Strona główna » Instytut » Pracownicy » Krzysztof Murawski prof. dr hab. Krzysztof MurawskiZakładZakład Astrofizyki i Teorii Grawitacji Stanowisko:profesor zwyczajny Kontakt:pokój: 309 telefon: (081) 537-61-98 e-mail: kmur@kft.umcs.lublin.pl strona www: http://kft.umcs.lublin.pl/kmur/ W IF UMCS od roku:2001 Specjalność naukowa:astrofizyka Funkcje pełnione w IF UMCS:kierownik Zakładu Astrofizyki i Teorii Grawitacji Zainteresowania naukowe:fizyka Słońca, komet, magnetosfer planet, wiatru slonecznego, plazma astrofizyczna, teoria fal (solitonów), numeryczne metody rozwiązywania równań różniczkowych Kariera zawodowa naukowo - dydaktyczna:mgr 1982 – UMCS doktor 1989 – Flinders University of South Australia doktor 1991 – UMCS doktor habilitowany 1995 – Uniwersytet Wrocławski prof. 2005 – UMCS
Studia doktoranckie w Uniwersytecie Flindersa w Południowej Australii -- 1986-1988 Studia podoktoranckie (post-doc) w Uniwersytecie Św. Andrzeja, Szkocja -- 1990-1992 Staż naukowy (fellowship) w Uniwersytecie Katolickim w Leuven, Belgia -- 1992-1994 Staż naukowy (fellowship) w Instytucie Południowo-Zachodnim w San Antonio, Texas, USA -- 1994-1995 Staż naukowy (visiting professorship) w Universitas de Illes Ballears, Hiszpania -- 5 miesięcy w latch 1995, 1996 i 1997 Staż naukowy (STA fellowship) w Communication Research Laboratory, Tokyo, Japonia -- 1 miesiąc w 1996 roku Kariera zawodowa - inne formy działalności:Działalność naukowa dotyczy symulacji numerycznych zjawisk falowych w atmosferze Słońca, Wenus, Marsa i komet. Przeprowadzone badania dotyczyły również zaawansowanych symulacji numerycznych procesów oddziaływania wiatru słonecznego z ciałami niebieskimi nie posiadającymi własnego pola magnetycznego. Rezultatem badań są też publikacje dotyczące oddziaływania fali f z turbulentnym polem prędkości występującym w warstwie konwekcyjnej Słońca. Prowadził badania nad wpływem pól stochastycznych na częstość i amplitudę fal akustycznych. Praca naukowa polegała na analitycznym wyprowadzeniu związków dyspersyjnych dla fal stochastycznych i weryfikacji tych zwiazków wynikami symulacji numerycznych.
Członkostwo w towarzystwach i organizacjach:Polskie Towarzystwo Fizyczne Polskie Towarzystwo Astrofizyczne Komitet Astronomii PAN Rada Naukowa Komitetu Badań Kosmicznych PAN Zainteresowania i pasje:Muzyka, podróże, sport - K. Murawski,Z. E. Musielak, Linear Alfvén waves in the solar atmosphere, Astronomy and Astrophysics A, 37(), 2010, 518
- K. Murawski,T. V. Zaqarashvili, Numerical simulations of spicule formation in the solar atmosphere, Astronomy and Astrophysics A, 519(), 2010, 1-9
- P. Konkol,K. Murawski, Vertical oscillations of a solar coronal loop in a gravitationally stratified solar corona: comparison of 3D and 2D cases, Acta Physica Polonica B, 41(), 2010, 1369-1381
- M. Selwa,K. Murawski,S. K. Solanki,L. Ofman, Excitation of vertical kink waves in a solar coronal arcade loop by a periodic driver, Astronomy and Astrophysics A, 512(), 2010, 1-8
- P. Konkol,K. Murawski,D. Lee,K. Weide, Numerical simulations of the attenuation of the fundamental slow magnetoacoustic standing mode in a gravitationally stratified solar coronal arcade, Astronomy and Astrophysics A, 521(), 2010, 1-6
- M. Gruszecki, K. Murawski, S. Fromang, R. Teyssier, Vertical oscillations of a curved coronal slab in an inhomogeneous plasma, Acta Physica Polonica B, 40(), 2009, 367-377
- R. Ogrodowczyk, K. Murawski, S. K. Solanki, Slow magnetoacoustic standing waves in a curved solar coronal slab, Astronomy and Astrophysics, 595(), 2009, 313-318
- A. Wasiljew, K. Murawski, Numerical simulations of the first harmonic kink mode of vertical oscillations of a solar coronal loop, Astronomy and Astrophysics, 498(), 2009, 863-868
- D. J. Pascoe, V. M. Nakariov, T. D. Arber, K. Murawski, Sausage oscillations in loops with a non-uniform cross-section, Astronomy and Astrophysics, 494(), 2009, 1119-1125
- N. Terada, H. Shinagawa, T. Tanaka, K. Murawski, K. H. Shinagawa, T. Tanaka, K. Murawski, K. Terada, A three-dimensional, multi-species, comprehensive MHD model of the solar wind interaction with the planet Venus comprehensive MHD model of the solar wind interaction with the planet Venus, Journal of Geophysical Research, 114(), 2009, A09208(1-11)
- Gruszecki, M. Murawski, K. McLaughlin, J., Influence of a dense
photospheric-like layer on vertical oscillations of a curved coronal slab, Astronomy and Astrophysics, 489(), 2008, 413-418
- Gruszecki, M. Murawski, K., Vertical oscillations of an arcade loop
in a gravitationally stratified solar corona, Astronomy and Astrophysics, 487(), 2008, 717-721
- Gruszecki, M. Murawski, K. Ofman, L., Standing fast magnetoacoustic
kink waves of solar coronal loops with field-aligned flow, Astronomy and Astrophysics, 488(), 2008, 757-761
- Zaqarashvili, T.V. Murawski, K., Torsional oscillations of longitudinally inhomogeneous coronal loops, Astronomy & Astrophysics, 470(1), 2007, 353-357
Abstract: Aims. We explore the effect of an inhomogeneous mass density field on frequencies and wave profiles of torsional Alfven oscillations in solar coronal loops.
Methods. Dispersion relations for torsional oscillations are derived analytically in limits of weak and strong inhomogeneities. These analytical results are verified by numerical solutions, which are valid for a wide range of inhomogeneity strength.
Results. It is shown that the inhomogeneous mass density field leads to the reduction of a wave frequency of torsional oscillations, in comparison to that estimated from mass density at the loop apex. This frequency reduction results from the decrease of an average Alfven speed as far as the inhomogeneous loop is denser at its footpoints. The derived dispersion relations and wave profiles are important for potential observations of torsional oscillations which result in periodic variations of spectral line widths. Conclusions. Torsional oscillations offer an additional powerful tool for the development of coronal seismology.
- Ogrodowczyk, R. Murawski, K., Numerical simulations of slow magnetosonic standing waves in a straight solar coronal slab, Astronomy and Astrophysics, 467(), 2007, 311-316
- Ogrodowczyk, R. Murawski, K., Numerical simulations of impulsively excited magnetosonic waves in two parallel solar coronal slabs, Astronomy and Astrophysics, 461(), 2007, 1133-1139
- Selwa, M. Murawski, K. Solanki, S.K. Wang, T.J., Energy leakage as an attenuation mechanism for vertical kink oscillations in solar coronal wave guides, Astronomy and Astrophysics, 462(), 2007, 1127-1135
- Gruszecki, M. Murawski, K. Solanki, S.K. Ofman, L., Attenuation of Alfvén waves in straight and curved coronal slabs, Astronomy and Astrophysics, 469(), 2007, 1117-1121
- Selwa, M. Ofman, L. Murawski, K., Numerical simulations of slow standing waves in a curved solar coronal loop, Astrophysical Journal, 668(), 2007, L83-L86
- Gruszecki, M.
Murawski, K.
Selwa, M.
Ofman, L., Numerical simulations of vertical oscillations of a multi- stranded coronal loop, Astronomy & Astrophysics, 460(3), 2006, 887-892
Abstract: Aims. We consider impulsively generated oscillations in a 2D model of a curved solar coronal arcade loop that consists of up to 5 strands of dense plasma. Methods. First we do a simulation for a loop which consists of two curved strands. We evaluate by means of numerical simulations the influence of the distance between the strands and their number on wave period, attenuation time, and amplitudes of standing kink waves. Results. The results of the numerical simulations reveal that only strands which are very close to each other (distance comparable to the strand width) considerably change the collective behavior of kink oscillations. More distant strands also exhibit weak coupling of the oscillations. However, their behavior can essentially be explained in terms of separate oscillating loops. We compare the numerical results with recent TRACE observational findings, and find qualitative agreement.
- Ogrodowczyk, R.
Murawski, K., Numerical simulations of impulsively generated magnetosonic waves in a coronal loop, Solar Physics, 236(2), 2006, 273-283
Abstract: We consider impulsively excited magnetosonic waves in a highly magnetized coronal loop that is approximated by a straight plasma slab of enhanced mass density. Numerical results reveal that wavelet spectra of time signatures of these waves possess characteristic shapes that depend on the position of the initial pulse: in the case of a pulse launched inside the slab, these spectra are of a tadpole shape, while for a pulse excited in the ambient medium these spectra display more complex structures with branches of long and short-period waves. These short period oscillations correspond to waves that are trapped inside the slab, and the long-period oscillations are associated with waves that propagate through the ambient medium and reach the detection point. These findings are compatible with recent theoretical studies and observations by the solar eclipse coronal imaging system (SECIS).
- Selwa, M.
Solanki, S. K.
Murawski, K.
Wang, T. J.
Shumlak, U., Numerical simulations of impulsively generated vertical oscillations in a solar coronal arcade loop, Astronomy & Astrophysics, 454(2), 2006, 653-661
Abstract: Aims. The main aims of the paper are to carry out numerical simulations of the vertical oscillations in a coronal loop in order to determine their dependence on various parameters and to compare them with recent TRACE observations. Methods. We consider impulsively generated oscillations in a solar coronal arcade loop. The two-dimensional numerical model we implement in the ideal MHD regime includes the effects of nonlinearity and line curvature. We perform parametric studies by varying both the position and the width/strength of the pulse. Results. A pulse launched below a loop is in general found to excite multiple wave modes, in particular a vertical oscillation with many properties of a kink mode, fast mode oscillations and a slow mode pulse ( or two slow mode pulses, depending on the location of the original pulse). From our parametric studies we deduce that wave periods and attenuation times of the excited waves depend on the position below the loop summit, as well as on the width of the pulse. Wider pulses launched closer to a foot-point and to the loop's apex trigger wave packets of longer period waves which are more strongly attenuated. A perturbed loop does not return to its initial state but is instead stretched, with its apex shifted upwards. As a result the perturbations propagate along the stretched loop and consequently stronger and wider pulses which stretch a loop more lead to longer period oscillations. A pulse located near one of the foot-points is found to excite a distortion mode leading to asymmetric oscillations which are distinct from the vertical or horizontal kink modes that have been identified in TRACE data.
- Murawski, K.
Selwa, M.
Rossmanith, J. A., Numerical simulations of vertical oscillations of a curved coronal loop, Solar Physics, 231(1-2), 2005, 87-94
Abstract: We consider an impulsively-started, vertical excitation of a solar coronal loop that is embedded into a potential arcade. The two-dimensional numerical model we implement includes the effects of line curvature and allows us to explore the effect of varying the initial pulse position. The results of the numerical simulations reveal kink mode oscillations with waveperiods that are reasonably close to the observational findings of Wang and Solanki (2004).
- Selwa, M.
Murawski, K.
Solanki, S. K.
Wang, T. J.
Toth, G., Numerical simulations of vertical oscillations of a solar coronal loop, Astronomy & Astrophysics, 440(1), 2005, 385-390
Abstract: We consider the impulsive excitation of fast vertical kink standing waves in a solar coronal loop that is embedded in a potential arcade. The two-dimensional numerical model we implement includes the effects of field line curvature and nonlinearity on the excitation and damping of standing fast magnetosonic waves. The results of the numerical simulations reveal wave signatures which are characteristic of vertical loop oscillations seen in recent TRACE observational data.
- Murawski, K.
Selwa, M.
Nocera, L., Numerical simulations of fast magnetosonic waves in a curved coronal loop, Astronomy & Astrophysics, 437(2), 2005, 687-690
Abstract: We consider an impulsive excitation of fast magnetosonic waves in a dense and highly magnetised curved loop that is embedded in a potential arcade. The results of our numerical simulations reveal that the period of the excited waves agrees with both the period of the fast kink mode in the arcade and with the observations by TRACE.
- Selwa, M.
Murawski, K.
Solanki, S. K., Excitation and damping of slow magnetosonic standing waves in a solar coronal loop, Astronomy & Astrophysics, 436(2), 2005, 701-709
Abstract: We consider slow magnetosonic standing waves that are impulsively excited in a solar coronal loop. The one- dimensional numerical model we implement includes the effects of nonlinearity, optionally thermal conduction, heating, and cooling of the solar plasma. We numerically evaluate excitation and damping times of a standing wave in hot coronal loops on the basis of a parametric study. Results of the numerical simulations reveal that initially launched impulses mainly trigger the fundamental mode and its first harmonic, depending on the location of these pulses in space. Parametric study shows that these standing waves are excited in a dozen or so wave periods corresponding roughly to 13 min and that they are strongly damped over a similar time-scale.
- Selwa, M.
Murawski, K., Numerical simulations of impulsively generated mass density perturbations in a solar coronal loop, Astronomy & Astrophysics, 425(2), 2004, 719-724
Abstract: Non-symmetric oscillations in a solar coronal loop are numerically studied in the limit of a two-dimensional plasma. The obtained numerical results show that impulses, which are launched in the ambient plasma, excite oscillations in mass density profiles that reveal asymmetry in a cross-section of the loop. This asymmetry is a consequence of the fact that the loop oscillates as a whole. This feature has not been observed in the earlier numerical studies of impulsively generated waves, and it is important for the detecion of the spatial location of the source of the loop oscillations. It is also important for the observations because the density perturbations can produce corresponding perturbations of EUV or thermal X-ray emission.
- Selwa, M.
Murawski, K.
Kowal, G., Three-dimensional numerical simulations of impulsively generated MHD waves in solar coronal loops, Astronomy & Astrophysics, 422(3), 2004, 1067-1072
Abstract: Impulsively generated magnetohydrodynamic waves in a typical EUV solar coronal loop are studied numerically using a three- dimensional FLASH code. Our results reveal several 3D effects such as distinctive time signatures which are collected at a detection point inside the loop. A slow magnetosonic wave generates a significant variation in the mass density profile with a time scale of the order of 40 s. A fast kink wave affects the mass density, too, but its magnitude is much lower than in the case of a slow wave. Time scales which are associated with the fast kink wave are generally shorter than in the case of a slow wave; the former are in the range of a dozen or so seconds. Temporal signatures of a fast sausage wave reveal similar to5-s oscillations in the quasi-periodic phase. Impulses which are launched outside the loop excite several- second oscillations in the mass density. Time signatures depend on the position of the detection point; they are usually more complex further away from the exciter.
- Selwa, A.
Skartlien, R.
Murawski, K., Numerical simulations of stochastically excited sound waves in a random medium, Astronomy & Astrophysics, 420(3), 2004, 1123-1127
Abstract: In turbulent acoustic media such as the solar envelope, both wave sources and the propagation characteristics (background density, refractive index, dissipation, etc.) are stochastic quantities. By means of numerical simulation of the Euler equations, we study two cases in a homogeneous stochastic medium in which the background density fluctuations and wave sources are 1) correlated and 2) uncorrelated. We find that in the uncorrelated case, the coherent (or mean) acoustic field is zero, leaving only an incoherent field. In the correlated case, the coherent field is nonzero, yielding both coherent and incoherent fields. We question the use of mean-field dispersion relations to determine frequency shifts in p-mode and f-mode spectra, since the coherent field can be non-existent or weak relative to the incoherent field. We demonstrate the importance of accounting for a stochastic wave source by showing that the two cases give very different frequency shifts.
- Murawski, K., Acoustic waves in random fields, Waves in Random Media, 14(3), 2004, 467-477
Abstract: The effect of space- and time-dependent random mass density, velocity, and pressure fields on frequencies and amplitudes of acoustic waves is considered by means of the analytical perturbative method. The analytical results, which are valid for weak fluctuations and long wavelength sound waves, reveal frequency and amplitude alteration, the effect of which depends on the type of random field. In particular, the effect of a random mass density field is to increase wave frequencies. Space-dependent random velocity and pressure fields reduce wave frequencies. While space-dependent random fields attenuate wave amplitudes, their time-dependent counterparts lead to wave amplification. In another example, sound waves that are trapped in the vertical direction but are free to propagate horizontally are affected by a space-dependent random mass density field. This effect depends on the direction along which the field is varying. A random field, which varies along the horizontal direction, does not couple vertically standing modes but increases their frequencies and attenuates amplitudes. These modes are coupled by a random field which depends on the vertical coordinate, but the dispersion relation remains the same as in the case of the deterministic medium.
- Selwa, M.
Murawski, K., Numerical simulations of impulsively generated MHD waves in a solar coronal loop, Acta Astronomica, 54(2), 2004, 211-220
Abstract: We study by numerical means the MHD oscillations in a solar coronal loop. Obtained numerical results show that impulses, which are launched in the ambient plasma, excite oscillations in mass density profiles that reveal asymmetry in a cross- section of the loop. This asymmetry is a consequence of the fact that while a part of the loop is compressed its other part is rarefied, resulting from transverse loop oscillations. Due to a lack of spatial resolution this feature has not been observed in the earlier numerical studies of impulsively generated waves. The numerical simulations reveal distinctive time-signatures which are collected at a detection point inside the loop. For the chosen set of physical parameters, these signatures reveal a dozen or so seconds oscillations in the mass density. These oscillations are usually more complex further out from the trigger. A sufficiently large slow magnetosonic wave generates shocks which temporal evolution we trace in mass density profiles.
- Murawski, K.
Nocera, L.
Pelinovsky, E. N., Frequency and amplitude alterations of sound modes in a space- dependent random mass density field, Waves in Random Media, 14(2), 2004, 109-117
Abstract: We investigate the effect of a space-dependent random mass density field on small amplitude acoustic modes that are settled in a semi-infinite medium of a temperature growing linearly with depth. Using a perturbation method, the dispersion relation is derived in the form of Hill's determinant. Numerical solutions of this equation lead to the following conclusions: (a) a weak random field (with delta(eff) = 0.05) essentially affects long waves which experience attenuation and a frequency reduction; (b) for a stronger random field (with delta(eff) = 0.1), high-order sound modes behave as sound waves as they are attenuated and their frequencies are increased; (c) for a sufficiently strong random field (with delta(eff) = 0.2), mode coupling occurs, as a result of which the dispersive curves cross each other, the sound modes loose their identities, and some modes are amplified. Here delta(eff) denotes the effective strength of a random field.
- Nocera, L.
Murawski, K., Frequency shift of acoustic gravity waves in a stratified, isothermal, turbulent atmosphere, Geophysical and Astrophysical Fluid Dynamics, 98(1), 2004, 63-84
Abstract: We study the modification of the frequency of small amplitude acoustic gravity waves which propagate in an isothermal turbulent atmosphere that is stratified by a homogeneous gravitational field. Using a Green's function method, the dispersion relation for the frequency of the waves is formulated as an integral eigenvalue equation and it is solved by perturbation techniques. We draw the following main conclusions: (a) for an arbitrary turbulent correlation spectrum the dispersion relation has a root with a negative imaginary part in the unphysical Riemann sheet of the dispersion function, leading to wave attenuation, much in the same way as it happens for Landau damping; (b) the real part of this root differs from the frequency of an acoustic gravity wave propagating in a nonturbulent medium and, for all forms of the turbulent correlation spectrum, the absolute value of this difference increases if gravity increases; (c) for a Gaussian turbulent correlation spectrum, this difference is always positive; (d) conversely if this frequency difference is known from observations, the auto-correlation function of the temperature fluctuations can be calculated through a simple inversion formula.
- Murawski, K.
Medrek, M., Numerical simulations of frequency shift and amplitude alteration of sound waves in random mass density and velocity fields, Waves in Random Media, 13(4), 2003, 287-301
Abstract: We investigate the effect of space- and time-dependent random mass density and velocity fields on frequencies and amplitudes of small sinusoidal acoustic waves. The dispersion relations and their approximate solutions, which are valid in the limit of weak random fields and long sound waves, are presented. These approximate solutions are verified by numerical simulations for the full set of hydrodynamic equations. The main findings are: (a) both the analytical and numerical results reveal that random fields affect the frequencies and amplitudes of sound waves. A space-dependent (time-dependent) random field leads to wave attenuation (amplification). Random mass density and time-dependent random velocity fields lift up frequencies of sound waves. A space-dependent random velocity field produces redshifts in frequencies; (b) in the limit of validity of analytical solutions the numerical results are close to the analytical data; (c) for a stronger random field and for shorter sound waves, numerical and analytical data depart from each other.
- Murawski, K.
Medrek, M., One-dimensional numerical simulations of random sound impulses, Waves in Random Media, 13(4), 2003, 311-320
Abstract: We perform one-dimensional numerical simulations of small- amplitude acoustic pulses in space- and time-dependent random mass density and time-dependent velocity fields. Numerical results reveal that: (a) random fields affect the speeds, amplitudes and, consequently, shapes of sound pulses; (b) for weak random fields and short propagation times the numerical data converge with the analytical results of the mean field theory which says that a space-dependent (time-dependent) random field leads to wave attenuation (amplification) and all random fields speed up sound pulses; (c) for sufficiently strong random fields and long propagation times numerical simulations reveal pulse splitting into smaller components, parts of which propagate much slower than a wave pulse in a non-random medium. These slow waves build an initial stage of a wave localization phenomenon. However, this effect can be very weak in a real three-dimensional medium.
- Nocera, L.
Medrek, M.
Murawski, K., Solar acoustic oscillations in a random density field (vol 373, pg 301, 2001), Astronomy & Astrophysics, 407(2), 2003, 759-759
- Murawski, K., Random sound waves in a weakly stratified atmosphere, Waves in Random Media, 12(4), 2002, 433-441
Abstract: The influence of random mass density and velocity fields on the frequencies and amplitudes of the sound waves that propagate along a constant gravity field is examined in the limit of weak random fields, small amplitude oscillations and a weakly stratified medium. Using a perturbative method, we derive dispersion relations from which we conclude that the effect of a space-dependent random mass density field is to attenuate sound waves. Frequencies of these waves are higher than in the case of a coherent medium. A time-dependent random mass density field increases frequencies and amplifies the sounds waves. On the other hand, a space-dependent random flow reduces the wave frequencies and attenuates the sound waves. The time-dependent random flow raises the frequencies of the sound waves and amplifies their amplitudes. In the limit of the gravity-free medium the above results are in an agreement with the former findings.
- Medrek, M.
Michalczyk, J.
Murawski, K.
Nocera, L., Numerical simulations of random sound waves, Waves in Random Media, 12(2), 2002, 211-221
Abstract: We perform one-dimensional numerical simulations of both driven and impulsively generated sound waves propagating through a medium whose mass density admits time-independent, random fluctuations. While the amplitude of both types of wave is always attenuated, driven sound waves can be either retarded or speeded up depending on their wavenumber and amplitude and on the strength of the random field. The speed of a pulse propagating in the random medium is also altered, in agreement with the findings for the driven waves. The concomitant action of nonlinearity and randomness results in wave speeding for wavenumbers which are of the order of the size of an average random density fluctuation, whereas it gives retardation for larger wavenumbers.
- Murawski, K.
Nocera, L.
Pelinovsky, E. N., Influence of wave noise on frequencies and amplitudes of the solar p-modes, Astronomy & Astrophysics, 387(1), 2002, 335-338
Abstract: The influence of space- and time-dependent random mass density field, associated with granules, on frequencies and amplitudes of the solar p-modes is examined in the limit of weak random fields and small amplitude oscillations. The p-modes are approximated by the sound waves which propagate in the gravity- free medium. Using a perturbative method, we derive a dispersion relation which is solved for the case of wave noise for which the spectrum E(k, w) similar to E(k)delta(w - c(r)k), where delta is the Dirac's delta-function and c(r) is the random phase speed. We find that at c(r) = w/k a resonance occurs at which the cyclic frequency w tends to infinity. For values of c(r) which are close to the resonance point, the frequency shift may be both negative or positive and the imaginary part of the frequency attains the negative (positive) sign for c(r) < w/k(c(r) > w/k).
- Murawski, K.
Nocera, L.
Medrek, M., The effect of time-dependent random mass density field on frequencies of solar sound waves, Astronomy & Astrophysics, 376(2), 2001, 708-712
Abstract: The effect of a time-dependent random mass density field on the frequencies and amplitudes of solar p-modes approximated as sound waves is considered by analytical perturbative means and numerical simulations for one-dimensional hydrodynamic equations. The analytical results, which are worked out for a Gaussian spectrum of the random mass fluctuations, show frequency increase and amplitude amplification, in agreement with numerical simulations.
- Murawski, K.
Oliver, R.
Ballester, J. L., Nonlinear fast magnetosonic waves in solar coronal holes, Astronomy & Astrophysics, 375(1), 2001, 264-274
Abstract: A coronal hole is modeled as a slab of cold plasma threaded by a vertical, uniform magnetic field. A periodic driver acting at the coronal base is assumed to drive the velocity component normal to the equilibrium magnetic field. Previous works indicate that, in the linear regime, only fast mode perturbations propagate, since Alfven waves are excluded from the model and the slow wave is absent in the cold plasma limit. However, in this work, it is shown that nonlinear terms in the magnetohydrodynamic (MHD) equations give rise to excitation of the velocity component parallel to the equilibrium B, with a lower amplitude than the normal component. Another consequence of nonlinearities is the generation of higher-frequency Fourier modes, which can be detected by Fourier analyzing the velocity variations above the photosphere. The nature of the nonlinear interactions in the MHD equations determines the frequency of those modes. These interactions are quadratic in the case of the parallel component, while they are cubic in the case of the normal component. Therefore, nonlinearly excited frequencies 2w(d), 4w(d), 6w(d), : : : are present in the parallel velocity, whereas frequencies 3w(d), 5w(d), 7w(d), : :: are present in the normal velocity, with w(d) the driving frequency.
- Nocera, L.
Medrek, M.
Murawski, K., Solar acoustic oscillations in a random density field, Astronomy & Astrophysics, 373(1), 2001, 301-306
Abstract: The influence of a space-dependent random mass density field on the development of solar p-modes is investigated using analytical and numerical means. Using a perturbative approach, which is valid for a weak random field and small amplitude waves, we derive a linear dispersion relation whose solutions correspond to attenuated oscillations. The real part of the frequency of these oscillations exceeds the one of waves propagating in a medium without random density. We give an interpretation of the "unphysical" nature of the frequency shift and of the amplitude attenuation which is similar to Landau damping. The analytical findings are compared with the results of the numerical solution of a model wave equation. We find that, for weak random fields and for wavelengths which are a few times the correlation length of the random density fluctuations, numerical results agree with the analytical theory. Two practical formulas for deriving the correlation spectrum of the random density field from observations are also given.
- Murawski, K.
Nakariakov, V. M.
Pelinovsky, E. N., Fast magnetoacoustic waves in a randomly structured solar corona, Astronomy & Astrophysics, 366(1), 2001, 306-310
Abstract: The propagation of fast magnetoacoustic waves in a randomly structured solar corona is considered in the linear and cold plasma limits. The random field is assumed to be static and associated with plasma density inhomogeneities only. A transcendental dispersion relation for the fast magnetoacoustic waves which propagate perpendicularly to the magnetic field is derived in the weak random field approximation. It is shown analytically that the fast magnetosonic waves experience acceleration, attenuation, and dispersion in comparison to the homogeneous case. These analytical findings are essentially confirmed by numerical simulations for a wide-spectrum pulse, except that the waves were found decelerated. It is concluded that the coronal Moreton waves can be applied to MHD seismology of the solar corona.
- Medrek, M.
Murawski, K.
Nakariakov, V., Propagational aspects of sunquake waves, Acta Astronomica, 50(3), 2000, 405-416
Abstract: We present the results of numerical simulations of impulsively generated seismic waves excited by a spatially localized impulse source which is connected with a nearby solar flare. The solar atmosphere is modeled as a two layer medium with constant temperature over the photosphere and linearly growing temperature below the photosphere. Effects of magnetic fields are neglected. Only two dimensional effects are considered. The source is localized slightly below the photosphere. The numerical results show that the initial pulse of enhanced pressure, which can be connected with the thermal energy release by interaction of flare-generated particles with the sub-photospheric medium in the flare-loop footpoint, generates an acoustic (seismic) wave. Interaction of the wave with the solar surface produces perturbations registered as sunquakes. Typical observationally registered features of the sunquakes, such as characteristic wave signatures and acceleration of the wave with the distance from the epicenter, are well reproduced with the model developed. It is found that the seismic waves are essentially dispersive and non-linear. The proposed model provides us with a theoretical basis for sunquake seismology of the solar interior.
- Murawski, K., Turbulent f-mode in a stratified solar atmosphere, Astronomy and Astrophysics, 360(2), 2000, 707-714
Abstract: The solar f-mode is a surface gravity wave which high horizontal wavenumber k and the frequency omega satisfy the dispersion relation omega(2) = gk, where g is the surface gravity of the Sun. However, the observations of this mode revealed deviations from this simple dispersion relation. According to these observations for high values of k the f-mode frequency is significantly lower than the frequency given by the simple dispersion relation and the line-width grows with k. We derive a general dispersion equation which is valid for arbitrary vertical profiles of the stratified solar atmosphere. As an illustrative example the case of isothermal atmosphere is considered. Solving this equation numerically for various parameters of the equilibrium and the turbulent flow we find that the frequencies and line-widths of the turbulent f-mode are close to those observed recently by the SOHO/MDI.
- Murawski, K.
Pelinovsky, E. N., The effect of random flow on solar acoustic waves, Astronomy and Astrophysics, 359(2), 2000, 759-765
Abstract: We examine the influence of a random flow that occurs in the convection zone on frequencies of the solar acoustic oscillations. Using a perturbative method the dispersion relation is derived in the case of a random flow that is parallel to the wave propagation. This relation is subsequently solved numerically for various parameters of the random field to find the frequencies of the random acoustic waves. The numerical results reveal that these waves can be both amplified and damped by the random flow: also their frequencies can be higher and lower than the frequencies of the coherent acoustic waves. The amplification and frequency increase are more pronounced for low spherical harmonic degree l, stronger random flow, and lower spatial correlation length, These analytical findings are tested against numerical simulations for the full set of linear equations, without introducing a weak random field approximation. Numerical results reveal for the first time wave amplification and frequency decrease of the sound waves.
- Murawski, K., Influence of coherent and random flows on the solar f-mode, Astrophysical Journal, 537(1), 2000, 495-502
Abstract: The solar f-mode is referred to as a surface gravity wave that exists right below the solar corona. For a high horizontal wavenumber k, the frequency omega of this mode is given by the dispersion relation omega(2) = gk, where g is the surface gravity of the Sun. However, the observations of this mode revealed deviations from this simple dispersion relation. According to these observations, for high values of k, the f- mode frequency is significantly lower than the frequency given by the simple dispersion relation and the line width grows with k. The purpose of this paper is to display some of the features that go into a theoretical description of the flow that occurs in the convection zone and its effect on the spectrum of the f- mode oscillations. In particular, we aim to consider coherent and random flows that may be associated with supergranules and granules. In the case of the coherent horizontal flow Vb, we derive a general dispersion equation that is valid for arbitrary equilibrium profiles. The space-dependent flow V-0(z) exhibits a singularity at the vertical position z = z(c), for which omega/k = V-0(z(c)). As a special example, the case of an isothermal atmosphere and a uniform flow is discussed in detail to show the Doppler effect. In the case of the random flow, we generalize the dispersion relation derived by Murawski & Roberts for a space- and time-dependent velocity field. While the effect of a space-dependent random held is to reduce frequencies and attenuate the f-mode, a time-dependent random flow can increase frequencies and amplify the f-mode. Solving the random dispersion equation numerically for various parameters of the equilibrium and random field, we find that the frequencies and line widths of the random f-mode are close to those observed recently by the Solar and Heliospheric Observatory (SOHO) MDI instrument.
- Murawski, K., Influence of a stochastic flow on acoustic waves, Acta Astronomica, 50(2), 2000, 269-277
Abstract: An influence of a random flow on frequencies and amplitudes of the acoustic oscillations is examined on the basis of numerical simulations for the linear hydrodynamic equations. The case of the random flow that is parallel to the direction of a wave propagation is discussed for various parameters of the random field. The numerical results reveal that the sound waves can be both amplified and damped by the random flow as well as their frequencies can be higher and lower than frequencies of the coherent acoustic waves. These effects are more pronounced for stronger random flow. Spectral properties of the sound waves depend on a flow pattern.
- Murawski, K., Spectrum of a random f-mode and the SOHO/MDI data, Astronomy & Astrophysics, 358(1), 2000, 343-346
Abstract: According to the SOHO/MDI observations (Duvall et al. 1998, Antia & Basu 1999) the solar f-mode frequency departs from the parabolic dispersion relation omega(2) = gk. We propose to explain this behaviour in terms of a space- and time-dependent random flow that occurs in the convection zone. A time- dependent random flow speeds up the f-mode while the effect of a space-dependent random flow is to slow it down. A competition between these two effects brings a reduction of frequencies and a line-width increase at low l. Theoretical estimation of the frequency shift and line-width leads to a conclusion that for l < 2000 the results are consistent with the recent SOHO/MdDI data.
- Murawski, K.
Diethelm, K., Randomly generated spectrum of the solar f-mode, Astronomy and Astrophysics, 358(2), 2000, 753-758
Abstract: We show that a random flow that depends on space and time generates a mode which is damped and move slower than the coherent f-mode as well as possesses properties which are consistent with the recent SOHO/MDI data (Antia & Basu 1999).
- Medrek, M.
Murawski, K., Influence of turbulent energy spectra on damping and frequency reduction of the solar f-mode, Astrophysical Journal, 529(1), 2000, 548-553
Abstract: This paper generalizes the random wave theory that was developed to explain the recently observed line width spreading and frequency reduction of the f-mode. The generalization is based on a replacement of the Gaussian energy spectrum by a more realistic spectrum such as von Karman, Reynolds, or exponential as well as on an averaging of the results over various granules. The f-mode reduces its frequency as it spends more time propagating against the flow than with the flow. As a result, its effective speed and consequent frequency omega are reduced. This reduction is revealed by the real part of omega. The negative imaginary part of the frequency represents the damping of the coherent f-mode held due to scattering by turbulent flow. The f-mode damping is a result of the generation of the turbulent field at the expense of the coherent held. Theoretical estimation of the line width and frequency shift leads to the conclusion that for high spherical degree the results are consistent with the properties of the f- mode obtained from the high-resolution Michelson Doppler Imager (MDI) data from the Solar and Heliospheric Observatory recently reported by Duvall et al. As a result of averaging, we have obtained a significant improvement of our theoretical results.
- Medrek, M.
Murawski, K.
Roberts, B., Damping and frequency reduction of the f-mode due to turbulent motion in the solar convection zone, Astronomy and Astrophysics, 349(1), 1999, 312-316
Abstract: Solar-f-mode properties were observed recently with high accuracy using high-resolution Michelson Doppler Imager (MDI) data from SOHO (Duvall et al. 1998). According to these observations, linewidths increase with wavenumber k and the f- mode frequency omega is significantly lower than the frequency omega(0) given by the simple dispersion relation omega(0)(2) = gk. This paper provides a possible explanation of these observations on the basis of the turbulent flow that is in the convection zone. The f-mode spends more time propagating against the flow than with the how. As a result, its effective speed and consequently frequency are reduced. This reduction is revealed by the real part of omega. A negative imaginary part of the frequency omega represents the damping of the coherent- f-mode field due to scattering by the turbulent flow. The f- mode damping is a result of the generation of the turbulent field at the expense of the coherent field.
- Murawski, K.
Boice, D. C.
Huebner, W. F.
DeVore, C. R., Two-dimensional MHD simulations of the solar wind interaction with Comet Halley, Acta Astronomica, 48(4), 1998, 803-817
Abstract: Numerical simulations are performed in the framework of nonlinear two-dimensional magnetohydrodynamics to investigate the solar wind interaction with Comet Halley at 0.83 a.u. corresponding to the Vega 2 encounter. The governing equations are solved by a flux corrected transport method. For a gas (water) production rate of 10(30) molecules/sec, it was found that a weak bow shock is formed at about 0.5 million km upstream the comet nucleus while a contact surface and an inner shock are located at about 20 000 km in the subsolar direction from the nucleus. We also found an enhancement of the ion-mass density just inside the contact surface, at the position of the inner shock. The model reasonably reproduced the locations of the bow shock and the diamagnetic cavity that were observed by recent missions to Comet Halley.
- Duvall, T. L.
Kosovichev, A. G.
Murawski, K., Random damping and frequency reduction of the solar f-mode, Astrophysical Journal, 505(1), 1998, L55-L58
Abstract: We present observations showing that the frequency of the high- degree f-mode is significantly lower than the frequency given by the simple dispersion relation, omega(2) = gk, and that the line width grows with the wavenumber k. We attempt to explain that this behavior is the result of the interaction with granulation, which we model as a random flow. Because the f- mode spends more time propagating against the how than with the flow, its effective speed and, consequently, frequency are reduced. Additionally, an eddy viscosity introduces the negative imaginary part of frequency. This negative imaginary part represents the damping of the coherent field due to scattering. The line width is proportional to the magnitude of the imaginary part of the frequency. We apply an analytical perturbation technique and numerical methods to estimate the line width and the frequency shift, and we show that the results are consistent with the properties of the f-mode obtained from the high-resolution Michelson Doppler Imager data from the Solar and Heliospheric Observatory.
- Murawski, K.
Aschwanden, M. J.
Smith, J. M., Impulsively generated MHD waves and their detectability in solar coronal loops, Solar Physics, 179(2), 1998, 313-326
Abstract: Impulsively generated magnetohydrodynamic waves in solar coronal loops, with arbitrary plasma beta, are studied numerically by a flux-corrected transport algorithm. Numerical results show that the total reflection which occurs in the region of low Alfven speed leads to trapped fast kink magnetosonic waves. These waves propagate along the slab and exhibit periodic, quasi-periodic, and decay phases. As a consequence of the difference in wave propagation speeds, the time signatures of the slow magnetosonic waves are delayed in time in comparison to the time signatures of the fast magnetosonic and Alfven waves. An interaction between the waves can generate a longer lasting and complex quasi-periodic phase of the fast wave. We discuss also the observational detectability of such Mi-ID waves in optical, radio, and soft X-ray wavelenghts.
- Nakariakov, V. M.
Roberts, B.
Murawski, K., Nonlinear coupling of MHD waves in inhomogeneous steady flows, Astronomy & Astrophysics, 332(2), 1998, 795-804
Abstract: The nonlinear coupling of MHD waves in a cold (beta = 0) compressible plasma with a smoothly inhomogeneous low-speed steady flow directed along the magnetic field is considered. The effect is similar to Alfven wave phase mixing in a static, inhomogeneous medium and leads to the production of steep transversal gradients in the plasma parameters, which increases dissipation. Transversal gradients in the total pressure, produced by phase mixing, lead to the secular generation of obliquely propagating fast magnetosonic waves, at double the frequency and the wavenumber of the source Alfven waves. The efficiency of the generation is defined by the Alfven wave amplitude and the transversal spatial scale of the flow inhomogeneity. The secular growth of density perturbations, connected with fast waves, takes place for flow speeds that are considerably below the thresholds of the Kelvin-Helmholtz and negative energy wave instabilities. The initial stage of the nonlinear generation of the fast waves is considered analytically and illustrated by numerical simulations.
- Oliver, R.
Murawski, K.
Ballester, J. L., Numerical simulations of impulsively generated MHD waves in a potential coronal arcade, Astronomy and Astrophysics, 330(2), 1998, 726-738
Abstract: Impulsively generated waves in coronal arcades are simulated numerically by an application of nonlinear ideal magnetohydrodynamic (MHD) equations. The simulations sire performed in the (x, z)-plane on a non-uniform Cartesian mesh. In this geometry the magnetic field can be expressed in terms of the vector potential. The governing equations, which are applied in the limit of low plasma-beta, are solved by a Aux corrected transport method. The model excludes the Alfven waves and, since the slow mode is absent in the cold plasma limit, the excited disturbances are fast magnetosonic waves. Numerical results show that for short times after the impulse is launched (i. e., in the linear regime), only motions normal to the equilibrium magnetic field get propagated away from the position of the initial displacement and that any velocity parallel to the unperturbed magnetic field lines remains essentially unchanged in time. In the nonlinear regime there is conversion between normal and. parallel flow and the two velocity components propagate from the site of the initial impulse. In addition, nonlinearities that are built in the MHD equations modify the shape and speed of the propagating wavefront, an effect that becomes most noticeable where the wave amplitude is larger The effect of nonlinearity on down- going perturbations is to speed up positive wave amplitudes and to slow down negative wave amplitudes (positive and negative refers to die sign of the normal velocity component). On the contrary, up-going positive and negative waves are slowed down and speeded up, respectively. Impulsively generated waves exhibit temporal signatures with characteristic time scales of the order of 10 s. Similar scales have been recently reported in radio observations. microwaves, and hard X-rays.
- Murawski, K.
Tanaka, T., Modern numerical schemes for solving magnetohydrodynamic equations, Astrophysics and Space Science, 254(2), 1997, 187-210
Abstract: In this paper several modern shock-capturing schemes for solving hydrodynamic and magnetohydrodynamic equations are reviewed. This review covers a wide range of explicit and implicit schemes as well as those in which adaptive mesh refinement methods are adopted. As these numerical schemes are based on Riemann solvers which use Godunov-type techniques, they are well suited for strong shocks and other discontinuities without oscillations in the flow variables. Some other numerical issues as grid generation, divergence cleaning, and an application of MHD schemes to several problems in coronal and interplanetary physics are discussed.
- Nakariakov, V. M.
Roberts, B.
Murawski, K., Alfven wave phase mixing as a source of fast magnetosonic waves, Solar Physics, 175(1), 1997, 93-105
Abstract: The nonlinear excitation of fast magnetosonic waves by phase mixing Alfven waves in a cold plasma with a smooth inhomogeneity of density across a uniform magnetic field is considered. If initially fast waves are absent from the system, then nonlinearity leads to their excitation by transversal gradients in the Alfven wave. The efficiency of the nonlinear Alfven-fast magnetosonic wave coupling is strongly increased by the inhomogeneity of the medium. The fast waves, permanently generated by Alfven wave phase mixing, are refracted from the region with transversal gradients of the Alfven speed. This nonlinear process suggests a mechanism of indirect plasma heating by phase mixing through the excitation of obliquely propagating fast waves.
- Tanaka, T.
Murawski, K., Three-dimensional MHD simulation of the solar wind interaction with the ionosphere of Venus: Results of two-component reacting plasma simulation, Journal of Geophysical Research-Space Physics, 102(A9), 1997, 19805-19821
Abstract: The large-scale solar wind interaction with the Venusian ionosphere is numerically simulated in the framework of two- component, three-dimensional magnetohydrodynamics (MHD). The finite volume total variation diminishing scheme is used to solve this problem. The impinging solar wind is represented by H+ ions, and the ionosphere is assumed to consist of O+ ions produced by photoionization of atomic oxygen in the Venusian upper atmosphere and by charge exchange of CO2+ ions. The O+ ions are lost by charge exchange with carbon dioxide molecules. The numerical simulations are performed for interplanetary magnetic field (IMF) perpendicular to the solar wind flow and for the solar wind parameters which correspond to maximum solar activity. Results of the calculation give the formation of the bow shock, the magnetic barrier, and the ionopause in the dayside region as a self-consistent state of the interaction processes. The dynamical behavior of the dayside ionosphere under the influence of the impinging solar wind and the IMF slipping over the pole results in the formation of wing-like bulges of the ionosphere and an accompanying poleward flow in the topside ionosphere. The model also reproduces several features of the nightside ionosphere that are predicted by earlier theories and observations, including complex structures such as a flattened ionotail, tail rays, and ionospheric holes, as a continuation of the wing-like bulge. It is also shown that slow plasma flow in the ionotail and nonideal MHD process play important roles in the formation of the induced magnetotail of the planet.
- Murawski, K.
Steinolfson, R. S., Numerical simulations of mass loading in the solar wind interaction with Venus, Journal of Geophysical Research-Space Physics, 101(A2), 1996, 2547-2560
Abstract: Numerical simulations are performed in the framework of nonlinear two-dimensional magnetohydrodynamics to investigate the influence of mass loading on the solar wind interaction with Venus. The principal physical features of the interaction of the solar wind with the atmosphere of Venus are presented. The formation of the bow shock, the magnetic barrier, and the magnetotail are some typical features of the interaction. The deceleration of the solar wind due to the mass loading near Venus is an additional feature. The effect of the mass loading is to push the shock farther outward from the planet. The influence of different values of the magnetic field strength on plasma evolution is considered.
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