multiple energetic particle injections into the inner magnetosphere is addressed using a rare opportunity of measuring the energetic particle fluxes at different radial distances under known electric and magnetic fields. During a strong substorm on February 10, 1991, the CRRES spacecraft measured {ital E} and {ital B} fields and high-energy particle fluxes near the magnetic equator at r{approximately}5R{sub e}, whereas particle injections, their azimuthal locations, and some other parameters were simultaneously monitored by three geostationary spacecraft and ground networks. We show a multitude of impulsive short-duration injection events which correlate with 1{endash}2 min long pulses of dawn-dusk electric field. The observations suggest that some {ital E} field pulses recorded deep in the inner magnetosphere were fast magnetosonic waves radiated by the current disruption region. This supports the concept of impulsive dissipation event as an elementary building block of substorm expansion. Furthermore, our modeling results indicate that most of the flux variations of energetic particles can be explained by the global convective transport and corresponding particle acceleration. However, we emphasize that, depending on particle spectra and radial flux gradient, one can observe either flux increase, or decrease, or no variation (often seen in different energy ranges simultaneously andmore » at the same point) as a response to the electric field pulse. Both the cloud of injected particles and magnetic field dipolarization region had a sharp inner boundary (injection front) which propagated inward at the convection speed. We document the complicated structure of this front, consisting of a diamagnetic hot proton layer followed by the dipolarization front which contains enhanced energetic electron fluxes. Further study is required to understand how common this structure is and, if common, how it may be formed. {copyright} 1998 American Geophysical Union« less

Recently, efforts to characterize and monitor the state of the magnetosphere have intensified, along with the rising interest in space weather. The latitude of the ion energy flux precipitation maxima (“b2i”), which almost invariably occurs near the equatorward edge of the nightside main auroral oval, has been suggested as one such parameterization. It has been suggested that b2i corresponds to the ion isotropy boundary (IB), which has been independently researched as a measure of the extent to which the magnetotail is stretched. By comparing simultaneous observations by the Defense Meteorological Satellite Program (DMSP) and NOAA spacecraft, we confirm a close association between b2i and the isotropy boundary of 30 keV protons. Using 2.5 years of simultaneous data from DMSP and GOES spacecraft, we verified that magnetic field inclination (the extent to which the magnetotail is stretched) strongly controls the b2i/IB latitude. Based on use of the b2i latitude, corrected for local time variation, as an index of magnetic stretching in the tail to show a considerable dawn-dusk asymmetry, we find that the magnetic field is more depressed and stretched at dusk than at dawn, and asymmetry increases with increasing magnetotail stretching. This asymmetry is consistent with the rotation of the symmetry line of the b2i(MLT) curve toward premidnight hours and suggests the growth of a so-called “partial ring current” system with increasing activity. Finally, the utility of the b2i/IB boundary as a characterization of the state of the magnetosphere is shown by demonstrating that the average pressure in the magnetotail is better specified by b2i than by Kp.

At 1130 UT on November 28, 1995, two spacecraft, Interball-Tail and Geotail, were in a favorable position to study the plasma sheet activity and an auroral breakup observed on the ground near the spacecraft ionospheric footpoints. Both spacecraft were near the neutral sheet, and they were nearly aligned along the magnetic meridian. During the auroral breakup observed at the equatorward half of the auroral oval (also registered as an AKR burst at Interball) both spacecraft simultaneously detected signatures of a reconnection pulse: The earthward plasma streaming and magnetic field dipolarization were observed at 12 R E at Interball, while the tailward energetic ion beam, then the tailward flow and the passage of a plasmoid were observed at 28 R E at Geotail. This pulse seem to proceed inside of the plasma sheet closed field lines, in the region of small (∼ 1nT) background magnetic field at the neutral sheet. At Interball position the onset of fast earthward ion flow, likely initiated by the reconnection pulse, was followed by other manifestations (dipolarization, enhancements of the magnetic turbulence and the energetic particle flux, the intensification of field-aligned currents). Auroral observations showed initial brightening delayed an approximately 1 min after the commencement of the reconnection pulse. The auroral intensification was not accompanied by a significant magnetic disturbance on the ground, and therefore the event can be classified as the pseudobreakup. We estimate magnetic flux transport characteristics and possible location of the onset region in the plasma sheet. We conclude that observations during this event are consistent with the initiation of an auroral breakup by some disturbance (e.g., Alfven wave) generated by the reconnection pulse that commenced in the neutral sheet at ∼15 R E distance.

We document a clear case of spontaneous substorm onset under conditions of steady southward interplanetary magnetic field on January 22, 1993. The substorm occurred at 0415 UT with an intensification at 0455 UT during an interval of convection bay activity but had most of the features associated with substorms. These features include high-latitude and midlatitude magnetic bays at ground magnetometers, and magnetic field compression and energetic particle injection at geosynchronous altitude. In addition, a plasmoid release was observed in the distant tail plasma sheet by the Geotail spacecraft at a GSM position (-96, 1, -6) R E . We conclude that spontaneous-onset substorms occur occasionally even during steady convection bay events. Such substorms and classical substorms progress in a very similar manner, irrespective of possible differences in their onset mechanism.

Substorm phenomena are reviewed with emphasis on the magnetospheric source region of the onset, on the morphology of the initial breakup and subsequent activations, and on the variable character of individual substorms. We provide evidence that before the substorm onset and during the following activations an intense, thin current sheet is formed at the interface between the quasi-dipolar and taillike magnetic field regions. We infer that the initial breakup, the following multiple activations, pseudobreakups, and other short-term activations during nonsubstorm times are all similar in morphology and have the same formation mechanism. We postulate that the elementary units of energy dissipation, impulsive dissipation events, which are localized in space and have a short lifetime of ∼1 min, are the manifestations of tail reconnection. We also emphasize the evidence that previous authors have presented in favor of this time dependence and localization. On the basis of the above, we suggest that there are two basic magnetospheric processes responsible for energy storage and dissipation during both substorm and nonsubstorm times: A global and slow quasi-static tail reconfiguration responsible for the energy storage, and a sequence of local, sporadic, short-term energy dissipation events. These competitive processes can be observed most the time in some part of the plasma sheet; their relative intensity determines the type of large-scale dynamic evolution. In this scenario, the various dynamical situations are interpreted as variations in the balance between the two competing processes.

Recent studies have shown that most Earthward transport hi the midtail, high-beta plasma sheet takes place in the form of short-lived, high-speed plasma flow bursts. Bursty bulk flows are observed both when the plasma sheet is thin, such as during substorm expansion, and when it is thick, such as during substorm recovery. We present multi-instrument observations from the ISEE1 and ISEE 2 spacecraft to argue that when the plasma sheet becomes thick and close to its equilibrium state, the plasma and magnetic field signatures of high-speed flow events are consistent with the theoretically predicted signatures of plasma-depleted flux tubes or “bubbles” [Pontius and Wolf, 1990; Chen and Wolf, 1993]. These signatures consist of a decrease in the plasma pressure and an increase in the Bz-component of the magnetic field accompanying the high speed flow. We show that the Earthward moving bubbles are separated from the plasma ahead of them by a sharp tangential discontinuity. The layer ahead of the bubbles exhibits flow and magnetic field shear consistent with flow around an Earthward moving obstacle. The bubble is in approximate total pressure balance with the surrounding medium. We show that there is a systematic difference in the orientation of the discontinuity measured at ISEE 1 and 2, implying a small (about 1–3 RE) cross-tail size of the bubbles.

Optical images of the auroral bulge as seen by the Viking UV imager were compared in several cases with the substorm current wedge (SCW) upward and downward field-aligned currents (FAC) whose positions were determined using the inversion algorithm based on the substorm-related magnetic variations observed at midlatitudes. With reasonable accuracy (better than 0.5 hours MLT) the estimated longitudes of the upward FAC generally pointed to the surge or to the brightest luminousity region in the western half of the bulge. The latter feature may imply a more complicated structure of the net FACs than can be described by the simple substorm current wedge scheme. Similarly, the estimated positions of the downward FAC pointed close to the eastern termination of the bulge. The associated optical signatures of this current system ranged from the well-defined emission depletion regions to new auroral intensifications. The downward current appears to correspond in some cases at least with the division between the morning sector portion of the double oval and the nightside portion connected more directly to the substorm bulge. The results in general confirm the expected association between the auroral bulge and the SCW, as well as showing a reasonably good results from the inversion algorithm based on midlatitude magnetic observations. Our results, however, also indicate that one must be careful in interpreting the apparent motion of SCW-related field-aligned currents inferred from midlatitude observations in terms of a true westward or eastward expansion of the SCW or of the auroral bulge. The observed changes may instead sometimes be related to the redistribution of the net FACs within than a shift or expansion of the simple current system.

[1] We examine the pitch angle distribution (PAD) of suprathermal electrons (>40 keV) inside the flux pileup regions (FPRs) that are located behind the dipolarization fronts (DFs), in order to better understand the particle energization mechanisms operating therein. The 303 earthward-propagating DFs observed during 9 years (2001–2009) by Cluster 1 have been analyzed and divided into two groups according to the differential fluxes of the >40 keV electrons inside the FPR. One group, characterized by the low flux (F 40 keV electrons as the low-flux situation may lead to large uncertainties in computing the anisotropy factor that is defined asA = F⊥/F∥ − 1 for F⊥ > F∥, and A = −F∥/F⊥ + 1 for F⊥ 0.5), and 44 events have field-aligned distribution inside the FPR (A < −0.5). The perpendicular distribution appears mainly inside the growing FPR, where the flow velocity is increasing and the local flux tube is compressed. The field-aligned distribution occurs mainly inside the decaying FPR, where the flow velocity is decreasing and the local flux tube is expanding. Inside the steady FPR, we observed primarily the isotropic distribution of suprathermal electrons. This statistical result confirms the previous case study and gives an overview of the PAD of suprathermal electrons behind DFs.

[1] We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) and GOES observations to investigate the plasma sheet evolution on 28 February 2008 between 6:50 and 7:50 UT, when there developed strong magnetic field oscillations with periods of 100 s. Using multispacecraft analysis of the plasma sheet observations and an empirical plasma sheet model, we determine both the large-scale evolution of the plasma sheet and the properties of the oscillations. We found that the oscillations exhibited signatures of kinetic ballooning/interchange instability fingers that developed in a bent current sheet. The interchange oscillations had a sausage structure, propagated duskward at a velocity of about 100 km/s, and were associated with fast radial electron flows. We suggest that the observed negative gradient of the ZGSM magnetic field component (∂BZ/∂X) was a free energy source for the kinetic ballooning/interchange instability. Tens of minutes later a fast elongation of ballooning/interchange fingers was detected between 6 and 16 RE downtail with the length-to-width ratio exceeding 20. The finger elongation ended with signatures of reconnection in an embedded current sheet near the bending point. These observations suggest a complex interplay between the midtail and near-Earth plasma sheet dynamics, involving localized fluctuations in both cross-tail and radial directions before current sheet reconnection.

[1] We address the long-standing problem of the location and origin of the equatorwardmost Pre-Breakup auroral Arc (PBA) by combining energetic particle observations from NOAA Polar Operational Environmental Satellites (POES) overpasses of prebreakup arcs with auroral imaging and magnetospheric observations from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. The prebreakup arc was observed within a few minutes of auroral breakup and ∼1–2 hours in MLT from the breakup meridian. For three ideal conjunctions out of 16 PBA crossings, we also construct a dynamically-adapted magnetospheric model after adding concurrent magnetic observations by the GOES spacecraft. Model-predicted isotropy boundaries of energetic particles are compared with observations, informing us about model uncertainties. Direct mapping with adapted models as well as particle flux comparisons between the ionosphere and the magnetosphere confirm that the PBA source lies within the region of the steep equatorial magnetic field gradient, where the equatorial field is also small (5–20 nT). That equatorial location, at roughly 8–10 Re, is likely the earthwardmost edge of the thin cross-tail current sheet. From observations we find that the prebreakup arc nearly coincides in latitude with the energy-dispersed, 30–300 keV electron isotropy boundary. Here the non-adiabatic electron precipitation from the high flux region of the outer radiation belt near its outer edge produces a narrow, intense energetic electron precipitation region, called the energetic electron arc (EEA). Two fortuitous conjunctions with DMSP also confirm that energetic (>20 keV) EEA electrons and lower energy, inverted-V electrons associated with the PBA are collocated. We suggest that EEA formation is an inherent part of the PBA formation process. By creating an enhanced conductance strip, the EEA (a seed arc) produces ionospheric polarization that leads to field-aligned current generation and associated field-aligned electron precipitation. We also discuss implications of our findings for the substorm onset mechanism.

[1] Since the work of Dungey (1961), the global circulation pattern with two (dayside and nightside) reconnection regions has become a classic concept. However, the contributions of dayside and nightside sources to the cross-polar cap potential (PCP) are not fully understood, particularly, the relative role and specifics of the nightside source are poorly investigated both in quantitative and qualitative terms. To fill this gap, we address the contributions of dayside and nightside sources to the PCP by conducting global MHD simulations with both idealized solar wind input and an observed event input. The dayside source was parameterized by solar wind–based “dayside merging potential” Φd = LeffVBt sin4(θ/2), whereas to characterize the nightside source we integrated across the tail the dawn-dusk electric field in the plasma sheet (to obtain the “cross-tail potential” Φn). For the idealized run we performed simulations using four MHD codes available at the Community Coordinated Modeling Center to show that contribution of the nightside source is a code-independent feature (although there are many differences in the outputs provided by different codes). Particularly, we show that adding a nightside source to the linear fit function for the ionospheric potential (i.e., using the fit function Φfit = KdΦd + KnΦn + Φ0) considerably improves the fitting results both in the idealized events as well as in the simulation of an observed event. According to these simulations the nightside source contribution to the PCP has a fast response time (

[1] Although the substorm current wedge (SCW) is recognized as a basic 3-D current system of the substorm expansion phase, its existing models still do not extend beyond a cartoon-like sketch, and very little is known of how well they reproduce magnetic variations observed in the magnetosphere during substorms. A lack of a realistic quantitative SCW model hampers testing model predictions against large sets of spacecraft data. This paper (1) presents a computationally efficient and flexible model with a realistic geometry of field-aligned currents, conveniently parameterized by the SCW strength, longitudinal width, and position, all derived from ground-based midlatitude magnetic variations; and (2) tests the model against INTERMAGNET network observations during substorms and compares its predictions with space magnetometer data. The testing demonstrated significant and systematic discrepancies between the observed and predicted magnetic variations, depending on spacecraft location, concurrent magnetotail configuration, and substorm phase. In particular, we found that the net SCW current derived from the midlatitude field variations corresponds to only a relatively small and variable fraction of the distant 3-D substorm current, inferred from spacecraft data in the lobe and at geosynchronous distance. The discrepancy can be partly attributed to additional region 2 polarity field-aligned currents in the same longitudinal sector, associated with azimuthal diversion of the earthward plasma flow when it encounters the region of strong quasi-dipolar field in the inner magnetosphere.

[1] We analyze an isolated substorm on 29 March 2009 observed by the Thermal Emission Imaging System (THEMIS) and well monitored by ground-based observatories at and near station Gillam. The event provides a rare opportunity for monitoring the substorm magnetic topology thanks to fortuitous clustering of the THEMIS probes, complemented by the GOES 12 spacecraft. The neutral sheet position was found to be displaced by ∼0.5 RE northward from its average location. The peak cross-tail current density was estimated to be ∼20 nA/m2 at the end of the growth phase, revealing the formation of a thin current sheet during the last 15 min prior to the expansion onset. The fortuitous spacecraft conjunction allowed us to construct an adjusted time-varying model based on magnetic field and pressure observations during the substorm. We then used the adjusted model to map the location of the spacecraft to the ionosphere and the breakup from the ionosphere to the equatorial region. Significant time-dependent differences between this and the standard models (e.g., T96) do exist, resulting in breakup mapping to ∼22 RE, compared to 12 RE if classical models are used. Moreover, we find that spacecraft footprints in the ionosphere move significantly equatorward (2°) over tens of minutes during the growth phase but jump poleward (2°–4°) after expansion onset. Since such motions are also typical for auroral arcs during substorms, we infer that magnetic field reconfiguration during various substorm phases, rather than plasma motion in the equatorial magnetosphere, is largely responsible for the observed motion of the aurora.

[1] We present a fortuitous case of an isolated substorm on 29 March 2009, observed by the Time History of Events and Macroscale Interaction during Substorms (THEMIS) probes clustered at ∼11–14 RE with simultaneous coverage by the THEMIS ground network. The four probes are at roughly the same radial and azimuthal location, with one probe staying near the neutral sheet during entire growth phase and during the ensuing transition to the substorm expansion phase. Prior to substorm onset, THEMIS observed the damping of earthward convection and development of an embedded tail current sheet with half thickness ≤ 0.15 RE and current density ∼20 nA/m2, while the total magnetic field closest to the neutral sheet was below 2 nT. Tail activity was observed to start prior to substorm onset tailward of the THEMIS probes (

Simultaneous measurements of the low-altitude energetic particle flux by NOAA spacecraft and the geostationary magnetic field by GOES 2 spacecraft are used to test the recently proposed isotropic boundary algorithm (IBA) method to evaluate the instantaneous magnetospheric configuration. According to the IBA method, the equatorward boundary of the isotropic proton precipitation, in brief the isotropic boundary (IB), corresponds to the boundary separating adiabatic and chaotic regimes of particle motion in the tail current sheet and is controlled by the properties of the equatorial magnetic field. In this study we confirm some of the fundamental features of the IBA method. First, we show that the low-altitude IB position of 30- to 300-keV protons is strongly controlled by the equatorial magnetic field in the tail. (The corresponding correlation coefficient exceeds 0.9.) Second, the MLT dependence of the nightside IB latitude is in good agreement with that computed using magnetospheric models. Third, the observed magnetic field and the field predicted by the IBA method using the measured IB position have similar values and are well correlated with a correlation coefficient of at least 0.84 for the main components and a standard deviation of only about 10% of the dynamic range of these components. This shows that the threshold condition separating the two particle motion regimes is fulfilled in the proximity of the IB field line. We argue that the remaining inconsistencies between the calculated and observed magnetic fields are mainly due to the fact that the available magnetospheric models seem to underestimate the amount of tailward stretching of both the tail field lines during active conditions as well as field lines starting from the dayside. In view of its good capabilities to remotely determine the instantaneous magnetic field, we expect that the IBA method will find wide applications in the mapping of magnetic field lines and in testing of existing and new magnetospheric models.

The spatial dispersion pattern of particle precipitation during an event of sustained moderate magnetic activity was studied using observations of energetic (>30 kev) and auroral (0.3-20 keV) particles made by two low-altitude NOAA 6 and NOAA 7 spacecraft. The polar cap boundary (PCB, boundary between closed and open flux tubes) was identified on the nightside using the north-south asymmetry of solar energetic particle fluxes over the polar caps. The energy and species dependency of the latitude of the boundary of energetic particles in the vicinity of the PCB was found to be consistent with a simple scheme, described by (1) the accelerator which provides the >30-keV particle population operates at the outer boundary of the plasma sheet and (2) the particles that are transported across magnetic flux tubes equatorward (earthward) by the convection electric field during their flight over a distance of approximately 50 to 100 R[sub E] from the equatorial region down to the ionosphere. These findings agree with the concept of a far-tail transient reconnection process which gives rise to suprathermal particles. The poleward boundary of the total energy flux of keV electrons (at a threshold of 0.01 erg cm[sup [minus]2] s[sup [minus]1]) was often encountered at significantlymore » lower latitudes than the energetic proton boundary, however. The equatorward shift of the auroral electron boundary was as expected for the 5- to 10-keV protons, suggesting that there is not enough dense cold plasma in the outer tail plasma sheet to neutralize the electric space charge arising from the different convection trajectories of the earthward moving auroral protons and electrons; it appears that the auroral particles of both signs are forced to move together. The apparent convective displacement of auroral electrons can be as large as 2[degree] to 3[degree] in latitude, which could shift their poleward boundary some 200-300 km equatorward of the actual PCB. 28 refs., 3 figs.« less

[1] It is difficult to establish the degree to which global magnetospheric mapping models are accurate, because there exists no definitive, independent method of validating such models. Toward that end we use the isotropy boundary (IB) of precipitation of energetic particles, as determined by low-altitude spacecraft. These particles are observed at ionospheric altitudes but their precipitation is governed by the magnetic field near the equator. Precipitating and trapped fluxes measured at the ionosphere can thus be used to determine the equatorial field strength, which can in turn be compared with predictions of magnetospheric models. By using hundreds of IB observations at the ionosphere during THEMIS major tail conjunctions in 2008 we report on the mapping accuracy obtained using three models: T96, AM-01, and AM-02. The first model is driven by the simultaneous solar wind and Dst measurements, whereas the latter two are obtained by fitting model data to THEMIS observations. The AM-02 and T96 models show comparable agreement with proton IB locations, with error estimates of about 1° in latitude. However, the AM-02 outperforms T96 in predicting electron IB locations. Mapping errors increase with magnetic activity and have significant magnetic local time dependence. We conclude that event-based magnetospheric models can be as good as or better than solar wind-based models, provided that a number of distributed magnetotail spacecraft are used to constrain model parameters.

[1] Changes in plasma moments and entropy during dipolarizations are studied using Time History of Events and Macroscale Interactions during Substorms observations made near the neutral sheet at 6–12 RE on the nightside. Plasma tube entropy (pV5/3) at the observation point is estimated for the data set of 147 dipolarizations using the formula of Wolf et al. (2006). Plasma pressure began to increase shortly before dipolarization and proceeded without considerable entropy change until the arrival of the dipolarization front, after which major entropy changes occurred. We have found that on average, postdipolarization plasma pressure changes very little from predipolarization plasma pressure at r = 10–12 RE and increases only slightly in the near-Earth region. Although the associated plasma tube entropy always decreases in the region downtail of r = 8 RE, this decrease becomes smaller closer to the Earth. The local entropy estimate pn−5/3 shows a large increase, however, suggesting ∼40% reduction in flux tube particle content after dipolarization. Our statistical results provide a constraint for dipolarization theories and support the bubble model of dipolarizations.

[1] On 17 March 2008 around 0912 UT the five THEMIS spacecraft P1–P5 were in the plasma sheet between 2200 and 2300 h magnetic local time (MLT), covering radial distances between 15 Earth radii (Re) (P1) and 9 Re (P5). All the spacecraft consecutively observed a bursty bulk flow (BBF) that traveled earthward, slowed down from 400 km/s to 50 km/s between P1 and P5, and then turned in the opposite direction. The most tailward-located spacecraft, P1 and P2, detected thinning and then thickening of the plasma sheet around the time of the flow direction change. Meanwhile, the other three THEMIS spacecraft, which were located in a more dipolar region, observed plasma sheet thickening and then thinning. Observations indicated that the thinning/thickening was stronger around the BBF funnel. Further, during the interaction of the earthward-flowing BBF plasma with the Earth's dipolar field lines, the BBF was deflected by about 70° at a scale of about 5 Re. The radial pressure gradient was substantially increased when the BBF reached the shortest radial distance to the Earth and substantially decreased after the tailward plasma flow. We conclude that the tailward pressure pulse produced by the enhanced radial pressure gradients after the earthward BBF stopped could be responsible for the observed tailward plasma flows.

[1] We report on the evolving ion distributions associated with the arrival of an earthward propagating dipolarization front in the near-Earth magnetotail using Time History of Events and Macroscale Interactions during Substorms (THEMIS). Ion distributions exhibit steady duskward anisotropy well before the front arrival, suggesting thin current sheet formation at ∼11 RE, during the growth phase of a moderate geomagnetic substorm. As the dipolarization front moves closer, an additional, earthward streaming ion population appears, resulting in an earthward velocity moment. This population eventually overwhelms the preexisting duskward anisotropy and merges with the earthward convecting bulk flow once the dipolarization front arrives. Test-particle simulations show that the observed ion evolution is consistent with a picture of ions reflected and accelerated by the approaching front and moving ahead of it.

[1] Using auroral TV data and particle precipitation data from low-altitude satellites, we identify the ionospheric signature of magnetotail dipolarizations and substorm injections measured in the near-Earth near-equatorial plasma sheet by Time History of Events and Macroscale Interactions during Substorms (THEMIS). Field line mapping exploits a recently developed time-dependent adaptive model which minimizes the variance to THEMIS in situ magnetotail observations. We present strong evidence that the equatorward edge of the auroral bulge corresponds to the innermost extent of earthward propagating dipolarization fronts in the magnetosphere, whereas individual equatorward moving auroral enhancements correspond to the motion of individual injection fronts reaching at times distances as close to Earth as 5.5 RE. The region of tail dipolarization corresponds to the auroral bulge, a broad spatial region of enhanced but structured auroral emissions, bounded on the poleward side by discrete auroral forms and on the equatorward side by a sharp drop in auroral luminosity and particle precipitation. Particle precipitation within the bulge is enhanced considerably at the energies above 30 keV. Ionospheric protons are isotropic and electrons are anisotropic but with fluctuating fluxes which are below, but on occasion comparable with, trapped levels. The equatorward edge of the bulge, herein termed the “Equatorward edge of Auroral Bulge” propagates during substorm expansion toward lower latitudes, initially fast (corresponding to 100 km/s in space at r ∼ 7 RE) but with decreasing speed after onset. Our adaptive model mapping suggests that equatorial points at near-geosynchronous altitude can map to ionospheric magnetic latitudes up to 2°–3° off of predictions using standard T96 models. The offsets can be either toward lower latitudes due to field line stretching before auroral breakup or toward higher latitudes after breakup due to the near-Earth tail dipolarization.

[1] We suggest and test a simple procedure to adapt a magnetic field model by fitting it to observations made simultaneously by several spacecraft. This is done by varying input parameters of a standard model (T96) to find the best fit to the observed field at each time step. As a result we obtain a time-dependent model which can be used for evaluating the quality of the standard model and of the mapping at any particular time, to navigate in the magnetosphere and reproduce its variable configuration during large-scale dynamical events. This procedure was tested using observations made by five Time History of Events and Macroscale Interactions during Substorms (THEMIS) and other complementary (e.g., GOES) spacecraft during the tail season of THEMIS mission (January-March 2008), for which a simplest version of the adapted model was routinely calculated and has been made publicly available. We also use the proton isotropic boundaries observed by low-altitude NOAA spacecraft for independent evaluation of the obtained field models. We found that in quiet conditions deviations of ionospheric footprints between standard and adapted models are generally small (within 1° of latitude), whereas during substorms they may be as large as several degrees, because of stretching and dipolarizations of magnetospheric configuration. We found that the variable tilt of the tail current sheet, partly caused by variations of nonradial component of the solar wind flow, is an additional important factor influencing the modeling result and the mapping quality. By analyzing the adapted models constructed at the time of auroral breakup onset, we conclude that this simple approach is not yet sufficiently accurate to evaluate the source distance in the magnetotail.

We analyze observations of magnetotail current sheet dynamics during a substorm between 2330 and 2400 UT on 28 August 2005 when Cluster was in the plasma sheet at [-17.2, -4.49, 0.03] R-E (GSM) wit ...