Thermal overstability of hydromagnetic surface waves

We investigate the effects of radiative heat losses and thermal conductivity on the hydromagnetic surface waves along a magnetic discontinuity in a plasma of infinite electrical conductivity. We show that the effects of radiative heat losses on such surface waves are appreciable only when values of the plasma pressure on the two sides of the discontinuity are substantially different. Overstability of a surface wave requires that the medium in which it gives larger first-order compression should satisfy the criterion of Field (1965). Possible applications of the study to magnetic discontinuities in solar corona are briefly discussed.Collaborative programme in Astronomy and Astrophysics, Department of Physics, Indian Institute of Science, Bangalore 560012, India.     

Compressibility effects on hydromagnetic surface waves

The dispersion equation for hydromagnetic surface waves along a plasma-plasma interface has been solved as a function of the compressibility factor c ₁/v _A1, where c ₁ and v _A1 are the acoustic and Alfvén wave speed in one of the medium, for general wave propagation direction. Both slow and fast magnetosonic surface waves can exist. The nature and existence of these waves depends on the values of c ₁/v _A1 and , the angle of wave propagation. For low- plasmas only fast mode exists. The slow mode does not propagate below a critical value of c ₁. When c ₁ the phase velocity of the slow wave tend to the Alfvén surface wave velocity in the incompressible media and for large the phase velocity of the fast wave approaches this value. The phase velocity of the slow wave increases whereas for the fast wave it decreases with increase in the angle .     

Plane and cylindrical hydromagnetic shock waves

The Chisnell-Chester-Whitham method has been used to investigate the propagation of diverging plane and cylindrical shock waves through an ideal gas in presence of a magnetic field having only constant axial and variable azimuthal components, simultaneously for both weak and strong cases. Assuming an initial density distribution ₀=r ^–w , where is the density at the plane/axis of symmetry andw is a constant, the analytical expressions for shock velocity and shock strength have been obtained. The expressions for the pressure, the density, and the particle velocity immediately behind the shock have also been derived for both cases.     

Resonant interaction of hydromagnetic waves with charged particles

The effect of the resonant interaction of a distribution of hydromagnetic waves on a distribution of particles is described by a diffusion equation in momentum space. The diffusion coefficients and other coefficients describing systematic acceleration and diffusion in energy space are derived in general and for a number of particular cases. It is shown that the resonant acceleration of slow ions by hydromagnetic waves is ineffective. The time evolution of the energy spectrum for ultrarelativistic particles due to interaction with hydromagnetic waves is found and applied to the case of the Crab Nebula in an accompanying article.     

Transverse hydromagnetic waves as a radiation mechanism of pulsars

Transverse hydromagnetic waves, excited and amplified by firehose instability, probably exit in highly magnetised neutron stars. Such waves may leak out in the form of a beam of electromagnetic radiation, which will have a polarization pattern similar to that in the single vector model.     

Ducted propagation of hydromagnetic waves in the upper ionosphere

This paper presents a theoretical investigation of the ducting of Pc1 hydromagnetic waves in an ionospheric layer situated above the F₂ region. Theoretical calculations show that this upper ionospheric duct may also sustain horizontal propagation of Pc1 pulsations over appreciable distances. It is found that there is a low-frequency cutoff as in the case of the F₂ layer waveguide. The group velocity of waves in the upper ionospheric duct is considerably greater, and dispersion is more pronounced compared to the ducted propagation in the F₂ region.     

Localised compressional hydromagnetic waves in the magnetospheric ring current

It is shown that the field line resonance phenomenon that occurs for quasi-transverse signals in a non-uniform cold plasma generally occurs for disturbances with a compressional magnetic component in a hot plasma like the ring current. The equations describing such localised signals are derived and discussed in various limits. It is pointed out that recent observations of low frequency compressional oscillations on spacecraft may be an example of such localised signals.     

Simultaneous geomagnetic and ionospheric oscillations caused by hydromagnetic waves

During magnetically quiet or slightly disturbed nights, closely correlated oscillations of the geomagnetic field and the F-layer were observed by means of magnetometers and a vertical-icidence continuous-wave Doppler sounder at 3.57 MHz. The magnetic oscillations were mostly Pi2 pulsations with periods from 0.5 to 2 min, and an amplitude of 10^?9 T corresponding to a Doppler shift of the order of 0.3 Hz. The observations cannot be explained by a dynamo-motor hypothesis assuming that the magnetic and ionospheric oscillations are caused by alternating E-layer currents, but they agree well with the theory of downgoing hydromagnetic waves. In particular, this theory explains the observed effects due to sporadic E-layer ionization and ion-neutral collisions. The results are found to differ substantially from those of other authors.     

Internal atmospheric hydromagnetic planetary-gravity waves in zonal wind-magnetic shears

Internal atmospheric hydromagnetic planetary-gravity waves propagating through a latitudinally sheared zonal flow and a zonal magnetic field sheared both latitudinally and vertically are studied.It is shown that the waves possess four critical latitudes. At two of them the waves propagate on one side only with their energy and momentum fluxes differing by an additive factor on the two sides of the critical latitude, whereas at the other two they are capable of propagating on both sides and exhibit the value effect behaviour, with their energy and momentum fluxes attenuated by an exponential factor as they cross the critical latitude.     

Circular polarization of synchrotron radiation in the presence of hydromagnetic waves

It is shown that relativistic electrons in the presence of circularly polarized hydromagnetic waves emit synchrotron radiation which is partially circularly polarized. The relation between the degree of polarization of the radiation and the energy density and wavelength of the waves is derived, and the factors determining the sense of polarization are discussed. Waves of the type required are generated by pitch angle anisotropies in a relativistic electron gas. An application of the result to conditions expected in quasistellar objects shows that the degree of circular polarization of synchrotron radiation in these objects may be of order 1% or greater.     

Propagation characteristics of hydromagnetic waves in various layers of the ionosphere

This paper derives the basic propagation characteristics of hydromagnetic waves in various layers of the ionosphere. It is shown that propagation in the upper ionosphere and the F₂ layer is largely isotropic. In the lower region of the ionosphere there are two possible modes of propagation, both being anisotropic. Propagation characteristics of waves in this lower region, however, are relatively independent of the direction of horizontal propagation. Calculations of intrinsic wave attenuation show that ducted propagation of Pc 1 signals over appreciable horizontal distances may only take place in the upper layers of the ionosphere.     

The structure of strong hydromagnetic shock waves in a thermally-radiating gas

The structure of strong shock waves in a thermally-radiating and electrically-conducting fluid is studied by method of asymptotic matching principle. It is assumed that upstream of the shock the gas is cold and does not radiate thermally while the Mach number (M) is very high so that after being shocked the gas now thermally radiates. The viscosity of the gas () depends on temperature (T) in the simple manner T where is a constant exponent. The problem is worthy of note in high-temperature phenomena in hypersonic flow.     

The effect of asymmetry on toroidal hydromagnetic waves in a dipole field

The meridional and azimuthal electric wave fields are considered as the characteristic toroidal and poloidal components. Neglecting the exchange of energy between these fields leads to a toroidal mode wave equation which retains the principal longitudinal or asymmetric contribution. The asymmetric spectrum appears as a logical extension of the results for the symmetric field line oscillations. The model for this study consists of a dipole field magnetized plasma, whose density is commensurate with conditions in the plasmapause. Eigenperiods are calculated for a broad range of asymmetric modes. Because of the similarity in the latitudinal variation between the symmetric and asymmetric periods, it is imperative to revise current idealized magnetospheric models and incorporate such similarity in future models.     

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.     

Umbral oscillations as resonant modes of magneto-atmospheric waves

Umbral oscillations in sunspots are identified as a resonant response of the umbral atmosphere to forcing by oscillatory convection in the subphotosphere. The full, linearized equations for magneto-atmospheric waves are solved numerically for a detailed model of the umbral atmosphere, for both forced and free oscillations. Resonant fast modes are found, the lowest mode having a period of 153 s, typical of umbral oscillations. A comparison is made with a similar analysis by Uchida and Sakurai (1975), who calculated resonant modes using an approximate (quasi-Alfvén) form of the wave equations. Whereas both analyses give an appropriate value for the period of oscillation, several new features of the motion follow from the full equations. The resonant modes are due to upward reflection in the subphotosphere (due to increasing sound speed) and downward reflection in the photosphere and low chromosphere (due to increasing Alfvén speed); downward reflection at the chromosphere-corona transition is unimportant for these modes.     

Over-reflection of hydromagnetic planetary-gravity waves at the solar helmet streamers and magnetic sectors

Over-reflection of propagating hydromagnetic planetary-gravity waves incident on the current sheets of the helmet streamers and magnetic sectors of the solar corona is investigated.It is shown that over-reflection arises only if the wavenumbers, or the energy fluxes per unit mass, perpendicular to the current sheet of the incident and the transmitted waves in both cases are in opposite directions. The over-reflected waves then draw magnetic energy from the sun's field and communicate it to the interplanetary magnetic field.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

On the energy dissipation of fast hydromagnetic shock waves in the solar chromosphere

MHD equations including dissipation terms are applied to study the most important irreversible processes occurring in fast hydromagnetic shock waves under the conditions of the outer solar atmosphere. The atmosphere is assumed to be permeated by a nearly horizontal, uniform magnetic field, the magnitude and inclination angle of which being parameters of the analysis. Numerical examples, corresponding to situations which might occur in the upper chromosphere, are computed in order to demonstrate the procedure.     

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.     

The instability of hydromagnetic planetary-gravity waves in a zonal flow and transverse magnetic field

Hydromagnetic planetary-gravity waves propagating on a β-plane through a zonal flow and transverse magnetic field are examined for instability. Such instabilities may be related to same physical phenomena in the atmospheres of the Sun and planets and in the Earth's core. It is found that the onset of instability depends on the directions of the vertical and transverse wave-numbers and the zonal flow. It is also shown that as the magnetic field intensity is kept uniform instability can onset provided that the zonal flow strength does not exceed a certain factor, which depends on the parameters of the medium, and then the zonal wavenumbers that can become unstable are limited to a given range. If the basic Alfvén wave speed is allowed to vary whereas the zonal flow is kept uniform the zonal wavenumbers that can exhibit instability are again limited but the basic Alfvén wave speed can assume any value.