Detailed information on transients is important to study the plasma dynamics. In the Earth's magnetosphere, these transients are reflected in very beautiful and changing forms of auroras, which give a unique opportunity to study processes in the magnetosphere-ionosphere system by their manifestations in spatial distribution of auroral luminosity. Precipitation of the auroral particles, their penetration in the high-latitude atmosphere and the subsequent response of the ionosphere play an important role in the energy balance of the magnetosphere-ionosphere plasma. Modern optical technique makes it possible to record the auroral forms with good temporal and spatial resolution. However, information about the spatial dynamics of auroral phenomena is still far from being fully used. Moreover, the causes and mechanisms of auroral structuring on a small scale, less than 10 km on the level of the ionosphere, are still the subject of debate.
The project is aimed at solving one of the most urgent tasks of the magnetosphere-ionospheric physics - the problem of the origin of auroral structures.
The main aims of the project::
(1) Precise optical observations of small-scale (0.1-10 km) auroral structures.
(2) Search of the statistical (including scaling) characteristics of small-scale auroral structures.
(3) Basing on the experimental results, testing different models of the auroral structuring.
Currently, the nature of the interaction of the solar wind with the Earth magnetosphere is well understood for large-scale processes, which are determined mainly by the orientation of the interplanetary magnetic field relative to the Earth's magnetic field and the dynamic pressure of the solar wind. However, the manifestations of this interaction on small scales require further research. Typically, the turbulent, chaotic nature of small-scale spatial and temporal perturbations in the magnetosphere-ionospheric system (Kintner, 1976; Kintner and Seyler, 1985; Antonova and Ovchinnikov, 1999; Borovsky and Funsten, 2003; Chang et al., 2004; Voros et al., 2004). This typically turbulent nature of perturbations in the plasma greatly complicates the interpretation of local observations by Earth satellites, both for the purpose of identifying plasma processes (instabilities and waves) associated with these perturbations, and for describing the dynamics of the system as a whole. This explains the interest in the observations of auroras, which gives a two-dimensional projection of the dynamic processes occurring in the magnetosphere-ionosphere system.
At present, all-sky camera systems are an integral part of the ground support of satellite observations (i.e. ALIS system, THEMIS ground support), but they are intended primarily for the study of large-scale processes (with characteristic length scales> 1 km and temporal scales> 10 c). Observations with a large spatial and temporal resolution are usually performed only when carrying out active influences on the atmosphere and/or sounding rocket projects.
Until recently, this was due to the complexity of storing and processing a large number of video data. At the same time, there are fundamental physical limitations on the possibility of recording the small-scale spatial-temporal structure of auroras, which are determined by the ratio of such factors as the luminescence intensity in the observed wavelength range, the sensitivity of the detector, the optical gain of the optical system, the lifetimes of the excited states of the atmospheric constituents, “movement" of auroral forms, etc.
Since early work, based on visual observations, many authors have noted self-similarity in the optical aurora (Hallinan and Davis, 1970; Oguti, 1975; Trondsen and Cogger, 1998). Thus, Oguti noted the similarity of auroral forms on different scales observed by high-sensitivity television cameras, which presupposes the existence of simple similarity laws, which are subject to auroral dynamics. In relatively recent works (Uritsky et al., 2002; Kozelov, 2003; Kozelov et al., 2004; Kozelov and Rypdal, 2007), self-similarity (scale invariance) in the dynamics of aurorae during the explosive substorm phase was considered as evidence of self-organized criticality and intermittent turbulence in the magnetospheric plasma. Such states are characterized by a power-law form of statistical dependencies of certain characteristics on the spatial/temporal scale.
The connection of the spatial structure of auroras with the perturbations of the plasma in the magnetosphere is still an open problem. Although it is known that the main energy of the most active substorm transients is contained in the magnetospheric plasma layer (Yahnin et al., 2006), the source of small-scale structures is not fully understood. A definite class of bright auroral structures located on closed lines of force is generated by fluxes of accelerated electrons. On sufficiently large scales (> 50-100 km) corresponding to inverted-V structures, electrons are accelerated in static electric fields arising at heights of ~ 1 Re in regions of strong field-aligned currents, and at small scales (~ 0.1-10 km) - in electric fields connected with waves. Some additional information on the structuring of plasma at small scales was given by observations of electric and magnetic fields by low-altitude satellites, in particular, the Freja and FAST satellites [Stasiewicz, 2000].
Beginning in the mid-1970s, a number of papers have given evidence (in most cases based on the power-law falling of the Fourier spectra of satellite data) that small-scale electric and magnetic fields in the high-latitude ionosphere are turbulent in nature (Kintner, 1976; Weimer et al., 1985; Basu et al., 1988; review (Antonova, 2002) and references there). Later, more correct methods of statistical physics and wavelet analysis have demonstrated that small-scale electric fields in the auroral zone and the polar cap are a manifestation of intermittent turbulence that develops in regions of large-scale field-aligned currents (Tam et al., 2005; Golovchanskaya et al., 2006; Kozelov and Golovchanskaya, 2006; Kozelov et al., 2008). This conclusion was based on the presence of the scale invariance (scaling) properties of the investigated fluctuations and the non-Gaussian form of the probability density function of the fluctuations.
When comparing scaling characteristics of auroral fluctuations and electric field fluctuations, it is necessary to take into account that in the first case the spatial and temporal nature of the fluctuations can be reliably distinguished, whereas in the case of fields observed by one satellite, one can distinguish between spatial and temporal fluctuations only by making additional assumptions. In a number of cases, such assumptions are justified (Kozelov et al., 2008). It should also be noted again that at present there is no universal theoretical model linking the fluctuations of the electric field observed on low-altitude satellites with mechanisms of longitudinal electron acceleration leading to optical aurora (Borovsky, 1993). The situation is complicated by fundamentally different physics, which determines the magnetospheric-ionospheric interaction on large and small scales. For large scales of several tens of kilometers, the magnetospheric generator is a voltage generator, the magnetospheric electric fields are slightly distorted when transmitted to the ionosphere, and the electrostatics approximation is performed with good accuracy (Lysak, 1985; Vickrey et al., 1986; Weimer et al., 1987). On the contrary, for small scales the magnetospheric generator is a current generator, static electric fields are strongly attenuated during transmission to the ionosphere, and waves play a significant role in the magnetospheric-ionospheric interaction (Weimer et al., 1985; Ishii et al., 1992). In this case, a special role belongs to dispersion (kinetic and inertial) Alfven waves, in which there is a longitudinal component of the electric field, capable of leading to an acceleration of electrons (Dubinin et al., 1988; Knudsen et al., 1990; Stasiewicz, 2000; Pokhotelov et al., 2003).
Another kind of auroral structures are the pulsating auroras observed in the morning sector, which are usually associated with generation of VLF chorus emissions, with variations in the amplitude of chorus emissions and auroras usually showing close quasiperiods of about 10-30 s. The mechanism of formation of pulsating auroral spots and their connection with VLF choruses was proposed in the works of V.Yu.Trakhtengerts and co-authors and is based on self-oscillatory processes in a flow cyclotron maser (Trakhtengerts, 1986; Demekhov and Trakhtengerts,1994). Note that the flow maser theory explains the pulsating spots in which the shape of the spots does not change, the so-called "pure pulsation patches" (Yamamoto. and Oguti, 1982). It has been experimentally established that other types of pulsating aurora are also observed in the morning sector (expansion and/or propagating pulsation, steaming pulsation, fast auroral waves). Since the whistler waves seem to play a decisive role in the morning rashes, it can be expected that different types of pulsating aurora are associated with different regimes of cyclotron instability in the magnetosphere. In the theory of a flowing cyclotron maser (Trakhtengerts et al., 1986, Demekhov, and Trakhtengerts, 1994), the presence of a fiber (duct) of increased concentration of cold plasma plays an important role. This duct plays the role of a resonator for whistler waves and, on the other hand, is distinguished with respect to the surrounding plasma by more favorable conditions for the development of cyclotron instability. In work (Pasmanik, 2004), a generalization of the classical quasilinear theory of the cyclotron interaction to the case of a cylindrical plasma waveguide is performed, taking into account the spatial distribution of the energetic particles and the mode structure of the excited waves. Depending on the parameters of the magnetospheric waveguide, these proper waveguide modes will have different spatial distributions of the wave fields inside the waveguide. In these cases, certain spatial forms of the precipitating electrons can be expected. The elucidation of this question requires detailed information on the space-time variations of the luminescence in various types of pulsating aurora.