C. Selvaraj

A comparative study of atmospheric Maxwell current and electric field from a low latitude station, Tirunelveli

Simultaneous measurements of atmospheric Maxwell current and electric field, using horizontal long wire antenna and passive horizontal wire antenna system at 1 m above the Earth’s surface, were carried out at Tirunelveli (8.7°N, 77.8°E), India, during January/February 2002. The objectives of the present work have been to understand the nature of the measured atmospheric electrical parameters and explore the possibility of detecting the signature associated with the global thunderstorm activity. As the measurements indicate, the atmospheric electrical parameters at Tirunelveli are severely masked by the locally induced current components during disturbed weather conditions, for example, severe convective activity. During the selected fair weather conditions, the hourly averaged diurnal variation curves of Maxwell current and electric field are characterized by two peaks: The first peak is the local “sunrise effect” and the second peak occurs at times close to 1900 UT, the time of maximum global thunderstorm activity as noted in the famous “Carniege curve”. The correlation coefficient between the measured Maxwell current and electric field has a high value (more than 0.8) for all the fair weather days. These results support our view that this site is free from local pollution during the fair weather conditions and is suitable for long-term measurements of atmospheric electrical parameters.

First results from imaging riometer installed at Indian Antarctic station Maitri

Cosmic noise absorption (CNA) measurred by imaging riometer, is an excellent tool to passively study the high latitude D-region ionospheric conditions and dynamics. An imaging riometer has been installed at Indian Antarctic station Maitri (geographic 70.75°S, 11.75°E; corrected geomagnetic 63.11°S, 53.59°E) in February 2010. This is the first paper using the imaging riometer data from Maitri. The present paper introduces the details of this facility, including its instrumentation, related CNA theory and its applications. Sidereal shift of around 2 hours in the diurnal pattern validates the data obtained from the newly installed instrument. Moreover, the strength of cosmic noise signal on quiet days also varies with months. This is apparently due to solar ionization of D-region ionosphere causing enhanced electron density where collision frequency is already high. The main objective of installing the imaging riometer at Maitri is to study magneotspheric–ionospheric coupling during substorm processes. In the current study, we present two typical examples of disturbed time CNA associated with storm-time and non-storm time substorm. Results reveal that CNA is more pronounced during storm-time substorm as compared to non-storm time substorm. The level of CNA strongly depends upon the strengthening of convectional electric field and the duration of southward turning of interplanetary magnetic field before the substorm onset.

Diurnal variation of atmospheric Maxwell current over the low-latitude continental station, Tirunelveli, India (8.7°N, 77.8°E)

Observations of atmospheric Maxwell current from the low-latitude continental station, Tirunelveli (8.7°N, 77.8°E), over a period of 8 years are presented in this study. The horizontal long-wire antenna is used as a sensor for picking up charges from the atmosphere. The objectives of the present work have been to understand the antenna system in response to different meteorological conditions and on fairweather days. We define a fairweather day as a day when there is no snowfall/rainfall at the measuring site, when there are high clouds less than 3 octas throughout the day, and when the wind speed is less than 10 m s−1. A sunrise effect is observed in the measured Maxwell current during fairweather days in all seasons. The measured current exhibits an increase that commences about 30 min before sunrise and lasts for nearly 1.5 h. The sunrise effect is found to be inhibited on days when there is pre-dawn convection and during the presence of rain, haze, or cloud cover. Barring the sunrise effect, the rest of the variations on fairweather day appear to follow a trend typical of the Carnegie curve. On clear cloudless conditions, reasonably good data are obtained during equinox and winter months but not in summer when strong winds associated with the onset of the southwest monsoon make the measurement of Maxwell current difficult and limited data are available.

Measurement of atmospheric air-earth current density from a tropical station using improvised Wilson’s plate antenna

We have developed an experimental set-up to measure the atmospheric air-earth current (conduction current). Data obtained with the continuous measurements of Wilson’s plate are used to study of air-earth current density, with the aim of gaining an understanding of the experimental set-up’s response to different meteorological conditions, including fair-weather days. This paper is a part of the on-going Global Electric Circuit (GEC) studies from Tirunelveli (8.7°N, 77.8°E), a measurement site in the tropical and southern tip of the Indian peninsula. Attempts have been made in past few years to obtain the global signature in this region with this sensor, but on most of the occasions it has been impossible to obtain the global signature during fair-weather days. The data used for February–April, 2007 have the well-defined nature of this global signature, which is in agreement with the well-established classical Carnegie curve of GEC. This paper also deals with very important observations made at sunrise and during those hours when fog existed. It is noted that the resistivity of the atmosphere increased significantly with the onset of fog and later decreased as the fog disappeared, based on the measured value of conduction current density when compared with the electric field measured by horizontal passive wire antenna. Also, during fair-weather conditions, conduction current and electric field variations are similar because the conductivity during this period is more or less constant at this site. Observations made during different meteorological conditions, such as different wind speeds, humidities, and temperatures, are also discussed.

Anomalous diurnal variation of atmospheric potential gradient and air-Earth current density observed at Maitri, Antarctica

The scope of this paper is to explore the mechanisms operating over Maitri (70.76oS, 11.74oE, 117u2009m AMSL), a coastal Antarctic station, that produce an anomalous fair-weather diurnal pattern of the atmospheric electric Potential Gradient (PG) and air-Earth current density (AEC). The anomaly in the diurnal variations of AEC and the PG is displaying an ostensible minimum at ~10 UT and a diminished response to the thunderstorm over the African continent in the 14-16 UT time frame. The data sets (2005-2014, except 2012) of the PG, and to some extent, AEC, from Maitri, are used to explore this anomaly. It follows that the fair-weather electrical phenomena over Maitri can be ascribed to global electrified convection on the one hand and to regional phenomena like convection due to the replacement of warm air by katabatic winds on the other hand. The katabatic winds originate on the polar plateau, and blow from ~130° at Maitri are likely to transport various elements from the mountain slopes and space charge from the polar plateau is expected to produce various disturbances in the PG and AEC monitored over the coastal Antarctica. This mechanism may be responsible for peaks in the early UT hours and so also for the anomalous behavior of atmospheric electrical parameters observed at Maitri. Maitri data is compared with that of Carnegie cruise and Vostok to explain the source of anomaly.