T. A. Kornilova

On separation of the potential and vortex parts of the magnetotail electric field

Abstract Development of magnetospheric substorms is controlled by variations of two components of the magnetospheric electric field: a potential field (Epm) and a vortex one (Evm). It is shown that the motion of auroral arcs located at closed magnetic field lines is determined by the Epm, field only, while the motion of arcs located at the field lines passing the reconnection (neutral) line in the plasma sheet is determined by the Evm, field only. The regularities of the auroral arc motion at various phases of the substorm are studied, and the energy budget of magnetospheric substorms is discussed.

Fine structure of auroras during auroral breakup according to the ground-based and satellite observations

The dynamics and fine structure of auroras before and during 60 auroral breakups, including pseudobreakups and breakups at moderate and high auroral activity, have been studied using the developed method for processing television images. The IMAGE and POLAR satellite and simultaneous ground images of auroras, ground magnetic data, and measurements of IMF and solar wind plasma parameters have been analyzed. The signatures that can be precursors of breakup have been found out in the auroral dynamics and morphology in the spatial—temporal vicinity of breakup. The morphological characteristics of auroral structures have been analyzed statistically. The directions of motion of weak subvisual structures have been determined. The velocities of motion of such structures are presented. The relation of the initial auroral arc bright-ening during breakups and pseudobreakups to the beginning of magnetic activation and formation of rayed structures has been analyzed.

Wave Structure of Magnetic Substorms at High Latitudes

A new type of high-latitude magnetic bays is revealed at geomagnetic latitudes higher than 71°, called “polar substorms.” It is shown that polar substorms differ from both classical substorms and high-latitude geomagnetic disturbances of the type of polar boundary intensifications (PBIs). While classical substorms start at latitudes below 67° and then expand poleward, polar substorms start almost simultaneously in the evening-night polar region of the oval. In contrast to PBIs, accompanied by auroral streamers expanding southward, polar substorms are accompanied by auroral arcs quickly traveling northward. It is shown that polar substorms are observed before midnight (20–22 MLT) under weak geomagnetic activity (Kp ∼ 2) during the late recovery phase of a magnetic storm. It is shown that a typical feature of polar substorms is the simultaneous excitation of highly intensive Pi2 and Pi3 geomagnetic pulsations at high latitudes, which exceed the typical amplitude of these pulsations at auroral latitudes by more than an order of magnitude. The duration of pulsations is determined by the substorm duration, and their amplitude decreases sharply at geomagnetic latitudes below ∼71°. It is suggested that pulsations reflect fluctuations in ionospheric currents connected with polar substorms.

Auroral intensification structure and dynamics in the double oval: Substorm of December 26, 2000

Based on results of the simultaneous TV observations at Barentsburg high-latitude observatory and Lovozero auroral observatory and using the IMAGE auroral luminosity images, the auroral fine structure and dynamics has been studied during the substorm of December 26, 2000, when the auroral luminosity distribution represented a double oval. It has been indicated that the interaction between the processes proceeding in different magnetospheric regions, the projections of which are the poleward and equatorward edges of the double oval, is observed in auroras in the process of substorm development.

Unusual spatial-temporal dynamics of geomagnetic disturbances during the main phase of the extremely strong magnetic storm of November 7–8, 2004

The spatial dynamics of geomagnetic variations and pulsations, auroras, and riometer absorption during the development of the main phase of the extremely strong magnetic storm of November 7–8, 2004, has been studied. It has been indicated that intense disturbances were observed in the early morning sector of auroral latitudes rather than in the nighttime sector, as usually takes place during magnetic storms. The unusual spatial dynamics was revealed at the beginning of the storm main phase. A rapid poleward expansion of disturbances from geomagnetic latitudes of 65°–66° to 74°–75° and the development of the so-called polar cap substorm with a negative bay amplitude of up to 2500 nT, accompanied by precipitation of energetic electrons (riometer absorption) and generation of Pi2–Pi3 pulsations, were observed when IMF Bz was about −45 nT. The geomagnetic activity maximum subsequently sharply shifted equatorward to 60°–61°. The spatial dynamics of the westward electrojet, Pi2–Pi3 geomagnetic pulsations, and riometer absorption was similar, which can indicate that the source of these phenomena is common.

High-latitude geomagnetic disturbances during the initial phase of a recurrent magnetic storm (from February 27 to March 2, 2008)

A complex of geophysical phenomena (geomagnetic pulsations in different frequency ranges, VLF emissions, riometer absorption, and auroras) during the initial phase of a small recurrent magnetic storm that occurred on February 27–March 2, 2008, at a solar activity minimum has been analyzed. The difference between this storm and other typical magnetic storms consisted in that its initial phase developed under a prolonged period of negative IMF Bz values, and the most intense wave-like disturbances during the storm initial phase were observed in the dusk and nighttime magnetospheric sectors rather than in the daytime sector as is observed in the majority of cases. The passage of a dense transient (with Np reaching 30 cm−3) in the solar wind under the southward IMF in the sheath region of the high-speed solar wind stream responsible for the discussed storm caused a great (the AE index is ∼1250 nT) magnetospheric substorm. The appearance of VLF chorus, accompanied by riometer absorption bursts and Pc5 pulsations, in a very long longitudinal interval of auroral latitudes (L ∼ 5) from premidnight to dawn MLT hours has been detected. It has been concluded that a sharp increase in the solar wind dynamic pressure under prolonged negative values of IMF Bz resulted in the global (in longitude) development of electron cyclotron instability in the Earth’s magnetosphere.

Auroral fading structure before breakup: A review

This review is devoted to auroral fading before beginning of the substorm active phase. This initial stage of the active phase called breakup is accompanied by a sharp brightening of auroras and their rush toward the pole. Auroral fading before breakup was first detected in discrete auroras in the nightside sector and consisted in that a short-term decrease in brightness of an arc moving toward the equator below the level observed during the preliminary phase was observed during the substorm preliminary phase 2–3 min before breakup. During fading, the velocity of equatorward motion of auroral arcs decreased up to their complete stoppage. Auroral fading in the noon sector was registered simultaneously with fading on the Earth’s nightside before the beginning of the active phase. Short-term background fading was also observed both equatorward and poleward of an arc on the nightside. It was subsequently indicated that similar fading is observed in various geophysical phenomena. It was detected that a radar aurora signal fades before breakup, if auroral substorm is observed in a radar pattern and substorm source is located under good aspect conditions. Riometer absorption decreases simultaneously with auroral fading. Geomagnetic pulsations decay on dayside and nightside immediately before breakup. Such a multiform manifestation of fading in various geophysical phenomena indicates that fading is related to some global processes proceeding in the magnetosphere when energy accumulation in this region comes to the end before its explosive release into the polar ionosphere.

East‐west type precursor activity prior to the auroral onset: Ground‐based and THEMIS observations

Using ground-based optical observations, we study an auroral breakup event, focusing on the wave-like signatures of the east-west (E-W) type auroral activities which appear before breakup. By conjunction with the Time History of Events and Macroscale Interactions during Substorms (THEMIS) P2 and P5 measurements, it is shown that the underlying wave mode can be identified as the ballooning mode. Considering the similarity of the wave-like characteristics derived from ground-based auroral and THEMIS spacecraft observations, we argue that the E-W activities under study may be related to ballooning waves propagating in the plasma sheet. The implications for mechanisms of substorm triggering are discussed.

On the physical nature of auroral breakup precursors as observed in an event on 5 March 2008

Using coordinated THEMIS spacecraft and all-sky imager observations, we studied an auroral breakup event on 5 March 2008, where auroral activities for 30–40 min before T0 were all of the East-West (E-W) orientation, and found that their dynamics infers a wave process. For the event under study, there were conjunctive measurements (with 3 s time resolution) of plasma, energetic particles, magnetic B and electric E fields by four THEMIS probes, positioned approximately along the tail. The THEMIS probe measurements, bandpass-filtered in the range 12–120 s, revealed the low-frequency wave activity in the considered time interval. The out-of-phase relation between variations in the magnetic and plasma pressures, along with a positive correlation between −∂Bx/∂t and z GSM component of ion velocity (flapping), indicated the ballooning mode. Considering the similarity of the wave-like characteristics derived from ground-based auroral and THEMIS spacecraft observations, we argue that the E-W auroral features preceding onset may be related to ballooning waves propagating in the plasma sheet, their wavefronts inclined at relatively small angles to the azimuthal direction. The implications for mechanisms of substorm triggering are discussed.

Peculiarities of the azimuthal propagation of perturbations in discrete auroral arcs during the substorm growth phase

The azimuthal propagation of luminosity inhomogeneities (of the bead type) within auroral arcs extended from east (E) to west (W) during the substorm growth phase is studied with high-precision groundbased optical observations at PGI observatories and THEMIS Canadian ground stations. The propagation velocities and directions are compared with the predictions of the known theories that were proposed in order to interpret this phenomenon. It is concluded that there is no unified theory capable of explaining the disturbance propagation peculiarities observed in different events.