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Investigations on Sun-Earth plasma connection have revealed that magnetic clouds moving with high speeds have higher core magnetic fields. Two most predominant causes of major storms are found to be intense southward interplanetary magnetic fields in the sheath region ahead of the fast interplanetary manifestation of solar coronal mass ejections (ICMEs) and the intrinsically high southward fields of magnetic clouds within ICMEs. Using the WIND magnetic field and plasma parameters and Polar UV imaging data, it was shown that The interplanetary shock impingements on the Earth’s magnetosphere can cause significant auroral zone dayside precipitation. It involves not only the dayside auroral brightening but an expansion of this aurora from noon to 03 and 21 MLT, associated with the shock down-tail propagation. Applying the knowledge of interplanetary and solar causes of storms gained from the space-age to the super intense storm of September 1-2, 1859, it has been possible to deduce that an exceptionally fast (and intense) magnetic cloud was the interplanetary cause of this geomagnetic storm with a Dst ~ -1600 nT.

The studies on the microstructure of the solar wind using Ulysses and Giotto magnetic field data have lead to the discoveries of Magnetic Decreases and arc-polarized waves near comets, respectively.


The study of Polar Plasma Wave Instrument data lead to the discovery of polar cap boundary layer (PCBL) waves, in the frequency range of a few mHz to several 100 kHz, on the auroral field lines that map to the low-latitude boundary layer (LLBL Analyses of high resolution POLAR plasma wave data show that PCBL waves are actually discrete electrostatic and electromagnetic modes. Fluid models based on nonlinear electron-acoustic modes, both in 1D and 2D (dromions) have been proposed to explain the bipolar structures. Nonlinear coupling between large amplitude ion (electron) cyclotron and ion (electron) acoustic waves has been studied. It can account for the high frequency component of the field-aligned bipolar electric field pulses observed within the broadband electrostatic noise in the auroral, polar and magnetotail regions of the Earth's magnetosphere. Works are also in progress for studying the width-amplitude variation patterns for ion (electron) acoustic waves and localized 2D structures like lower hybrid dromions in conjecture with the observations of Polar and FAST satellites. Fluid and particle codes are being developed to undertake the computer simulation studies of beam-driven instabilities and their evolution to explain the observations of nonlinear structures made by Polar, FAST, Geotail etc. It is noted that all wave modes can be generated by ~100 eV to ~10 keV auroral electrons and protons. It is shown that the cyclotron resonant interaction of energetic particles with the waves can lead to their rapid pitch angle scattering. The resulting precipitated energy flux to the ionosphere is sufficient to create the diffuse auroral oval.


It is found that the presence of sufficiently strong fluxes of ionospheric-origin oxygen ions in the near Earth plasma sheet can excite helicon mode and shear flow instabilities which can lead to substorm onset. This study clearly shows that ionosphere cannot be considered as a passive sink of energy during the substorms. On the contrary, the ionospheric feedback, in the form of build up of oxygen ion fluxes in the plasma sheet region, may control or regulate the substorm processes. In the ring current region, energetic hydrogen and oxygen ions can excite several types of quasi-electrostatic modes which can scatter the ions and electrons and deplete the ring current leading to its decay.

R. V. Reddy, S. S. Ghosh, Satyavir Singh, Amar Kakad, G. S. Lakhina

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