M. V. Kubyshkina

Earthward flow bursts, auroral streamers, and small expansions

Earthward flow bursts associated with small auroral expansions, including pseudobreakups, and auroral streamers are studied by using Geotail plasma and magnetic field data and Polar ultraviolet imager data. These flow bursts are accompanied by dipolarization and decrease in the plasma pressure, which are consistent with the characteristics of so-called bubbles, and have a timescale of 2.5 min on average. Based on a statistical study of the flow bursts, it is shown that the location of the flows are centered about 0.4 hour magnetic local time east of the center of auroral expansion and are localized with a width of 3 – 5 RE. This relationship supports the idea that a dawn-to-dusk polarization electric field is created in the bubble to enhance the flows. The flow bursts associated with the small expansions, which are mainly observed in the region earthward of 15 RE, show more distinct signatures of compression at the front side of the flow, which possibly leads to the stopping of these flows. Flow bursts related to auroral streamers, which are observed mainly tailward of 15 RE, take place during relatively thick plasma sheet configurations, and are accompanied by stronger flow shear.

Flow bursts and auroral activations: Onset timing and foot point location

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.

Ionospheric current signatures of transient plasma sheet flows

The plasma flow in the central plasma sheet of the magnetospheric tail often includes short impulsive bursts. Here we investigate the ionospheric signatures of such bursts. The Wind satellite recorded several transient fast flows in the plasma sheet (at geocentric distances of ∼ 12 RE) on December 21–22, 1995. The data are compared with magnetic field observations made in the Scandinavian sector, at the ionospheric conjugate point of the satellite. Superposed epoch analysis of the satellite data suggests that most of the events are Earthward flow bursts accompanied by magnetic dipolarizations, increases in the convection electric field, and drops in the plasma pressure and density. Occasionally, also isolated tailward flow bursts within closed flux tubes can be observed. We demonstrate that in both cases the transient plasma sheet flows are systematically associated with distinct ground magnetic field variations which (after 90° rotation) have specific vortex-like spatial distributions. The vortex patterns have similar duration to that of the flows at Wind and their longitudinal extent (≤1 hour in local time) is consistent with the azimuthal scale sizes (∼3 RE) of the transient flows reported earlier. In many cases the sense of flow rotation observed at Wind and at Winds ionospheric footpoint agree with our expectation. Despite the care taken in accounting for the instantaneous and local currents that affect the mapping, uncertainties in the footpoint location may still be responsible of the absence of a higher degree for compliance with theory.

Toward adapted time‐dependent magnetospheric models: A simple approach based on tuning the standard model

[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.

Two distinct substorm onsets

At times, substorms occur in a particular sequence, where a clear growth phase is followed by a first onset at lower latitudes and a second one involving all latitudes between 60° and 70°. The second onset is followed by a clear recovery phase. In the present paper we present nine such sequences in the form of a superposed epoch analysis. Comparison with solar wind data shows that the second onset occurs when the interplanetary magnetic field turns northward. Determination of the change in the open polar cap magnetic flux using the magnetogram inversion technique method indicates that the first onset occurs during an interval of continuous loading of the tail with new open flux merged at the dayside, Hence, despite showing clear ionospheric signatures of substorm expansion, this onset likely involves the reconnection of closed plasma sheet field lines only, while during the second onset and expansion phase, reconnection clearly proceeds from closed plasma sheet to open lobe field lines.

Substorm and convection bay compared: Auroral and magnetotail dynamics during convection bay

Using observations from eight spacecraft and a ground network, we study two subsequent bay-like disturbances on December 10, 1996, initiated by southward interplanetary magnetic field intervals, one being a classic substorm and another one a convection bay. Both events showed enhanced convection and Dst decreases as well as Pi2 pulsations in the auroral zone. Contrasting to the well-defined substorm signatures of the first event (poleward auroral expansion, substorm current wedge, strong particle injection to 6.6 R E ) resulting from energy loading/unloading and near-Earth reconnection in the tail, these signatures were virtually absent during the convection bay (CB). Distinctive features of the CB event were the same as those during the Steady Magnetospheric Convection intervals: (1) wide double oval at the nightside; (2) thick plasma sheet, relaxed lobe field, and enhanced magnetic flux closure (large B z ) and multiple bursty earthward flows (BBFs) in the midtail; (3) sporadic narrow soft injections to 6.6 R E ; (4) auroral streamers associated with both BBFs and narrow injections. We emphasize the development of multiple and sporadic auroral streamers which start at the poleward oval boundary, propagate equatorward (in 3-8 min) and end with a long-duration bright spot in the equatorward oval. We conclude that the plasma sheet and auroral dynamics during the convection bay was formed by sporadic narrow (a few R E wide) plasma streams (plasma bubbles) which transported the plasma sheet material from the distant magnetotail reconnection regions to the inner magnetosphere and may significantly contribute to the magnetospheric circulation on the nightside. We modeled the nightside tail configuration using magnetotail magnetic observations and low-altitude particle boundaries to show that at the beginning of the convection bay the increase of magnetic flux tube volume with distance was small in the midtail. Therefore the pressure crisis in the tail was significantly reduced during the convection bay, and the efficient earthward transport by sporadic narrow plasma streams was probably able to balance the magnetospheric circulation to avoid the large-scale instability of the magnetotail.

Entry of plasma sheet particles into the inner magnetosphere as observed by Polar/CAMMICE

Statistical results are presented from Polar/CAMMICE measurements of events during which the plasma sheet ions have penetrated deeply into the inner magnetosphere. Owing to their characteristic structure in energy-time spectrograms, these events are called “intense nose events.” Almost 400 observations of such structures were made during 1997. Intense nose events are shown to be more frequent in the dusk than in the dawn sector. They typically penetrate well inside L = 4, the deepest penetration having occurred around midnight and noon. The intense nose events are associated with magnetic (substorm) activity. However, even moderate activity (AE = 150–250 nT) resulted in formation of these structures. In a case study of November 3, 1997, three sequential inner magnetosphere crossings of the Polar and Interball Auroral spacecraft are shown, each of which exhibited signatures of intense nose-like structures. Using the innermost boundary determinations from these observations, it is demonstrated that a large-scale convective electric field alone cannot account for the inward motion of the structure. It is suggested that the intense nose structures are caused by short-lived intense electric fields (in excess of ∼1 mV/m) in the inner tail at L=4–5.

Period and damping factor of Pi2 pulsations during oscillatory flow braking in the magnetotail

[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)[

Contribution from different current systems to SYM and ASY midlatitude indices

Using empirical magnetospheric models, we study the relative contribution from different current systems to the SYM and ASY midlatitude indices. It was found that the models can reproduce ground-based midlatitude indices with correlation coefficients between the model and real indices being ∼0.8–0.9 for SYM-H and ∼0.6–0.8 and ∼0.5–0.7 for ASY-H and ASY-D, respectively. The good agreement between the indices computed using magnetospheric models and real ones indicates that purely ionospheric current systems, on average, give modest contribution to these indices. The superposed epoch analysis of the indices computed using the models shows that, nominally, the cross-tail current gives the dominant contribution to SYM-H index during the main phase. However, it should be remembered that the model region 2, partial ring current, and cross-tail current systems are not spatially demarcated (the systems are overlapped in the vicinity of geostationary orbit). For this reason, this result should be taken with a precaution. The relative contribution from symmetric ring current to SYM-H starts to increase a bit prior or just after SYM-H minimum and attains its maximum during recovery phase. The ASY-H and ASY-D indices are controlled by interplay between three current systems which close via the ionosphere. The region 1 FAC gives the largest contribution to ASY-H and ASY-D indices during the main phase, though, region 2 FAC and partial ring current contributions are also prominent. In addition, we discuss the application of these results to resolving the long-debated inconsistencies of the substorm-controlled geomagnetic storm scenario.

Event study combining magnetospheric and ionospheric perspectives of the substorm current wedge modeling

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.