Accurate approximate formulae for toroidal standing hydromagnetic oscillations in a dipolar geomagnetic field

Calculations of the toroidal eigenmodes of oscillation of the magnetospheric plasma have been important in explaining the nature of Pc3, Pc4 and Pc5 geomagnetic pulsations. In this paper perturbation solutions of the governing equations are presented. These are much more accurate than the WKB approximation which has often been used, and much simpler to compute than the numerical solutions which have been used. A method of including the finite ionospheric conductivity is also presented.     

The relationship between ULF geomagnetic pulsations and ionospheric Doppler oscillations: Model predictions

This paper is a continuation of the work of Sutcliffe and Poole (1989, J. geophys. Res. 94, 13,505) which described the mechanisms which connect ground-based geomagnetic pulsation measurements with simultaneously observed Doppler velocity oscillations (V^*) in the ionosphere. We concentrate on a presentation of model predictions that show the extreme variability of V^* with parameters such as radio sounding frequency, electron concentration profile, magnetoionic mode, geomagnetic field inclination, scale length and pulsation frequency. The potential to use these results in a diagnostic fashion is also discussed.     

Associated geomagnetic and ionospheric variations

An investigation has been conducted into the association between geomagnetic micropulsation activity and ionospheric disturbances. It has been found that both correlated oscillations and associated irregularities may be observed at various times.     

A hydromagnetic theory of geomagnetic storms and auroras

A theory of geomagnetic storms, auroras and associated effects is further developed. It depends on motions in the Earth's exosphere or magnetosphere initiated by a combination of pressure and frictional drag of the solar wind and modified and extended by electric fields and currents in the ionosphere. Motion may be non-divergent, streamline flow opposed only by Lorentz forces in the ionosphere and not propagating to Earth, or divergent, non-streamline motion opposed by Lorentz forces in the Earth. The two types of motion are coupled in the E region where the former is identified with free flow of Hall current and the generation of non-streamline motion. The latter is identified with blockage of Hall current, the creation of a polarization field and hence the generation of streamline motion.

A theory of all components of a geomagnetic storm is given in terms of combinations of these motions, and their distant, ionospheric and earth currents. This includes a new theory of the preliminary reverse part of the DS field and the transition from the sudden commencement to the main phase of the DS field. It is extended to introduce briefly a theory of auroras based mainly on ionospheric drifts caused by the magnetospheric motions.     

Standing hydromagnetic oscillations in the magnetosphere

Many types of ULF pulsations observed at geosynchronous orbit exhibit properties of standing shear Alfvén waves. Observation of the harmonic mode, polarization state and azimuthal wave number is crucial for determining the source of energy responsible for excitation of these waves. In recent years it has become possible to identify the harmonic mode of standing waves from dynamic spectral analysis, as well as simultaneous observations of electric and magnetic fields of the waves or a comparison between plasma mass density estimated from the frequency of the waves and that observed by direct measurement. It is then more reasonable to classify pulsations according to their physical properties, including the harmonic mode, polarization state, azimuthal wave number, and localization in occurrence, than according to the conventional scheme based on the wave form and period range. From analysis of magnetic pulsations observed at geosynchronous orbit, at least two distinctively different types of waves have been identified. One is azimuthally polarized waves simultaneously excited at the fundamental and several harmonics of a standing Alfvén wave which are observed throughout the day side. They have relatively small azimuthal numbers (less than 10) and propagate tailward. They are likely to be excited by the interaction of the solar wind with the magnetopause or bow shock. Another type is radially polarized waves most strongly excited at the second harmonic. They are observed mainly on the afternoon side. Bounce resonance of a few keV ions has been suggested as the mechanism for excitation of the radially polarized waves.     

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.     

Multisatellite observations of resonant hydromagnetic waves

Most measurements of long period ULF pulsations have come from ground based and single satellite observations. The observations have given strong support to the idea that these waves are resonant standing hydromagnetic waves on geomagnetic field lines. Simultaneous ground-satellite observations provide further details of the pulsation structure and are useful for examining the effect of the ionosphere on the transmission of the waves to the ground. Recently, multisatellite observations have been used to provide further insight into the nature of pulsations and we review the results obtained using this technique. Among the results presented are those from the ISEE 1 and 2 spacecraft which are closely spaced in identical orbits, making it possible to distinguish temporal from spatial structure in waves. The ISEE spacecraft have made measurements of resonant region widths and resonance harmonics. In addition, examples are shown of recent multisatellite observations of the global nature of some pulsations and the localization of Pi2 pulsations in space.     

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 .     

Simultaneous storm-time increases of the ionospheric total electron content and the geomagnetic field in the dusk sector

Using the Faraday rotation technique with the ATS-3 satellite, it has been possible to monitor changes in the total electron content (N_T) of the mid-latitude ionosphere during the first day of 20 geomagnetic storms. Our analysis has shown that during the positive phase (ΔN_T > 0) of ionospheric storms the absolute magnitude of the increase in N_T exhibits a very pronounced maximum near sunset. The mean value of ΔN_T at 17:00 LT is more than five times the average ΔN_T value at local noon. This effect is basically independent of the storm commencement time and is usually associated with substantial local enhancements of the total geomagnetic field. The N_T enhancements are discussed in terms of a contraction and draining of the plasmasphere. A model is presented in which the dawn-dusk electric field responsible for the magnetospheric convection slows down the corotational motion of the plasmaspheric ionization in the dusk sector. This braking action causes a ‘pile up’ of the plasma and the magnetic field along the entire dusk sector.     

Artificial oscillations of geomagnetic field line

Forced oscillations of a geomagnetic field tube are theoretically studied. The oscillation source is the ionosphere the conductivity of which is modulated artificially. The amplitude-frequency characteristics of the tube are calculated in the Pc3–5 frequency range.     

Further studies of ionospheric and geomagnetic effects of sudden impulses

From riometer records for the sudden impulse event of 4 February 1969, it is shown that ionospheric absorption accompanying a sudden impulse has the same type of latitude and longitude variations found for sudden commencement events. In addition, an examination of magnetograms at College, Alaska shows that some positive sudden impulses may trigger negative bays around local midnight, similar to the recent results for sudden commencements.     

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.     

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.     

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.     

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.     

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.     

Dynamo currents representing geomagnetic L variation demonstrated by a multi-layer ionospheric model

A multi-layer ionospheric model and lunar (2,2) tidal mode have been used to calculate dynamo current systems representing lunar geomagnetic semidiurnal variations. Since both the height variation of the ionospheric conductivities and latitudinal dependence of the height of the conductivity peaks have been taken into account, the dynamo current systems agree with equivalent ones (estimated from geomagnetic data) better than those for a thin shell model of the ionospheric conductivity, especially in the polar region.     

Radiation-hydrodynamic waves and global solar oscillations

We investigate nonadiabatic hydrodynamic waves in a nongrey, radiating, thermally conducting, homogeneous atmosphere in LTE with a finite mean free path of photons. Avoiding the Eddington approximation the remaining simplifications in the basic equations are discussed, the generalized dispersion relation is analysed, and some wave properties in a grey model are studied. The properties of waves in a stratified atmosphere are analysed as well. In connection with the predicted properties of the nonadiabatic waves we discuss observations ofp-modes by measuring brightness fluctuations.