Vol 59, No 3 (2016)

We study the possible origin of long-lasting soft X-ray flares on the AD Leo star, which were observed onboard the Extreme Ultraviolet Explorer (EUVE) spacecraft for the period of 1993-2000 [1]. These flares have relatively long rise and decay times of the radiation intensity (τ_R ≈ 10⁴ s and τ_d ≈ 5 · 10³, respectively), as well as a relatively large emission measure EM ≈ 10⁵¹cm⁻³, which exceeds by 1–3 orders of magnitude the emission measure of soft X-ray flares on the Sun. Assuming that the radiation appears in magnetic loops and basing on the observed values of the emission measure and radiation decay time, the authors of [1] determined the typical length \( \overline{l}\approx 1.5\cdot {10}^{10} cm \), electron number density \( \overline{n}\approx 3\cdot {10}^{11}c{m}^{-3} \), and plasma temperature \( \overline{T}\approx 2.5\cdot 107 \) K of the loops. This paper considers plasma heating due to dissipation of the electric currents in the coronal magnetic loops of the star induced by the photospheric convection. The large inductance of the loop as an equivalent electric circuit determines the long time of the current rise in the source and explains the observed time of plasma heating and the rise time of the X-ray radiation intensity. It is shown that the parameters of the X-ray sources in the AD Leo atmosphere agree with the parameters calculated under the assumption of simultaneous emission of a great number of loops (about 50) with electric currents greater than 10¹³ A, which exceeds the electric currents in the solar coronal magnetic loops by 1–3 orders of magnitude. Such an exceeding can be related to the higher photospheric convection velocities on the late-type stars compared with the Sun.

We consider the problem of generator parameterization in the global electric circuit (GEC) models. The relationship between the charge density and external current density distributions inside a thundercloud is studied using a one-dimensional description and a three-dimensional GEC model. It is shown that drastic conductivity variations in the vicinity of the cloud boundaries have a significant impact on the structure of the charge distribution inside the cloud. Certain restrictions on the charge density distribution in a realistic thunderstorm are found. The possibility to allow for conductivity inhomogeneities in the thunderstorm regions by introducing an effective external current density is demonstrated. Replacement of realistic thunderstorms with equivalent current dipoles in the GEC models is substantiated, an equation for the equivalent current is obtained, and the applicability range of this equation is analyzed. Relationships between the main GEC characteristics under variable parameterization of GEC generators are discussed.

We develop a physical model allowing one to analyze reflection of the inhomogeneous beams of surface acoustic waves from metal strips in a planar waveguide on the piezoelectric substrate. Analytical relationships for determining the coefficients of scattering and mutual conversion of the transverse waveguide modes during their interaction with the spatially limited Bragg reflectors are obtained. The waveguide-reflector characteristics are shown to depend on the ratio of the waveguide aperture to its maximum value for which only the fundamental transverse mode is excited. It is established that the developed model strictly corresponds to the energy conservation law, i.e., in the absence of dissipation, the power of the inhomogeneous beam, which is incident on the finite reflector, is equal to the total power of all the scattered fields of the discrete and continuous waveguide spectra.

We present the results of analytical and numerical studies of propagation of longitudinal acoustic waves in micro-inhomogeneous media with different-modulus nonlinearity and relaxation. The solutions for evolution of the profiles and spectral characteristics of the initially harmonic waves are obtained.