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SYNOPSISSynthesis of dihydroxybisimide monomers and their subsequent polycondensation with dichlorophenylphosphine oxide by use of a phase-transfer catalyst are reported. The monomers were characterized by melting point, elemental analysis, and IR spectroscopy. The polyimidophosphonates were characterized by viscosity measurement, molecular weight determination, elemental analysis, IR, ESCA, 'H-NMR, 3'P-NMR, and X-ray diffraction. Thermal stability and glass transition temperature (T,) of the polymers were evaluated by TGA, DTA, and TMA. These polymers are self-extinguishing and readily soluble in highly polar solvents like DMF, DMSO, DMAC, NMP, HMPA, etc.

Abstract. X-ray photons emitted during solar flares cause ionization in the lower ionosphere (~60 to 100 km) in excess of what is expected to occur due to a quiet sun. Very low frequency (VLF) radio wave signals reflected from the D-region of the ionosphere are affected by this excess ionization. In this paper, we reproduce the deviation in VLF signal strength during solar flares by numerical modeling. We use GEANT4 Monte Carlo simulation code to compute the rate of ionization due to a M-class flare and a X-class flare. The output of the simulation is then used in a simplified ionospheric chemistry model to calculate the time variation of electron density at different altitudes in the D-region of the ionosphere. The resulting electron density variation profile is then self-consistently used in the LWPC code to obtain the time variation of the change in VLF signal. We did the modeling of the VLF signal along the NWC (Australia) to IERC/ICSP (India) propagation path and compared the results with observations. The agreement is found to be very satisfactory.

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