S. C. Chakravarty

Vertical structure of mesospheric echoes from the Indian MST radar

An experiment has been carried out using the Indian MST Radar located at Gadanki (13.5degreesE, 79.2degreesN) to investigate the radio wave (53 MHz) scattering characteristics from mesospheric region and associated dynamical phenomena. The main objective is to study the temporal variation of the vertical structure of the radar return echo power and the dynamics of the scattering regions within the mesosphere based on the observations conducted during the period March 1998-July 1999 with radar time provided for 2 days per month. The results presented here show general agreement with those obtained earlier using the Jicamarca radar. However, some of the new features brought out by the analysis of data include (a) significant height variation of the main scattering field/layer occurring between 68 and 76 km with lowest in winter and highest in summer months, (b) the intensity of radar return echo shows periodic variations indicating generation and decay of major scattering centres caused by neutral turbulence and (c) the absolute magnitudes of the fine-of-sight velocities are found to be anti-correlated with the strength of the return echo power. The mechanisms leading to the fluctuations in the refractive indices responsible for mesospheric echoes are discussed and a synthesised view is presented for possible explanations to these results

Influence of aerosols on middle atmospheric conductivities

Existing models of atmospheric conductivities and ion densities derived from the ion production rates in the middle atmosphere due to cosmic rays do not take into account the effects produced by aerosols and their particle size distribution. A modified 13 ion model by Mitra is used to compute the small ion densities in the troposphere and stratosphere as against the simple 6 ion scheme which is applicable to the D-region of the ionosphere. Incorporating the measured height profile of integrated aerosol concentrations into the 13 ion scheme along with the laboratory data on aerosol-neutral attachment, aerosol ion-small ion and aerosol positive and negative ion recombination coefficients, the equilibrium condition small ion densities are computed and compared with the small ion densities in the absence of aerosols. On an average it is found that the small ion densities are reduced by about 4% in the presence of normal aerosol particles. The computations when repeated to take into account the aerosol size distributions by assuming a suitable power law distribution, the influence of aerosol appears to be reduced to about 0.1%. However, it is found that for relatively large size aerosol particles the effect continues to be appreciable. This indicates that during a volcanic eruption when the aerosol size distribution gets altered considerably towards bigger size aerosol particles, a large hole in the small ion densities may appear till this anomalous condition returns to normal aerosol condition after a period of about a few months.

Middle atmospheric electrodynamics at low lattitude over India

Abstract Low latitude middle atmospheric electrodynamics has a number of interesting features such as low flux of cosmic rays due to high cut-off rigidities, higher solar electromagnetic radiation intensities and mesospheric ionisation, large scale convection and wide-spread lightning phenomena, higher tropopause level and special characteristics associated with the equatorial electrojet. These special features play a vital role in governing the electrodynamics of the global middle atmosphere. In India, a well coordinated multi-institutional campaign was organised under MAP (Middle Atmosphere Programme) to carry out balloon and rocket borne experiments to measure electrical parameters of the middle atmosphere. The measured parameters include electron/ion densities and mobilities, polar ion conductivities and electric fields. The experiments were carried out during different seasons and solar activity epochs. The results obtained from these investigations are discussed and compared with similar measurements over the middle latitude stations to assess the integrated effect of the global atmospheric electrodynamic phenomenon.