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We present the results on numerical testing of the Boundary Control Method in the sound speed determination for the acoustic equation on semiplane. This method for solving multidimensional inverse problems requires no a priory information about the parameters under reconstruction. The application to the realistic Marmousi model demonstrates that the boundary control method is workable in the case of complicated and irregular field of acoustic rays. By the use of the chosen boundary controls, an `averaged' profile of the sound speed is recovered (the relative error is about $10-15\%$). Such a profile can be further utilized as a starting approximation for high resolution iterative reconstruction methods., Comment: 27 pages

A thin current sheet (TCS), with the width of an order of thermal proton gyroradius, appears a fundamental physical object which plays an important role in structuring of major magnetospheric current systems (magnetotail, magnetodisk, etc.). The TCSs are nowadays under extensive study by means of space missions and theoretical models. We consider a simple model of the TCS separating two half-spaces occupied by a homogenous magnetic field of opposite sign tangential to the TCS; a small normal component of the magnetic field is prescribed. An analytical solution for the electric current and plasma density in the close vicinity of the TCS has been obtained and compared with numerical simulation. The number density and the electric current profiles have two maxima each. The characteristic spatial scale $z_S$ of the maxima location was investigated as a function of initial pitch-angle of an incoming charge particle. The effect of the thermal dispersion of the incoming proton beam have been taken into consideration., Comment: 33pages, 5figures in Journal of Geophysical Research: Space physics, 2015

We present the results of numerical testing on determination of the sound speed $c$ in the acoustic equation $u_{tt}-c^2\Delta u=0$ by the ${\it boundary}$ ${control}$ ${method}$. The inverse data is a response operator (a hyperbolic Dirichlet-to-Neumann map) given on controls, which are supported on ${\it a part}$ of the boundary. The speed is determined in the subdomain covered by acoustic rays, which are emanated from the points of this part orthogonally to the boundary. The determination is ${\it time-optimal}$: the longer is the observation time, the larger is the subdomain, in which $c$ is recovered. The numerical results are preceded with brief exposition of the relevant variant of the BC-method., Comment: 35 pages, 17 figures

Petschek-type time-dependent reconnection (TDR) and quasi-stationary reconnection (QSR) models are considered to understand reconnection outflow structures and the features of the associated locally generated turbulence in the solar wind. We show that the outflow structures, such as discontinuites, Kelvin-Helmholtz (KH) unstable flux tubes or continuous space filling flows cannot be distinguished from one-point WIND measurements. In both models the reconnection outflows can generate more or less spatially extended turbulent boundary layers (TBDs). The structure of an unique extended reconnection outflow is investigated in detail. The analysis of spectral scalings and break locations show that reconnection outflows can control the local field and plasma conditions which may play in favor of one or another turbulent dissipation mechanisms with their characteristic scales and wavenumbers., Comment: 5 figures

The exact mechanism by which astrophysical jets are formed is still unknown. It is believed that necessary elements are a rotating (Kerr) black hole and a magnetised accreting plasma. We model the accreting plasma as a collection of magnetic flux tubes/strings. If such a tube falls into a Kerr black hole, then the leading portion loses angular momentum and energy as the string brakes, and to compensate for this loss, momentum and energy is redistributed to the trailing portion of the tube.} {We found that buoyancy creates a pronounced helical magnetic field structure aligned with the spin axis. Along the field lines, the plasma is centrifugally accelerated close to the speed of light. This process leads to unlimited stretching of the flux tube since one part of the tube continues to fall into the black hole and simultaneously the other part of the string is pushed outward. Eventually, reconnection cuts the tube, the inner part is filled with new material and the outer part forms a collimated bubble-structured relativistic jet. Each plasmoid can be considered like an outgoing particle in the Penrose mechanism: it carries extracted rotational energy away from the black hole while the falling part with the corresponding negative energy is left inside the ergosphere., Comment: 10 pages, 5 figures

A new kind of magnetohydrodynamic instability and waves are analyzed for a current sheet in the presence of a small normal magnetic field component varying along the sheet. These waves and instability are related to existence of two gradients of the tangential and normal magnetic field components along the normal and tangential directions with respect to the current sheet. The current sheet can be stable or unstable if the multiplication of two magnetic gradients is positive or negative. In the stable region, the kink-like wave mode is interpreted as so called flapping waves observed in the Earth's magnetotail current sheet. The kink wave group velocity estimated for the Earth's current sheet is of the order of a few tens kilometers per second. This is in good agreement with the observations of the flapping motions of the magnetotail current sheet., Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett

Relativistic magnetohydrodynamics can be reformulated in terms of magnetic flux tubes which turn out to obey equations of non-linear strings. The string approach is applied to study how a test flux tube falls into a Kerr black hole. Analytical treatment and numerical simulations show that the leading portion of the falling tube loses angular momentum and energy as the string breaks, and to compensate for this loss, momentum and energy has to be generated to conserve energy and momentum for the tube. Inside the ergosphere the energy of the leading part can be negative, and the rest of the tube then extracts energy from the hole. Increasing centrifugal forces eject the part of the tube with extra positive energy from the ergosphere after a time producing a relativistic jet., Comment: 9 pages, 3 figures

Observed delays in the ground response to solar wind directional discontinuities have been explained as the result of larger than expected magnetosheath propagation times. Recently, Samsonov et al. (2017, https://doi.org/10.1002/2017GL075020) showed that the typical time for a southward interplanetary magnetic field (IMF) turning to propagate across the magnetosheath is 14 min. Here by using a combination of magnetohydrodynamic simulations, spacecraft observations, and analytic calculations, we study the dependence of the propagation time on solar wind parameters and near-magnetopause cutoff speed. Increases in the solar wind speed result in greater magnetosheath plasma flow velocities, decreases in the magnetosheath thickness and, as a result, decreases in the propagation time. Increases in the IMF strength result in increases in the magnetosheath thickness and increases in the propagation time. Both magnetohydrodynamic simulations and observations suggest that propagation times are slightly smaller for northward IMF turnings. Magnetosheath flow deceleration must be taken into account when predicting the arrival times of solar wind structures at the dayside magnetopause.

Previous observations have shown an approximately 10-15 minute time delay in the ionospheric response to solar wind directional discontinuities marked by either southward or northward interplanetary magnetic field (IMF) turnings. We have studied one southward IMF turning observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) and GOES in the dayside magnetosphere. Using a global MHD (MagnetoHydroDynamics) model, we have reproduced the magnetopause motion in this event. We find that the observed delay in the ground response can be completely explained by deceleration of the directional discontinuity in the subsolar magnetosheath. We show that the speed of the discontinuity significantly decreases in the vicinity of the magnetopause where the magnetic barrier formed during the previous northward IMF interval. The southward turning can reach the magnetopause only after complete disruption of the magnetic barrier. The disruption or dissipation occurs via magnetosheath reconnection, as confirmed by high-speed jets in the magnetosheath. The magnetopause moves sunward as the directional discontinuity transits the magnetosheath. This sunward motion is followed by the earthward motion when the discontinuity strikes the magnetopause and magnetopause reconnection begins.

The magnetic barrier is a significant fraction of the magnetosheath region where magnetic forces react back on the plasma flow. We use a magnetohydrodynamic approach to calculate the solar wind flow around the magnetosphere. The use of so-called frozen-in coordinates, where the flow lines and the magnetic field are coordinate axes, is found to be most appropriate. Numerical solutions are presented for the problem of the flow around a blunt body. The results near the magnetopause are used as input parameters for a model of the reconnection of magnetic field lines based on shock-type structures.

Extensions of Petschek's (1964) analysis are reviewed and used to investigate the steady-state and time-dependent reconnection in a current sheet geometry of the type observed at the magnetopause. It is shown that steady-state reconnection appears as a very special case in a time-dependent analysis. A single theoretical framework is proposed for interpreting reconnection phenomena at the magnetopause and for investigating the characteristics of dayside reconnection.