Calculation of Classical Polar-Activity Indices Using Their Modern Variants

At present, the auroral-electrojet AE, AU, and AL indices are calculated using a smaller number of stations than earlier. This influences the accuracy of estimates of the magnetospheric activity components, which are performed using the relationships with modern indices AE₈ (obtained from the data of 8 stations) substituted instead of the classical indices AE₁₂ (obtained from the data of 12 stations). In this paper, we develop an artificial neural-network (ANN) algoritm for recalculating the classical polar-activity indices from modern ones. It is demonstrated that the developed ANN is capable of retrieving the AE₁₂-index from data of 8 stations (AE₈) with efficiency reaching 90% in some cases. The results obtained allow one to estimate the error in the representation of the auroral-electrojet activity by modern AE₈ indices as compared with the ones introduced into geophysical research by their creators. We also develop a retrieval technique for polar indices, which employs data on solar-wind parameters.     

Scattering of an Alfvén wave by an inhomogeneity in the solar wind density

The interaction between a nonlinear Alfvén wave and intense inhomogeneities in the density of interplanetary plasma is considered in the magnetohydrodynamic (MHD) approximation. The cold-plasma approximation was used to carry out a more correct study of the interaction since the thermal pressure can introduce pronounced changes into the shape of specified inhomogeneities in the plasma density. Results of numerical solution of the well-known MHD equations are presented in the form of three films demonstrating different scenarios of development of the nonlinear dynamics. The films allow us to observe the dynamic evolution of the form of an Alfvén perturbation and the changes in the density inhomogeneities. For small-amplitude Alfvén waves this corresponds to the process of linear transformation by the density inhomogeneities, which does not lend itself to comprehensive analytical study. Numerical simulation reveals the phenomena of reflection from regions of sharp density variation, which are very sensitive to the spatial scales of the interacting objects. The same method is used to investigate the scattering of strong waves. After reversible changes in shape in a high-density region (where oscillations of the shock-wave front are attenuated), a moderate-amplitude Alfvén wave is recovered in a more rarefied medium. A strong scattered Alfvén wave brings about irreversible changes in the shape of the density inhomogeneity. The results obtained illustrate the process of interaction between Alfvén waves and strong density perturbations related to piston or explosive shock waves in the solar wind. State Pedagogikal University, Nizhny Novgorod, Russia; Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 2, pp. 152–163, February, 1998.     

X-Ray studies of phase diagrams of the system Mn-Cr-O in air

X-Ray analysis was made of solid solutions and heterogeneous compositions of the general formula Mn_3?xCr_xO_m (0?x ? 3, Δx = 0.05). Subsolidus portions of the high temperature phase equilibria diagram and the diagram of water-quenched phases of the system Mn-Cr-O in air were established in the temperature range 700–1400°C. Comparative analysis of these diagrams has revealed the mechanism of the processes occurring upon rapid cooling of the system (in water and in air in a ceramic crucible) from high temperatures to room temperature. Rapid cooling is shown to preserve the phase composition and crystal structure of nonspinel phases, but for equilibrium solid solutions with spinel or hausmannite structure, this is not always the case. Depending on the quench temperature and the solution composition the following phenomena are observed: (1) tetragonal distortion of a high temperature phase with spinel structure; (2) merging of a tetragonally distorted spinel and spinel and formation of a homogeneous phase with hausmannite structure; (3) decomposition of the homogeneous phase with spinel structure into two spinel-type phases with spinel and hausmannite structures. Possible causes of these phenomena are discussed.     

Influence of the solar-wind magnetic field on the magnetosheath turbulence behind the bow shock

The experiment Sura–-WIND (1996–1997) on radio-raying of geodisturbed solar-wind region is interpreted in terms of modern knowledge of an interaction between the magnetized solar wind and the Earth's magnetosphere. Characteristics of the scattered signal at 9 MHz, determined by a plasma turbulence level with scales about 100 km, are statistically related to in situ measurements of solar wind parameters such as plasma density and the orientation and magnitude of the interplanetary magnetic field (IMF) onboard WIND spacecraft. The dependence of the scintillation index of the detected scattered signal, characterising the average turbulence level of the Earth's magnetosheath behind the bow shock, on the IMF orientation and magnitude is revealed. To verify the relation obtained, modern nonlinear correlation techniques based on the theory of artificial neural networks (ANN) are applied. The results obtained using a three-layer ANN with error backpropagation confirm an essential IMF influence on the plasma turbulence in the magnetosheath.