T. A. Kornilova and I. A. Kornilov
Polar Geophysical Institute, Apatity, Murmansk region, Russia
M. I. Pudovkin and O. I. Kornilov
Institute of Physics, St. Petersburg State University, St. Petersburg, Russia
As known, diffuse luminosity is connected with unstructured precipitation of electrons and protons and it makes the main contribution to the energy balance of atmospheric radiation in the auroral zone [Ponomarev, 1985]. Diffuse luminosity dynamics and its relationship with electrojets and geomagnetic pulsations Pc1-2, IPDP, and Pi1 were investigated, for example, in Solovjev  and references therein. Large scale undulations often appear at the equatorial edge of diffuse luminosity, and their relation with Kelvin-Helmholtz instability was considered in Yamamoto et al. . Auroral torch structures at the polar boundary of diffuse luminosity were studied in Tagirov  and references therein. During the last decades, numerous measurements of characteristics of auroral energetic particle intrusions were carried out [Galperin and Feldstein, 1989; Lui et al., 1977; Valchuck et al., 1979; Winningham et al., 1975]. It allowed us to find out the peculiarities of the structure in the global picture of electron and ions injections, their link with field aligned currents, and different manifestations of activity in the magnetosphere. A lot of papers were devoted to the problem of discrete and diffuse auroras mapping into the magnetosphere [Galperin and Feldstein, 1989; Lui et al., 1977; Valchuck et al., 1979; Vorobjev et al., 2000; Winningham et al., 1975; Yahnin et al., 1997]. Nevertheless, this problem remains unsolved. Opinions differ as to the question in what regions of the magnetosphere are the polar aurora oval and diffuse luminosity projected? However, independently of the region where the diffuse luminosity is mapped, the processes occurring inside diffuse luminosity surely are connected with substorm development. Moreover, we suppose that they are different for various substorm onsets. In the present paper on TV auroral data, we investigate the behavior of diffuse luminosity during the growth phase of substorms and its relationship with explosive phase onset.
This paper is organized as follows. In section 2, we describe TV camera characteristics and method used for processing of TV data. In section 3, we present experimental data. The summary and conclusion are given in section 4.
Auroral data have been recorded by SIT-vidicon TV cameras with Nicon all-sky lens. No aurora emissions filters were used, so camera spectral response was about 3500-7000Å, with a maximum near 5000Å. Computer framegrabber videocard and special software allowed us to digitize TV frames (up to 720512 pixels) and arbitrary chosen frame fragments. Keograms along the stable profile of any width, shape, size, orientation or moving together with an auroral arc are also available with a time resolution up to 20 milliseconds. Data are always digitized with a maximum possible resolution (50 TV fields per second) and before they are recorded in computer memory they are summarized and averaged both cross keogram profile and in keogram time interval, which allows strongly expanded dynamic range of the data and enhances weak details of aurora luminosity afterwards [Kornilov and Kornilov, 1997]. Different image processing methods also can be used (gradient, convolution, filtering, warp image geometry, and TV tube field sensitivity corrections and so on) [Kornilov, 1999]. Filtering methods reveal a lot of details often invisible in initial data and use information obtained during the bad observational conditions (fog, clouds, twilight, city lights, etc).
TV data discussed in this paper have been processed by a very simple and efficient gradient filtering. Displayed keogram matrix is a result of subtraction of the initial matrix and matrix shifted in vertical or horizontal direction for the desired number of pixels. If, for example, an initial keogram profile was oriented from north to south, the resulting vertical gradient image amplifies weak details of auroral forms moving in a north-south direction, and horizontal gradient emphasizes temporal variations of auroral luminosity.
The TV auroral data used in this study were obtained at Porojarvi (65.6o N, 105.9o E), Kalkkoaivi (65.6o N, 105.4o E), and Lovozero (64.07o N, 114.7o E) during Russian-Finnish experiments in 1993-1995, 1997 as well as during continuous observations in 1996-1998 at Loparskaya (64.7o N, 116.3o E) and Lovozero. (Coordinates of stations are corrected geomagnetic coordinates). Dynamics of auroral forms and diffuse luminosity behavior were analyzed for the growth and explosive phases of 18 substorms. The onset of explosive phase of substorm was defined by auroral data as brightening and poleward expansion of the equatorial arc. Magnetic disturbance was controlled by magnetic data of Loparskaya, Lovozero, and Scandinavian IMAGE Magnetometer Network. Also available, POLAR UVI data were attracted to control the ground observation of auroras and define their place in the global auroral picture.
The data analysis and special TV images processing described in section 2 allow us to reveal fine effects in the dynamics and structure of diffuse luminosity, and some common features have been found as well.
3. An interesting feature of diffuse luminosity is pulsations. We have found three types of auroral pulsations occurring in diffuse luminosity in the course of the growth phase: (A) arc-like filaments pulsating with the period ~3-10 s, moving or motionless inside diffuse luminosity, (B) auroral pulsations of the southern boundary of diffuse luminosity with the period of ~30-40 s, (C) auroral arc-like filaments pulsating with the period 30-50 s inside a well localized region, which we called "resonator." "Resonator," usually situated on the south boundary of diffuse luminosity, represents a band of width of 100-200 km stretched along geomagnetic latitude. Pulsating, arc-like filaments emerge every ~30-70 s at the southern boundary of "resonator" and move to its northern one with the velocity 0.2-0.8 km s -1. Simultaneously, the same pulsating filaments appear at the northern boundary of "resonator" and move southward. As a result, an intricate interference picture arise that looks like waves moving behind the ship and reflecting from the walls of channel.
Figure 4d shows the interval of 1920-1925 UT just before the breakup onset (1923:48 UT) and ~2 min after it. From the moment of breakup onset, the arc in the zenith begins to move poleward, the curls drifting westward along the arc, and variations of luminosity intensity are manifested in the time gradient keogram as pulsations with the period of ~10-17 s. Inside the diffuse luminosity southward from the arc situated near zenith there are fast pulsations with the period of ~5 s. The southern boundary of diffuse luminosity pulsates with the average period of ~36 s.
The data analysis shows that in the course of growth phase in diffuse luminosity, one or two types of pulsations may be observed simultaneously. Some attempt was done to reveal a connection between auroral pulsation types during the growth phase and the poleward expansion types according to classification given in Kornilova et al. [. As shown in Kornilova et al. [, the poleward expansion may be of two types. The first one is represented by leaped formation of new arcs into more polar latitudes. The poleward expansion during the second type looks like spreading of shining auroral region having irregular structure consisting of patches or arc fragments without a definite orientation. The patches and arc fragments are embedded within the diffuse luminosity region. Unfortunately, we do not have enough data for a decisive conclusion, but nevertheless, the analysis of available data has shown that before the first type of poleward expansion pulsations of (B) and (C) types, more often are observed. The pulsations of (A) type precede the second poleward expansion type.
The present work is, to our knowledge, the first where the fine structure of diffuse luminosity is studied using the methods of TV image filtering. This method allowed us to reveal unknown earlier fine affects in the dynamics and brightness variations of diffuse luminosity. Results from these studies can be summarized as follows:
1. During the substorm growth phase, variations of diffuse luminosity brightness with the period of ~15-20 min are observed.
2. An interchange of homogenous diffuse luminosity and appearance of structures inside it with the period of ~10 min for the moderate magnetic activity and about 15 min for the weak one take place.
3. During the growth phase, three types of pulsating structures are observed in diffuse luminosity: (A) arc-like filaments pulsating with the period ~3-10 s inside diffuse luminosity, (B) auroral pulsations of the southern boundary of diffuse luminosity with the period ~30-40 s, and (C) auroral arc-like structures pulsating with the period of 30-50 s and appearing with the period of 30-70 s at the south boundary of the localized "resonator" region and spreading northward simultaneously with the same structures appearing at the north boundary and spreading southward.
4. During the weak auroral activity, pulsations of brightness and their motion are observed in subvisual diffuse background.
5. Different types of poleward expansion during substorm explosive phases are preceded by different types of pulsations at the growth phase. Poleward expansion represented by leaps of arcs into higher latitudes is usually preceded by pulsations of (B) and (C) types; poleward expansion looking like spreading of bright diffuse region with internal structure is preceded by pulsations of (A) type.
We suppose that study of auroral pulsation types observed in diffuse luminosity during substorm growth phase allow investigation of wave processes in the plasma inhomogeneities regions and their connection with the explosive phase onsets. Different pulsation types and their localization inside diffuse luminosity can be a result of plasma gradients existence (density, temperature, energy, and so on). Probably, these pulsations manifest an interaction between different magnetosphere regions (for example, the inner boundary of the plasma sheet and the plasmapause) and so can give important information about magnetosphere structure.
Galperin, Yu. I., and Ya. I. Feldstein, Diffuse auroral zone. X. The nightside diffuse auroral zone, auroral oval of discrete forms and diffuse luminosity poleward from the oval, as the projections of the plasma domains of the magnetospheric tail, Kosmich. Issled. (in Russian), 27 (6), 890, 1989.
Kornilov, I. A., Some methods of TV data processing. Physics of auroral phenomena, Abstracts of the XXII Annual Apatity Seminar "Physics of auroral phenomena," Apatity, 23-26 March 1999, p. 49, 1999.
Kornilov, I. A., and O. I. Kornilov, Computer-based equipment for TV, VLF, magnetic and other videotape recorded data processing, in Instruments and methods of geophysical experiment, pp. 91-96, Polar Geophysical Institute, Apatity, 1997.
Kornilova, T. A., I. A. Kornilov, M. I. Pudovkin, and O. Kornilov, Two types of auroral breakups, Proc. International Conference on Substorm 5, St. Petersburg, Russia, 16-20 May, 2000, pp. 307-310, 2000.
Lui, A. T. Y., et al., Simultaneous observations of particle precipitation and auroral emission by the ISIS 2 satellite, J. Geophys. Res., 82, 2210, 1977.
Ponomarev, E. A., Mechanisms of the Magnetospheric Substorms (in Russian), 159 pp., Nauka, Moscow, 1985.
Solovjev, S. I., Geomagnetic pulsations and fine structure of the magnetospheric disturbances, Ph.D. thesis, St. Petersburg, 354 pp., 1996.
Tagirov, V. R., Auroral torch structures: Results of optical observations, J. Atmos. Terr. Phys., 55, 1775, 1993.
Valchuck, T. E., et al., Diffuse auroral zone, IV. Latitudinal picture of auroral particles precipitation into the ionosphere and the plasma sheet structure in the magnetotail, Kosmich. Issled. (in Russian), 17 (4), 559, 1979.
Vorobjev, V. G., et al., Variations of location of plasma penetration boundary and auroral luminosity in the night sector, Geomagn. Aeron. (in Russian), 40 (3), 79, 2000.
Winningham, J. D., F. Yasuhara, S.-I. Akasofu, and W. J. Heikkila, The latitudinal morphology of 10 eV to 10 KeV electron fluxes during magnetically quiet and disturbed times in the 2100-0300 MLT sector, J. Geophys. Res., 80, 3148, 1975.
Yahnin, A. G., V. A. Sergeev, B. B. Gvozdevsky, and S. Vennerstrom, Magnetospheric source region of discrete auroras inferred from their relationship with isotropy boundaries of energetic particles, Ann. Geophys., 15, 943, 1997.
Yamamoto, T., K. Makita, and C.-I. Meng, A particle simulation of large-amplitude undulations on the evening diffuse auroral boundary, J. Geophys. Res., 96 (A2), 1439, 1991.