Global Electric Circuit
Earth’s Global Electric Circuit – an integrated framework involving the lower atmosphere, the ionosphere and the magnetosphere:
The Earth’s electrical environment can be regarded as a single giant electrical circuit that connects currents and electric fields in the magnetosphere, ionosphere and the Earth’s lower atmosphere. The classical picture of the global electric circuit (GEC) presupposes that the thunderclouds are the only source and the fair-weather fields are set up by the upward flowing currents, from the thundercloud system, which charges the ionosphere to a few hundred kilovolts with respect to Earth. It is now recognized that there are at least two other important sources that contribute to the global fields. The solar wind/magnetosphere dynamo generates 30-150 kV potential drop across the northern and southern polar ionospheric caps which when mapped down to the surface can produce a ±20% changes in the air-Earth current and vertical electric field. A smaller contribution comes from the ionospheric dynamo originating in 100-150 km altitude range due to tidal forcing. Potential difference of 5-10kV are generated by this source in the ionosphere, and when mapped to the surface can produce perturbations typically few percent in the observed fields. The study of GEC provides a good framework for exploring the interconnections and coupling of various regions of the atmosphere. Long-term measurements of electrical parameters, namely, the atmospheric vertical electric field, conductivity and total current density, that are required to obtain an adequate description of the GEC, will be useful in such an integrated approach involving the various atmospheric regions.
Suitability of Antarctic station Maitri, for GEC studies
The Indian Antarctic station, Maitri, has been chosen as our instrument site for several reasons. Most importantly, the station is situated on the Antarctic plateau, a region where the atmosphere is more suited for making measurements of GEC parameters. The global electric circuit involves lower atmosphere and upper atmosphere generators, the latter being significant over polar caps. The Antarctic plateau supports a desert-like-climate with clear skies and very low atmospheric aerosol content. In summer, the prevailing winds are light, flowing in a nearly constant direction, and the atmosphere is relatively free of turbulent and convective motions. Thus the wind-induced conductivity fluctuations that contribute to meteorological noise in the measured atmospheric electric parameters are minimized when experiments are conducted in Antarctica. Atmospheric electrical measurements made in Antarctica are useful in investigating large scale electrical processes that are unique at high latitudes. The downward air-Earth current being delivered to the surface of the Antarctic plateau is larger than the global average owing to its location in high latitudes. Therefore, because of its orography, the Antarctic plateau is strongly coupled to the GEC. The ice surface is flat and is void of any obstructions and the electrical conductivity of the ice surface is several orders of magnitude higher than that of the air. Hence, the surface is essentially an infinite conducting plane at ground potential. Maitri due to its sub-auroral position is an ideal location for these studies. During disturbed magnetic conditions, the auroral oval expands to encompass the ionosphere over Maitri and the dawn-to-dusk magnetospheric electric field that maps into the polar cap region along the geomagnetic field lines gets enhanced. These externally impressed electric fields generate large-scale currents and plasma drifts in the high latitude ionosphere. Part of the electric field is mapped vertically down through the leaky atmosphere. The magnetospheric electric field also has a fluctuating component depending on the solar wind conditions. When impressed on the polar cap region, such a field generates displacement current close to the surface. The Institute has developed suitable sensors designed to measure important electrical parameters, namely, the air-Earth current density, the polar conductivity and the vertical electric field close to the surface. With continuous measurements of atmospheric electricity parameters made from Maitri, there is plenty of scope for addressing to the problems related to the modulation of GEC by terrestrial (other than thunderstorm generator) and extraterrestrial sources, namely, the interaction of solar wind with geomagnetic field resulting in a potential drop across the polar caps, auroral effects, solar flares, solar magnetic sector boundary crossing, etc.
Precursor for the onset of Blizzard:
Our recent observations of the electric field and current density at Maitri indicate that they turn to zero or negative a few hours before the onset of a Blizzard. Can these observations be the precursor for the onset of blizzard? We are looking into more such cases before coming to a conclusion.
Dirunal variation of GEC parameters during pre-blizzard conditions.