I. A. Kornilov

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.

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.

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.

On the possibility of coupling satellite and ground-based optical measurements in the region of pulsating auroras

The possibility of comparing in detail CLUSTER satellite data with data on pulsating spots registered with a TV camera at the Lovozero Observatory when the satellite crossed the magnetospheric region related to the camera’s field of view is discussed. The satellite ionospheric projections were calculated using the T89, T96, and T01 models. It was shown that the projection allows us to judge with confidence whether or not a satellite will find itself in a region of pulsating auroras when only the level of geomagnetic activity and the characteristics of the interplanetary medium are a priori known. When different models are used in the projection, the spread is not less than the characteristic dimensions of the pulsating spots and can be as high as 100 km. The corresponding satellite flight time is ∼4 min. Such a large spatial and time uncertainty does not allow us to compare in detail the satellite data with ground-based optical measurements without a priori information on, e.g., the character of precipitation above a spot, as has been done by other researchers in the case of auroral arcs. The situation becomes even more complex if a satellite is in the region of greatly stretched magnetic field lines.

Spatial-temporal dynamics of auroras during the magnetic storm main phase

The structure and dynamics of auroras in the midnight sector during substorms, which develop during the magnetic storm main phase as compared to the characteristics of a typical auroral substorm, have been studied using the ground-based and satellite observations. It has been found out that a difference from the classical substorm is observed in auroras during the magnetic storm main phase. At the beginning of the storm main phase, the series of pseudobreakups with the most pronounced jump-like motion toward the equator shifts to lower latitudes. The substorm expansion phase can be observed not only as arc jumps to higher latitudes but also as an explosive expansion of a bright diffuse luminosity in all directions. During the magnetic storm main phase, auroras are mainly characterized by the presence of stable extensive rayed structures and by the simultaneous existence of different auroral forms, typical of different substorm phases, in the TV camera field of view.

Localization of the source of precipitating electrons in active arcs during breakup and in pulsating auroras

The sensitive method for detecting and measuring the velocity of a weak luminosity wave, traveling from bottom to top along an arc or isolated auroral beams, has been developed. This wave is caused by dispersion of precipitating electrons over velocities and by a differential atmospheric penetration of different-energy electrons, and the wave velocity gives information about the location of the electron acceleration region in the magnetosphere. The method was tested using different model signals and was used to study pulsating auroras and auroral breakup. A luminosity wave has been detected in pulsating auroras, and it has been estimated that the injection region is located at a distance of 5–6 Re. The application of the method to intensification of auroras during breakup indicated that such a wave is absent; i.e., breakup electrons being accelerated near the ionosphere at altitudes of 2000–8000 km. It has been assumed that the regions of anomalous resistance, generated in the ionosphere by field-aligned currents during the breakup phase, cause intense local field-aligned electric fields. These fields accelerate thermal electrons and form the auroral breakup pattern.

Nighttime subvisual high-latitude auroras

Special methods for processing TV images have been used to study the characteristics of nighttime auroras based on the observations at high-latitude observatories on Spitsbergen. Weak subvisual auroras (SVAs), originating 3°–4° north of brighter auroras in the auroral oval, have been detected in the interval 1900-0400 MLT. The average lifetime of SVAs is approximately 7 min, and the average velocity of the equatorward shift is ∼0.6 km/s. SVAs were observed during relatively quiet periods, when the IMF Bz component is mainly positive. However, SVAs are not polar-cap auroras since they are oriented from east to west rather than toward the Sun. The optical observations indicate that the SVA intensity is 0.2–0.5 and 0.1–0.3 kR in the 630 and 557.7 nm emissions, respectively. The average ratio of the emission intensities (I5577/I6300) is about 0.5. According to the direct satellite observations, the SVA electron spectrum has a maximum at 0.4–1.0 keV. In this case the energy flux of precipitating electrons is approximately an order of magnitude as low as such a flux in brighter auroral arcs in the auroral oval.