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The magnetospheric interaction of Io is characterized by a strong field-aligned current connecting Io to the Jovian ionosphere. It is shown here that the mass-loading effect near Io could be responsible for an enhanced current density in the shear flow region which produce a high level of kinetic Alfven waves for particle acceleration.

Considering the basic equations of plasma and neutral fluids, it is shown, theoretically, how neutral dynamics gives rise to various modes in inhomogeneous, magnetized, and partially ionized plasma observed in space as well as in laboratory plasma. The most general dispersion relation which can describe almost all the modes in the system has been obtained. We have shown that the presence of relative velocity between ions and neutrals, compressibility nature of neutrals and gravity gives rise to a few new low frequency modes. The sources of free energy for the onset of various modes are identified. The effect of coupling of neutral dynamics with plasma pinching force ( J × B ) has been highlighted. It is shown that the phase velocities and the growth rates are very sensitive to the ratio of neutrals with ions and the scale heights associated with inhomogeneities. The potential application of the theoretical results is discussed.

The generation of surface waves due to a strong gradient in the plasma density and its coupling to kinetic Alfven waves near the Io plasma torus are found to be a viable mechanism for particle acceleration leading to a beam of energetic electrons along the magnetic field. It is shown that acceleration to a few kiloelectronvolts is possible and this might explain the intense auroral hiss observed near the inner edge of the Io plasma torus by Voyager 1.

Using a three-fluid model, a linear local analysis of neutral induced low frequency (NILF) mode instability in an inhomogeneous weakly ionized plasma in a magnetic field has been carried out. It is concluded that this novel instability, electro-mechanical in nature, is seeded by the neutrals and is driven by electro-mechanical coupling through the frictional force between the plasma and neutral fluids. The condition for the onset of the instability requires a finite neutral diffusion flow. It is proposed that the NILF mode can provide a physical mechanism to understand the nature and cause of the observed irregularities in the D -region of the Earth's ionosphere [Thrane, E. V. and Grandal, B. (1981) J. atmos. terr. Phys. 43 , 179; Sinha, H. S. S. (1992) J. atmos. terr. Phys. 54 , 49]. However, its general application in any weakly ionized plasma with an ambient magnetic field is not restricted.

A new mechanism for the generation of ionospheric irregularities of scale sizes of the order of 100 m or more, has been developed on the basis of thermal energy source due to collisional current dissipation. The important role is played by the diffusion of particles along and across the magnetic field. The threshold condition for the instability has been obtained and it is shown that these irregularities can be generated in the F-region height of auroral ionosphere when the electric field is enhanced to about 10 mV m −1 .

Non-linear wave-particle interaction in the whistler mode in a non-uniform magnetic field is considered. The effect of the second order resonant particles arising due to nonuniformity of the ambient magnetic field is found to be dominant near the equatorial plane of the Earth. The equations describing the time development of the amplitude and phase of the wave packet have been solved numerically by computing the resonant particle current in a self-consistent manner. The growth of the waves because of trapped particles is found to be substantial for triggering an emission and the changes in phase lead to the frequency-time structure. It is capable of reproducing all kinds of frequency time structure as observed in the case of a morse pulse.

It is shown that a two-dimensional lower-hybrid wave structure can parametrically trigger the growth of VLF and ULF noises in a plasma. Analytical expressions for the increment and threshold of the instability are obtained. Application of our work to the auroral zones of the topside ionosphere is discussed.

The resonant interaction between the whistler mode waves and the energetic electrons near the plasmapause boundary has been studied in the presence of field aligned currents which seem to exist during substorm activity. It is shown that the electrons which carry the current along the direction of the magnetic field enhance the whistler mode growth considerably if the streaming velocity is small compared to the phase velocity of the wave. It is likely that this is one of the mechanisms explaining the intense VLF emissions observed near the plasmapause during substorm activity.