C. Panneerselvam

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.

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.

Network of observations on the atmospheric electrical parameters during geomagnetic storm on 5 April 2010

The effects of a geomagnetic storm on the variation of the atmospheric electric field over Maitri (70°45′S, 11°44′E), Dome C (75°06′S, 123°20′E), and Vostok (78°27′S, 106°52′E) Antarctic research stations are presented in this paper. For the first time, the paper reports the simultaneous observations of the atmospheric electric field/potential gradient (PG) over the three high-latitude stations at the Southern Hemisphere, and its associated changes due to a substorm phenomenon. PG data obtained from these three stations under fair-weather conditions on 5 April 2010 are analyzed. The duration of geomagnetic disturbance is classified into three intervals, which contains three consecutive substorms based on the magnetic records of the Maitri station. The substorm is directly related to an enhancement of the magnetospheric convective electric field at high latitude, generally controlled by the solar wind parameters. It is found that the variation in the amplitude of PG depends on the magnetic latitude during substorm onset. During the substorm expansion phase, when the convection cell is at overhead, PG is significantly enhanced due to the downward mapping of the ionospheric horizontal electric field. The present observation demonstrated the changes on PG due to the spatial extension of the convection cell from high latitudes up to middle latitudes.

Variation of surface electric field during geomagnetic disturbed period at Maitri, Antarctica

The paper discusses on the variations of the atmospheric vertical electric field measured at sub-auroral station Maitri (70∘75′S, 11∘75′E), and polar station Vostok (78.5∘S, 107∘E) during the geomagnetic disturbances on 25–26 January 2006. Diurnal variation of surface electric field measured at Maitri shows a similar variation with worldwide thunderstorm activity, whereas the departure of the field is observed during disturbed periods. This part of the field corresponds to the magnetospheric/ionospheric (an additional generator in the polar regions) voltage generators. Solar wind parameters and planetary indices represent the temporal variation of the disturbances, and digital fluxgate magnetometer variation continuously monitored to trace the auroral movement at Maitri. We have observed that the electrojet movement leaves its signature on vertical and horizontal components of the DFM in addition; the study infers the position of auroral current wedge with respect to Maitri. To exhibit the auroral oval, OVATION model is obtained with the aid of DMSP satellite and UV measurements. It is noted that the Maitri is almost within the auroral oval during the periods of disturbances. To examine the simultaneous changes in the vertical electric field associated with this magnetic disturbance, the dawn–dusk potential is studied for every UT hours; the potential was obtained from Weimer model and SuperDARN radar. The comparison reveals the plausible situation for the superposition of dawn–dusk potential on surface electric field over Maitri. This observation also shows that the superposition may not be consistent with the phase of the electrojet. Comparison of surface electric field at Maitri and Vostok shows that the parallel variation exhibits with each other, but during the period of geomagnetic disturbances, the influence is not much discerned at Vostok.

Variation of atmospheric electric field measured at Vostok, Antarctica, during St. Patrick's Day storms on 24th solar cycle

The influence of solar wind-magnetosphere interaction on the atmospheric electric field is investigated in connection with the two severe geomagnetic storms during 24th solar cycle. The observation was carried out at Vostok (78°27′S, 106°52′E), Antarctica during 17-18 March 2013 and 17-18 March 2015. Two consecutive substorms were observed at Vostok during the main phase of geomagnetic storms, where the disturbed ionospheric current is anti-sunward in the morning sector (~04:00-10:00 UT) and sunward in the noon-afternoon sector (~11:00-16:00 UT). Interplanetary magnetic field (IMF) and solar wind interaction enhance the ionospheric potential, which in turn couple with Potential Gradient (PG) measured at ground level. Eventually, for the first time, the slope of ~1.0 Vm-1 per kV has been demonstrated between Vostok PG and overhead ionospheric potential (Weimer_05) during intense (Kp=8) geomagnetic perturbation. The linear relation between PG and overhead potential is highly significant on positive coupling, i.e., positive ∆PG changes, whereas the offset of ~ 25V/m has been estimated with negative coupling. Ionospheric convection map from SuperDARN is more compatible with PG on positive coupling and for negative changes of PG, radar observation is more consistent than the Weimer_05 model. Ionospheric electric potential from radar observation and empirical model is highly compromised when a polar cap is dominated by a single negative potential region associated with IMF By <<0. It is inferred that superposed overhead ionospheric potential on Vostok PG is highly effective when IMF maintained a steady flow, whereas it is less significance for rapid changes of SW-IMF parameters.