Propagating slow sausage waves in a sunspot observed by the New Vacuum Solar Telescope

A sunspot is an ideal waveguide for a variety of magnetohydrodynamic waves, which carry a significant amount of energy to the upper atmosphere and could be used as a tool to probe the magnetic and thermal structure of a sunspot. In this study, we used the New Vacuum Solar Telescope and took highresolution image sequences simultaneously in both Ti O(7058±10 ?A) and Hα(6562±2.5 ?A) bandpasses.We extracted the area and total emission intensity variations of sunspot umbra and analyzed the signals with synchrosqueezing transform. We found that the area and emission intensity varied with both three and five minute periodicity. Moreover, the area and intensity oscillated in phase with each other, this fact hold in both Ti O and Hα data. We interpret this oscillatory signal as a propagating slow sausage wave. The propagation speed is estimated at about 8 km s~(-1). We infer that this sunspot's umbra could have temperature as low as 2800–3500 K.     

Uniform radial motion of sound in a relativistic fluid ball

We present a new class of spherically symmetric exact solutions of the general relativistic field equations. These solutions describe perfect fluid balls with infinite central pressure and central density though their ratio is finite. A member of the class has been studied in detail from which we have constructed a model of causal fluid ball with constant sound speed.     

On thermal waves in stars

It is shown by a certain combination of stella parameters and energy which was released by a outburst, the transfer of that energy outwards is carried out by the thermal wave.     

The generation of high-frequency gravitational waves in a perfect fluid

The problem of the generation of a gravitational wave by an acoustic one is studied in the high-frequency domain for a perfect fluid. The negative result which is found may be of some relevance for astrophysical and cosmological applications.     

Stability of magneto-acoustic waves in a thermally conducting compressible fluid

The stability of magneto-acoustic waves in an inviscid, perfectly conducting isothermal fluid, stratified under constant gravity and subjected to a horizontal magnetic field is investigated in the presence of thermal dissipation.     

Trapping of electromagnetic waves by nonlinear resonant interactions with ion sound waves

Resonant wave-wave interaction among one ion sound wave and two electro-magnetic waves in an isotropic plasma is studied. The emphasis is on the possibility of trapping the electromagnetic wave. Equations for the three-wave system are derived. One particularly interesting case is that for which the frequency of ion sound wave is much less than the frequency of electromagnetic waves. For this case it is shown that energy exchange takes place only between the two high frequency waves. The ion sound wave does not participate in the energy exchange process but acts as a kind of catalyst for the interaction. Simple solutions are obtained. It is found that the electromagnetic energy is trapped within a certain spatial region. The trapping width is found to depend, among other parameters, on the magnitude of ion sound wave perturbation. Possible application of the theory to topside ionospheric observations of field-aligned propagation is discussed.     

On viscosity, conduction and sound waves in the intracluster medium

Recent X-ray and optical observations of the Perseus cluster indicate that a combination of weak shocks at small radii  (≳20  kpc)  and viscous and conductive dissipation of sound waves at larger radii is responsible for heating the intracluster medium and can balance radiative cooling of cluster cores. We discuss this mechanism more generally and show how the specific heating and cooling rates vary with temperature and radius. It appears that this heating mechanism is most effective above  10⁷  K  , which allows for radiative cooling to proceed within normal galaxy formation but stifles the growth of very massive galaxies. The scaling of the wavelength of sound waves with cluster temperature and feedback in the system are investigated.     

Solitary waves of vortices within a rotating,fluid shell

We consider a spherical, solid planet surrounded by a thin layer of an incompressible, inviscid fluid. The planet rotates with constant angular velocity about a fixed axis. The motion imparted by this planetary rotation upon the fluid particles of the ocean has been assumed to be governed by a linear version of the Navier-Stokes equation.We study the vortex motion within this rotating ocean and establish that the propagation of vortices depends on a third-order partial differential equation for the stream function. We prove that, in the most general case, this vorticity equation cannot generate any solitary waves; however, should the vertical component of vorticity satisfy a certain functional relationship, then we have obtained a family of solitary waves of permanent form.Retired, U.S. Naval Research Laboratory, Washington, D.C., U.S.A.     

On the propagation of hydromagnetic waves in a plasma of thermal and suprathermal components

The propagation of MHD waves is studied when two ideal fluids, thermal and suprathermal gases, coupled by magnetic field are moving with the steady flow velocity. The fluids move independently in a direction perpendicular to the magnetic field but gets coupled along the field. Due to the presence of flow in suprathermal and thermal fluids there appears forward and backward waves. All the forward and backward modes propagate in such a way that their rate of change of phase speed with the thermal Mach number is same. It is also found that besides the usual hydromagnetic modes there appears a suprathermal mode which propagates with faster speed. Surface waves are also examined on an interface formed with composite plasma (suprathermal and thermal gases) on one side and the other is a non-magnetized plasma. In this case, the modes obtained are two or three depending on whether the sound velocity in thermal gas is equal to or greater than the sound velocity in suprathermal gas. The results lead to the conclusion that the interaction of thermal and suprathermal components may lead to the occurrence of an additional mode called suprathermal mode whose phase velocity is higher than all the other modes.     

Observations of a Propagating Disturbance in TRACE

TRACE observations from 13 June 1998 in 171 and 195 Å wavelengths show a propagating disturbance, initiated near the origin of a C-class flare. The wave moves through and disrupts diffuse, overarching coronal loops. Only these overlying structures are affected by the wave; lower-lying coronal structures are unperturbed. The front does not appear in contemporaneous Lyman-α observations. The disturbance creates two types of displacement: (1) that of the wave front itself, and (2) those of large anchored magnetic structures, which `bob' due to the wave and show transverse velocities an order of magnitude smaller than those of the front. Comparisons between the 171 and 195 Å data show that the front appears differently at different temperatures. Observations in 171 Å (approx. 0.95 MK) show strong displacement of individual magnetic structures, while 195 Å (approx. 1.4 MK) data reveals a strong wave front and associated dimming but resolve much less structural motion. There is also strong evidence of heating in the material engulfed by the wave front, and comparisons of the 171 and 195 Å data allow us to constrain the temperature of the plasma through which the wave is propagating to 1–1.4 MK. Examination of the trajectories and velocities of points along the front suggests that the disturbance is Alfvénic in nature but contains a compressive component. This is best explained by a fast-mode magnetoacoustic wave. A comparison of the motion of anchored structures to that of the wave front gives a constraint on pulse width. Comparisons with contemporaneous SOHO-EIT full-disk 195 Å data show evidence that the disturbance is contained within a set of transequatorial field lines, such that it propagates from a southern active region to a northern one with no extensive motion to the east or west. The associated transequatorial loops display residual motion for about a hour after they are initially disturbed. These results, coupled with the deflection of wave trajectories, lead us to speculate on field strength differences between the transequatorial loops and the region in the TRACE field of view.     

On thermal waves in stars,II

The regularities of strong thermal waves moving in non-uniform media are considered in some detail. The application of thermal wave theory to Nova outburst interpretation is discussed.     

On magnetoconvection in a rotating fluid layer of high thermal conductivity

This paper explores the thermal instability of a plane fluid layer rotating rapidly about a vertical axis in the presence of a uniform vertical magnetic field. The thermal diffusivity is taken to be large compared with the magnetic diffusivity . For a range of parameters it is shown that an increase of the magnetic field leads to adecrease in the critical Rayleigh number, and two quite distinct physical mechanisms appear to be involved.     

Nonlinear dispersive waves on the surface of a self-gravitating fluid layer

The evolution of two dimensional wave packets on the surface of a self-gravitating fluid layer is investigated and shown to be governed by a nonlinear Schrödinger equation. The wave train of finite amplitude is modulationally unstable. Obtained also are the dynamical equations for the second harmonic resonance. The analysis reveals that the general motion consists of both amplitude and phase modulated waves of which the pure phase and amplitude modulated waves, solitary waves, and phase jump are just the special cases.     

Sound and thermal waves propagation in a hydrogen plasma heated by an external radiation field

The thermal equilibrium of a hydrogen plasma heated and ionized by an external radiation field, diluted by a factorW _* and defined by an effective temperatureT _*, is studied. In addition, the problem of propagation of acoustic and thermal waves in the above plasma model is also analysed. It is found that an external radiation field has stabilizing effects against wave amplification. From the dispersion relation obtained, the phase velocity for sound and thermal waves and their respective scale-length of damping (or anti-damping) is calculated as a function of the frequency for representative values of the plasma temperature and the external radiation fieldT _*.     

Ion-acoustic shock waves in a plasma with weakly relativistic warm ions, thermal positrons and a background electron nonextensivity

A weakly nonlinear analysis is carried out to derive a Korteweg–de Vries-Burgers-like equation for small, but finite amplitude, ion-acoustic waves in a dissipative plasma consisting of weakly relativistic ions, thermal positrons and nonextensive electrons. The travelling wave solution has been acquired by employing the tangent hyperbolic method. Our results show that in a such plasma, ion-acoustic shock waves, the strength and steepness of which are significantly modified by relativistic, nonextensive and dissipative effects, may exist. Interestingly, we found that because of ion kinematic viscosity, an initial solitonic profile develops into a shock wave. This later evolves towards a monotonic profile (dissipation-dominant case) as the electrons deviate from their Maxwellian equilibrium. Our investigation may help to understand the dissipative structures that may occur in high-energy astrophysical plasmas.     

On the thermal instability of a nonuniformly rotating fluid layer heated from below

The thermal instability both as stationary convection and overstability of a nonuniformly rotating fluid layer of vertical extent is considered. It is shown that in both types of instability the effect of the variation of the rate of rotation is to introduce a new branch to the marginal instability curves of uniform rotation which departs from them towards a zero of the Rayleigh number as the horizontal wavenumber approaches zero. The relevance to the solar interior is discussed.     

Oscillations and waves in a sunspot

Observations have been made in H of the vertical velocity distribution in a sunspot. Over the umbra the pattern consists of structures of scale-size 2–3. The velocity distribution undergoes oscillations with a period of about 165 s and typical amplitude ±3 km s^–1, but the pattern breaks down after one or two cycles because the period of oscillation varies typically by ±20 s from place to place. Transverse waves develop in the outer 0.1 of the umbral radius and propagate outwards with a velocity of about 20 km s^–1, becoming gradually invisible by or before the outer penumbral boundary; the amplitude is about ±1 km s^–1 at the umbra-penumbra border.The penumbral waves are believed to be basically of the Alfvén type, with 3 × 10^–8 g cm^–3. The umbral oscillations presumably represent gravity waves. In both cases the fluxes are inadequate by two orders of magnitude to account for the sunspot energy deficit.