IONOSPHERIC PHENOMENA - STUDIES OF EQUATORIAL ELECTROJET AND IONOSPHERIC IRREGULARITIES
Over most of the Earth and in the atmosphere, it has been found on average there is a vertical current density which ranges between 1 and 3 picoamperes per square meter. The source of this current density has been the subject of debate for centuries. In 1920 C.T.R.Wilson suggested that thunderstorms act as electrical generators that drive currents upward and thereby charge the upper atmosphere positively with respect to the Earth's surface. Driven by a potential difference of about 200,000 volts, the charge in the upper atmosphere leaks back to ground through the intervening atmosphere. Based on this simple model, it is expected that the global circuit current density peaks in the UT (universal time) afternoon with a minimum in the early morning, in accordance with the postulate that the thunderstorm peak occurrence probability, when integrated over the globe, is in the late afternoon UT hours. To date, this simplified model on the giant electric circuit operating on the global scale has never been adequately tested. Complexities are introduced when the effects produced by the solar flares on the polar cap ionosphere and the currents within the ionosphere and magnetosphere are considered. The ionosphere and the magnetosphere are no more treated as 'passive' and their contributions are being incorporated in the development of models on the global electric circuit (GEC).
Since 1995 the vertical air-Earth current density has been continuously measured at EGRL using long-antenna wires with appropriate low-noise differential amplifier-filter circuitry. In recent years, plate and ball antenna systems measuring complementary air-Earth current data, a passive antenna system yielding vertical potential gradient measurements and an electric field meter have been inducted in the atmospheric electricity laboratory functioning at EGRL.
Identification of the global component in the measured air-Earth current is severely obscured by effects produced by local processes. With the complete automatic weather station supported by surface pressure observations carried out with the help of an array of microbarographs, a comprehensive study of the influence of local processes (turbulence associated with local convection and any other atmospheric perturbations that influence the electrical parameters) on the measured air-Earth current density is being pursued by the institute scientists at EGRL. The present study, aimed at exploring interconnections and coupling of various regions of the atmosphere in terms of the electrical parameters associated with the GEC, will be strengthened by balloon launchings from Hyderabad for the measurements of atmospheric conductivity and electric field, and simultaneous air-Earth current measurements from Antarctica.
With expertise available on making circuit design for the measurements of electric and magnetic fields, a new experiment has been attempted at EGRL, perhaps for the first time in our country. It has been known for quite some that the lightning-induced electromagnetic impulses propagate within the space separating the Earth and the ionosphere. The phase addition and cancellation of waves that has traversed the globe many times along multiple paths produce resonant lines called the Schumann resonances observable in the frequency range 7-50 Hz. The difficulties in detecting the ELF signals arise because of their very small signal strengths and the obscuring nature of man-made electromagnetic noise in this frequency range.
On successful commissioning of the experiment designed for measuring the Schumann resonance intensities, it is possible to examine the role of thunderstorm activity in the generation of the global electric circuit. Further, it has been shown in the recent past that the ELF signals associated with lightning and severe weather can be used to infer global tropospheric temperatures. With long-term data on Schumann resonance intensities, it is hoped that a continuous record of global temperatures will be made available for examining the global warming that is threatening the world today.