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Kuker M.,Leibniz Institute for Astrophysics Potsdam | Rudiger G.,Leibniz Institute for Astrophysics Potsdam | Kitchatinov L.L.,Institute of Solar Terrestrial Physics
Astronomy and Astrophysics | Year: 2011

A series of stellar models of spectral type G is computed to study the rotation laws resulting from mean-field equations. The rotation laws of the slowly rotating Sun, the rapidly rotating MOST stars Ïμ Eri and κ1 Cet, and the rapid rotators R58 and LQ Lup can be easily reproduced. We also find that differences in the depth of the convection zone cause large differences in the surface rotation law and that the extreme surface shear of HD 171488 can only be explained with an artificially shallow convection layer. We verify the thermal wind equilibrium in rapidly rotating G dwarfs and find that the polar subrotation (dΩ/dz < 0) is due to the baroclinic effect and the equatorial superrotation (dΩ/dr > 0) is caused by the Reynolds stresses. In the bulk of the convection zones where the meridional flow is slow and smooth, the thermal wind equilibrium holds between the centrifugal and the pressure forces. It does not hold, however, in the bounding shear layers including the equatorial region where the Reynolds stresses dominate. © 2011 ESO.

Fleishman G.D.,New Jersey Institute of Technology | Fleishman G.D.,RAS Ioffe Physical - Technical Institute | Kuznetsov A.A.,Institute of Solar Terrestrial Physics
Astrophysical Journal | Year: 2014

Currently there is a concern about the ability of the classical thermal (Maxwellian) distribution to describe quasi-steady-state plasma in the solar atmosphere, including active regions. In particular, other distributions have been proposed to better fit observations, for example, kappa- and n-distributions. If present, these distributions will generate radio emissions with different observable properties compared with the classical gyroresonance (GR) or free-free emission, which implies a way of remotely detecting these non-Maxwellian distributions in the radio observations. Here we present analytically derived GR and free-free emissivities and absorption coefficients for the kappa- and n-distributions, and discuss their properties, which are in fact remarkably different from each other and from the classical Maxwellian plasma. In particular, the radio brightness temperature from a gyrolayer increases with the optical depth τ for kappa-distribution, but decreases with τ for n-distribution. This property has a remarkable consequence allowing a straightforward observational test: the GR radio emission from the non-Maxwellian distributions is supposed to be noticeably polarized even in the optically thick case, where the emission would have strictly zero polarization in the case of Maxwellian plasma. This offers a way of remote probing the plasma distribution in astrophysical sources, including solar active regions as a vivid example. © 2014. The American Astronomical Society. All rights reserved.

Klimushkin D.Yu.,Institute of Solar Terrestrial Physics | Mager P.N.,Institute of Solar Terrestrial Physics | Pilipenko V.A.,Space Research Institute
Earth, Planets and Space | Year: 2012

The paper examines the ballooning instability in gyrokinetic approximation taking into account the effects of finite-β, magnetic field line curvature, and diamagnetic drift. We used a simple model with a constant curvature of magnetic field lines which enabled us to obtain analytical results. The possible plasma oscillatory modes comprise the poloidal Alfvén and drift compressional modes, coupled due to the magnetic field line curvature and plasma inhomogeneity. The frequencies of these modes depend on the westward current value. As this value grows, the frequencies of these two branches approach to each other, and the branches are merged at some critical value of the current. Then an instability develops which is called the drift ballooning coupling instability. There are three major differences of the drift ballooning coupling instability from the ordinary MHD ballooning instability: (1) the drift ballooning coupling instability is not aperiodic, there is a real part of the oscillation frequency of the order of the drift frequency, (2) only the mode with the same direction of the azimuthal phase speed as the velocity of the ion diamagnetic drift can be unstable, (3) the instability threshold depends on the diamagnetic drift frequency. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).

Zharkova V.V.,University of Bradford | Kuznetsov A.A.,Institute of Solar Terrestrial Physics | Siversky T.V.,University of Bradford
Astronomy and Astrophysics | Year: 2010

Aims.The paper aims are to simulate steady-state distributions of electrons beams precipitating in collisional and Ohmic losses with pitch angle anisotropy into a flaring atmosphere with converging magnetic field and to apply these to the interpretation of HXR photon spectra, directivity and polarization observed for different photon energies and flare positions on the solar disk. Methods.Summary approximation method is applied to a time-dependent Fokker-Planck equation by splitting the temporal derivative equally between the derivatives in depth, energy and pitch angles and finding the solutions in forward and backward directions for each variable. Results. For softer beams, there is a noticeable flattening of the photon spectra at lower energies caused by the self-induced electric field that increases for larger viewing angles. For the models with an electric field, the HXR emission with lower energies (30 keV) becomes directed mainly upwards at upper atmospheric levels owing to the increased number of particles moving upwards, while in deeper layers it again becomes directed downwards. The polarization maximum shifts to higher energies with every precipitation depth approaching 25 keV for the models with pure collisions and 100 keV for the models with return currents. At deeper layers, the polarization decreases because of the isotropization of electrons by collisions. The maximum polarization is observed at the viewing angle of 90., becoming shifted to lower angles for softer beams. The integrated polarization and directivity shows a dependence on a magnetic field convergence for harder beams, while for softer beams the directivity is strongly affected by the self-induced electric field changing from a downward motion to an upward one at upper atmospheric depths. Conclusions. The proposed precipitation model for an electron beam with wider pitch angle dispersion of 0.2 taking into account collisional and Ohmic losses allowed us to fit the double power law HXR photon spectra with a spectrum flattening at lower energies observed in the flares of 20 and 23 July 2002. The observed directivity of HXR photons of 20 keV derived for a large number of flares located from the disk center to limb is also reproduced well by the theoretical directivity calculated for an electron beam with a very narrow pitch angle dispersion of 0.02. The simulated polarization of this narrowly-directed electron beam fits up to 90% of all the available polarimetric observations carried out at various locations across the solar disk. © ESO 2010.

Mikhailova O.S.,Institute of Solar Terrestrial Physics
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2014

The paper is devoted to the spatial structure of ULF (ultra-low frequency) Pc1 oscillations with the admixture of heavy ions taken into account. Due to dip of the Alfvén velocity in the plasmapause region and the admixture of heavy ions, the wave is located in the two-dimensional resonator located near the plasmapause on the radial coordinate and near the magnetic equator along the magnetic field line. For the purposes of a qualitative analysis of the parallel resonator, the WKB approximation in the coordinate along the magnetic field line was used to obtain the wave eigenfrequencies. © 2013 Elsevier Ltd.

Kuznetsov A.A.,Institute of Solar Terrestrial Physics | Kontar E.P.,University of Glasgow
Solar Physics | Year: 2015

We investigated in detail the 21 May 2004 flare using simultaneous observations of theNobeyama Radioheliograph,theNobeyama Radiopolarimeters,theReuven Ramaty High Energy Solar Spectroscopic Imager(RHESSI), and theSolar and Heliospheric Observatory(SOHO). The flare images in different spectral ranges reveal a well-defined single flaring loop in this event. We simulated the gyrosynchrotron microwave emission using the recently developed interactive IDL tool GX Simulator. By comparing the simulation results with the observations, we deduced the spatial and spectral properties of the non-thermal electron distribution. The microwave emission has been found to be produced by the highenergy electrons (>100 keV) with a relatively hard spectrum (δ"2); the electrons were strongly concentrated near the loop top. At the same time, the number of high-energy electrons near the footpoints was too low to be detected in the RHESSI images and spatially unresolved data. The SOHOExtreme-ultraviolet Imaging Telescopeimages and the lowfrequency microwave spectra suggest the presence of an extended “envelope” of the loop with lower magnetic field. Most likely, the energetic electron distribution in the considered flare reflects the localized (near the loop top) particle acceleration (injection) process accompanied by trapping and scattering. © Springer Science+Business Media Dordrecht 2014.

Fleishman G.D.,New Jersey Institute of Technology | Fleishman G.D.,RAS Ioffe Physical - Technical Institute | Kuznetsov A.A.,Armagh Observatory | Kuznetsov A.A.,Institute of Solar Terrestrial Physics
Astrophysical Journal | Year: 2010

Radiation produced by charged particles gyrating in a magnetic field is highly significant in the astrophysics context. Persistently increasing resolution of astrophysical observations calls for corresponding three-dimensional modeling of the radiation. However, available exact equations are prohibitively slow in computing a comprehensive table of high-resolution models required for many practical applications. To remedy this situation, we develop approximate gyrosynchrotron (GS) codes capable of quickly calculating the GS emission (in non-quantum regime) from both isotropic and anisotropic electron distributions in non-relativistic, mildly relativistic, and ultrarelativistic energy domains applicable throughout a broad range of source parameters including dense or tenuous plasmas and weak or strong magnetic fields. The computation time is reduced by several orders of magnitude compared with the exact GS algorithm. The new algorithm performance can gradually be adjusted to the user's needs depending on whether precision or computation speed is to be optimized for a given model. The codes are made available for users as a supplement to this paper. © 2010. The American Astronomical Society. All rights reserved.

Plyusnina L.A.,Institute of Solar Terrestrial Physics
Solar Physics | Year: 2010

There are two types of active longitudes (ALs) in terms of the distribution of sunspot areas: long-lived and intra-cyclic ALs. The rotation period of the long-lived ALs has been determined by a new method in this paper. The method is based on the property of ALs to be maintained over several cycles of solar activity. The daily values of sunspot areas for 1878 - 2005 are analyzed. It is shown that the AL positions remain almost constant over a period of about ten cycles, from cycle 13 to cycle 22. The rotation period was found to be 27.965 days during this period. The dispersion in AL positions is about 26° from cycle to cycle, which is half of the dispersion observed in the Carrington system. The ALs in the growth phase of the activity cycle are more stable and pronounced. The excess in solar activity in the ALs over adjacent longitudinal intervals is about 12 - 14%. It is shown that only one long-lived AL can be observed at one time on the Sun, as a rule. © Springer Science+Business Media B.V. 2010.

Kuznetsov A.A.,Armagh Observatory | Kuznetsov A.A.,Institute of Solar Terrestrial Physics
Astronomy and Astrophysics | Year: 2011

Context. The electron-cyclotron maser instability is responsible for the generation of the auroral kilometric radiation of the Earth and similar phenomena at other magnetized planets of the Solar System. The recently discovered radio emission from ultracool dwarfs has many similarities with the planetary auroral radio emission. The in situ measurements in the terrestrial magnetosphere indicate that the radiation is produced by nonthermal electrons with a horseshoe-like distribution. Kinetic simulations of the electron-cyclotron maser instability for these distribitions have not yet been performed. Aims. We investigate the amplification of plasma waves by the horseshoe-like electron distribution, as well as the relaxation of this distribution due to the electron-cyclotron maser instability.We determine the parameters of the generated plasma waves, the timescales of the relaxation process, and the conversion efficiency of the particle energy into waves. Methods. We developed a kinetic relativistic quasi-linear 2D code to simulate the coevolution of an electron distribution and highfrequency plasma waves. The code includes the processes of wave growth and particle diffusion,which are assumed to bemuch faster than other processes (particle injection, etc.). A number of simulations were performed for different parameter sets that seem to be typical of the magnetospheres of ultracool dwarfs (in particular, the plasma frequency is much less than the cyclotron one). Results. Our calculations indicate that the fundamental extraordinary mode dominates strongly. The generated waves have a frequency that is slightly below the electron cyclotron frequency and propagate across the magnetic field. The final intensities of other modes are negligible. The conversion efficiency of the electron energy into the extraordinary waves is typically around 10%. Complete relaxation of the unstable electron distribution takes much less than a second. Conclusions. Energy efficiency of the electron-cyclotron maser instability is more than sufficient to provide the observed intensity of radio emission from ultracool dwarfs. On the other hand, the observed light curves of the emission are not related to the properties of this instability and reflect, most likely, the dynamics of the electron acceleration process and/or geometry of the radiation source. © 2011 ESO.

Kobanov N.I.,Institute of Solar Terrestrial Physics | Pulyaev V.A.,Institute of Solar Terrestrial Physics
Solar Physics | Year: 2011

We find that oscillations of the LOS velocity in Hα vary within facula regions. The power spectra show that the contributions of low-frequency modes (1.2 - 2 mHz) increase at the network boundaries. Three- and five-minute periods dominate inside cells. The spectra of photospheric and chromospheric LOS-velocity oscillations differ for most faculae. We detected several cases where oscillations in faculae seem to propagate horizontally with phase velocities of 50 - 70 km s-1. Their location in space and time coincided with the local maximum of the longitudinal magnetic field. © 2010 Springer Science+Business Media B.V.

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