Fair-weather atmospheric electricity study at Maitri (Antarctica)
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 25 February 2013
Accepted: 17 September 2013
Published: 6 December 2013
Results of near-surface measurements of atmospheric electric field and meteorological parameters at the Indian Antarctic station, Maitri, during 12 fair-weather days of January and February, 2005, are presented. Data are analyzed to study the diurnal variation of the electric field and its departure, if any, from the global electric fields. Fair-weather days are classified into two groups depending upon the average of the hourly surface temperature. Group one, when the average of the hourly surface temperature is mostly above the freezing point, and group two, when the same is below the freezing point. The role of different ion sizes on the Maxwell current density and the air-Earth current density for the two groups are quite different under different conditions. To study the effect of ions on the atmospheric electric fields, ions are grouped as small ions, intermediate ions and large ions. We find that the small and the large ions largely influence the air-Earth current density with a correlation coefficient higher than 70%. The intermediate ions have a negative correlation in the case of group one fair-weather days, whereas for group two days no correlation is found. The diurnal variations of the Maxwell current density and the electric field show a peak between 1800 UT and 2000 UT and the nature of the variation can be attributed to the variation in worldwide thunderstorm activity. The correlation coefficient between the measured electric field and the electric field from the Carnegie curve is 0.93 with a <0.0001 significance level. Thus, the observed electric field at Maitri represents the global electric field. The results show that a wind velocity of less than 10 m/s and a surface temperature of lower than +7°C have almost no impact on the electric field and Maxwell current density.
It is known that a weak current of about 1–6 pA/m2 flows between the ionosphere and the Earth’s surface, which is maintained by lower atmospheric electric generators (mostly thunderstorms and strongly electrified clouds occurring around the globe) and upper atmospheric electric generators (which are in the polar caps). Thunderstorms generate an upward current of ~1000 A and maintain the vertical potential gradient (~250 kV) between the ionosphere and the ground (Alderman and Williams, 1996; Burns et al., 1995; Rycroft et al., 2000; Singh et al., 2004; Siingh et al., 2005a, 2007a, 2008). The ionospheric potential shows ±40 kV of diurnal variation in universal time; because, as the Earth rotates, thunderstorm activity maximizes successively in the late afternoon-evening local time over the non-uniformly distributed land masses around the globe. The global component of the variation of the atmospheric electric field is obtained by integrating over the globe and it follows universal time. The effect is different for variation originating by global and local processes (Williams and Heckman, 1993; Williams, 1994; Sheftel et al., 1994; Rycroft et al., 2000; Troshichev et al., 2004). The local component of a variation is governed by local meteorological processes and follows local time.
The influence of solar activity on the atmospheric electric field differs between mid-latitudes and high latitudes / Polar Regions. The measurement of fields at mid-latitudes contains variations due to local effects, in addition to global components. A local diurnal variation does not exist in the Polar Regions (Israel, 1973) and hence the measurement of field in these regions could represent the global variation. However, drifting snow and the polar cap potential may affect atmospheric field parameters. The solar wind interaction with the Earth’s magnetic field generates a cross polar cap potential difference in the range 30–60 kV (Papitashvili et al., 1999; Rycroft et al., 2000), and which, during enhanced solar activity, may exceed 100 kV and expand equatorwards (Boyle et al., 1997; Rycroft et al., 2000; Siingh et al., 2011a; Williams and Mareev, 2013). This part of the field corresponds to a horizontal potential and may affect measured field parameters. The Maitri station is outside the auroral oval during magnetically quiet times and is encompassed by the auroral oval under magnetically disturbed conditions. During a sunspot maximum period, one would expect more magnetic storms that may produce large day-to-day variations in the electric fields and the Maxwell currents. Based on measurements at the Maitri station, Panneerselvam et al. (2007a) concluded that during magnetically quiet and moderate conditions, the variations of measured atmospheric electric field parameters are similar to the Carnegie curve. However, the diurnal pattern during magnetically disturbed conditions differs from the Carnegie curve due to contributions from the ionosphere/magnetosphere. The solar activity was at its deep minimum during 2005 with unusually long periods without sunspots and a relatively very weak dipolar field strength (Nandy et al., 2011). As a result, the polar cap potential would be relatively small and its effect outside the auroral oval is expected to be small, which is likely to be further reduced during the averaging of data for the fair-weather days. In the present study, the effect of the polar cap potential on the atmospheric electrical field parameters is considered to be small and is therefore ignored.
The presence of drifting snow in the atmosphere is governed by wind velocity. The larger the wind velocity, the larger will be the number of particles. Minamoto and Kadokura (2011) have analyzed atmospheric electric field data at the Syowa station, Antarctica, and defined fair-weather days to be those with a wind speed of less than 6 m/s and a cloud coverage level of less than 10%. However, based on measurements at Maitri, Deshpande and Kamra (2001) considered fair-weather days to be those when the wind speed is less than 10 m/s. They showed that the percent deviation in the electric field lies within ±50% for wind speed lower than 10 m/s and all the other criteria of fair-weather are satisfied. This factor remains mostly positive and may exceed +50% when the wind speed exceeds 10 m/s. Based on these facts, we have considered a wind speed of ≤10 m/s as the criteria for fair-weather days at Maitri.
Ground-based measurements are often contaminated by the presence of ground radioactivity, atmospheric aerosols, turbulence and convection currents. Even columnar resistance, electrical conductivity, seasonal variations, ionospheric horizontal fields, and active thunderstorms in the area may influence the universal electric fields at a regional level (Takagi, 1977). However, measurements made at clean places such as over oceans (Mauchly, 1923; Ruttenberg and Holzer, 1955; Morita, 1971; Kamra et al., 1994), over high mountains (Cobb, 1968; Reiter, 1992), over exchange layers (Anderson, 1967; Markson, 1977), in the polar regions (Kasemir, 1972; Park, 1976; Cobb, 1977, Burns et al., 1995; Bering et al., 1998; Fuellekrug et al., 1999) and in the Antarctic (Despande and Kamra, 2001; Panneerselvam et al., 2007a; Minamoto and Kadokura, 2011; Jeeva et al., 2011) have been used to study the global variation of atmospheric fields and to verify the global circuit concept postulated by Wilson (1925) and to confirm the unitary diurnal variation in the electric field. Alderman and Williams (1996) studied the air-Earth current density at Mauna Loa, Hawaii, which is free of pollution, and observed a single peak at 1900 UT. Apart from the global universal time variations, land stations also exhibit local time variations typical of their locations.
The global electric field model (Wilson, 1920) includes charging current sources in thunderstorms and discharging current at all other sites. The experimental support for this model is that the universal daily variation of the electric field measured over the ocean regions (Carnegie curve) is consistent with the diurnal variation of thunderstorm activity integrated over the globe (Alderman and Williams, 1996; Bering et al., 1998; Siingh et al., 2005a, 2007a). Observations by Mauchly (1923) over open oceans support the hypothesis proposed by Wilson that thunderstorms feed the global electric circuit. These observations are repeated at poles and at clean places to verify these results as they are free of any local effects that may mask the global variation. The Antarctic region is one of the best places to verify the global electric circuit (GEC) as there is little human activity and the region is free from any anthropogenic sources of pollution. The Antarctic plateau supports a desert-like continent during the observation period as the winds are light, mostly flowing in a nearly constant direction and the atmosphere is relatively free of turbulent and convective motions (Byrne et al., 1993; Deshpande and Kamra, 2001).
In this paper, we report the measurements of various atmospheric electric parameters made at the Maitri station (lat. 70°45′52″S, long. 11°44′03″E; 130 m above mean sea level) situated on the Antarctic plateau, during January–February, 2005. Our objective is to study the diurnal variation of fair-weather electric fields at a relatively pollution-free place, and to find out the extent to which the measured field represents the universal Carnegie fields. Section 2 describes briefly the instruments used, the measurement site and the weather conditions at Maitri. Section 3 presents the data selection. Section 4.1 discusses the diurnal variation of the electrical parameters. The role of ions of different sizes on atmospheric electrical parameters is presented in Section 4.2. Section 5 discusses the dependence of atmospheric electrical parameters on the surface temperature and wind velocity. The diurnal variation of conductivity is discussed in Section 6. Section 7 compares the measured electric fields with the earlier measurements of fair-weather fields at other locations. Finally, the results are summarized in Section 8.
2. Instruments, Measurement Site and Weather
The atmospheric electric field is measured with a vertical alternating current (AC) field mill which is made out of nonmagnetic stainless steel to reduce the contact potential (Willett and Bailey, 1983). The details of the AC field mill has been discussed by Deshpande and Kamra (2001). The sensor plates and rotor used in the field mill are of 8.5 cm diameter. The shaft of the AC motor (220 V, 50 Hz, 1500 rpm) is grounded with a carbon brush. The weak signal is amplified with an amplifier circuit which is fitted inside the field mill. The time constant of the field mill is 1 s. For the electronic circuit, military grade components are used which maintain their characteristics even in subzero temperatures. The amplified signals are fed through teflon-insulated coaxial cables to a PC-based data acquisition system, which is kept inside a hut 6 m in height and 35 m away from the the field mill. The sampling rate for each instrument is 1 per second but the data are stored on 1 min-average and 1-hour average modes during the entire measurements. The cleaning of insulators and other maintenance of field mill was undertaken daily. The zero shifts were checked every 3 hours and, if found to be appreciable, were corrected.
A long wire of length 41.5 m and 3 mm in diameter kept horizontal (stretched parallel) at a height of 2 m from the Earth’s surface is used to measure the Maxwell current. The wire is mechanically supported by masts, using teflon rods at their ends for insulation from the supporting masts. The input signal is fed through the electrometer (model AD-549; military grade). A unit gain operational amplifier (LM 308) amplifies the electrometer output signal, and the amplified signal passes through the shielded coaxial cable and is fed to a PC-based data logger. The data are recorded at a sampling interval of one minute. Details concerning measurement and antenna information have been discussed by Panneerselvam et al. (2003, 2007a, b). The Maxwell current consists of a field-dependent current, convection current, lightning current and displacement current (Krider and Musser, 1982; Siingh et al., 2008). In the absence of lightning discharges, only field-dependent and convection current components exist, which vary rather slowly. When the electric field is zero, the field-dependent component becomes zero. The convection current is produced by the mechanical transport of net space charge by air motion/precipitation.
Dimensions and other parameters of three condensers of the ion-counter.
Length of the outer electrode (m)
Length of the inner electrode (m)
Diameter of the outer electrode (m)
Diameter of the inner electrode (m)
Potential applied (V)
Critical mobility (m2 V−1 s−1)
0.766 × 10−4
1.2 × 10−6
0.97 × 10−8
Flow rate (l s−1)
The air-Earth current is measured with a 1-m2 flat plate antenna kept flush with the ground shown in Fig. 2. The inputs from the three condensers of the ion counter and the air-Earth current plate are amplified with separate amplifiers placed close to the sensors and then fed through the coaxial cables to a data logger placed in a nearby hut. The air-Earth current is the total vertical current flowing to the plate and may contain contributions from the turbulent current and displacement current. In fair-weather days, overhead thunderstorm/charged cloud activities are almost absent and hence the displacement current contribution may be almost zero. The contribution from turbulence during fair-weather conditions is negligibly small and the measured vertical current is considered to be the air-Earth current (Panneerselvam et al., 2007a; Siingh et al., 2007b).
3. Data Selection
The atmospheric electric field parameters near the Earth’s surface are disturbed by the local weather. Therefore, it becomes almost essential to identify and exclude data contaminated by local effects or connected with some large-scale systems. During the data selection process, we could identify 12 fair-weather days within two months, i.e. January and February, 2005. These days were January 3, 4, 7, 8, 9, 10, 11, 12, 15, 25, and February 1, 5. Days are considered to be fair-weather days when there is no rain or snowfall, the wind speed is moderate (less than 10 m/s), low level clouds are absent, and high level clouds are less than 3 octa (Deshpande and Kamra, 2001; Panneerselvam et al., 2007a). On some of the selected days, the wind speed exceeded 10 m/s for a very brief period (~an hour) but the measured electric field parameters were found to be within the considered normal range. The air-Earth current measurement could be affected by the blowing space charges present in this lofted dust. However, such days have been included in the fair-weather days. We consider the convention that on the fair-weather days the electric field pointing downward is negative (i.e. the potential gradient in fair-weather days is positive).
Careful analysis of the data showed a peculiar feature that all the measured parameters had relatively higher values during the last three fair-weather days (January 25, February 1, 5). When the average of the hourly surface temperature was considered, it was found to be mostly above the freezing point (zero degrees centigrade) for fair-weather days before January 15 and below the freezing point after January 15. Accordingly we grouped the data into two groups: group one contains data for the fair-weather days from January 3 to January 15 and group two contains data for the days January 25, February 1 and 5. Even though, there are only three days data in group two, we have performed the analysis because the diurnal variations on each day are similar. As the data in group two are quite small, detailed studies have not been carried out. The analysis is made just to compare and validate the results of group one, and no major conclusions could be derived based on the group two data.
4. Fair-weather Atmospheric Electrical Parameters
4.1 Diurnal variation
Comparing Figs. 4(a) and 4(b), it is observed that the air-Earth current density follows a superimposed variation of the small and the large ions; the variation of the intermediate-ion concentration is independent, and seems to follow the variation of the surface air temperature in both groups. This also indicates that the sources of intermediate ions may be different from that of the small and large ions. The small dip around 0300 UT in the potential gradient of group one decreases when data are averaged for all 12 fair-weather days and the potential gradient and the Maxwell current density show a single maxima at 1800–1900 hrs and a minima at 0200 hrs. Burns et al. (1995) reported a peak in the electric field at Davis, Antarctica, between 1900 UT and 2200 UT. The diurnal variation of the potential gradient and the Maxwell current is attributed to the diurnal variation in the global thunderstorm activity. Panneerselvam et al. (2007a) measured the potential gradient and the Maxwell current at the Maitri station during 2001–2004, analyzed the data for 69 fair-weather days, and reported the diurnal variation to have a single periodic feature with a maximum at 1900 UT and a minimum at 0300 UT.
4.2 Role of different sizes of ions
5. Effect of Wind Speed and Surface Temperature on the Atmospheric Electricity Parameters
Variation of correlation coefficient (R), standard deviation (SD) (significance level, P) of electrical parameters with the surface temperature and wind speed.
Group 1 R, SD (P)
Group 2 R, SD (P)
Group 1 R, SD (P)
Group 2 R, SD (P)
Maxwell Current Density
Air-Earth Current Density
Diurnal variation and dependence on surface temperature and wind velocity of atmospheric electrical parameters.
Maxwell Current Density
Diurnal variation with a maximum between 1800 and 2000 UT
Diurnal variation with a maximum between 1800 and 2000 UT
Air-Earth Current density
No diurnal variation
no effect (Group 1
Decreases (Group 2
No diurnal variation
Diurnal variation with a flat maximum between 800 and 1800 UT
No diurnal variation
Less dependence (Group1)
No Dependence (Group 1)
6. Diurnal Variation of Conductivity
7. Comparison of the Maitri Electric Field to Global Values
The electric fields measured at continent exhibit two types of variations: the single-oscillation type, such as the winter time fields at Vassijaure (68.4°N; 18.2°E) (Norinder, 1916), the Carnegie curve derived from measurements over oceans (Whipple and Scrase, 1936), the present work at Maitri (70°45′52″S, 11°44′03″E), earlier measurements at Maitri (Panneerselvam et al., 2007a), and the winter time fields at Marsta (59.9°N, 17.6°E) during 1993–1998 (Israelsson and Tammet, 2001); and the double-oscillation type at Uppsaloa (59.8°N, 17.6°E) during the summer (Norinder, 1917), and at Marsta during the summer. The Carnegie curve is considered to reflect the true nature of global variations because the local diurnal variation is strongly suppressed over the oceans (Israel, 1973; Williams and Heckman, 1993). The diurnal variation of continental measurements typically follows the local time. However, Paramonov (1950) found that the averages over a large number of stations spread over the globe follow the global nature like the oceanic measurements.
Figure 9(b) presents the scattered plots of the electric fields reported in the Carnegie curve and the present study and there is a very good correlation (~93%), which shows that the trend obtained by the Carnegie curve is mirrored in our observations also. The field magnitude cannot be compared because this depends on the environment of the observation site and the change in local meteorological parameters. Israelson and Tammet (2001) attempted to determine the contribution of local meteorological parameters to the changes in global measurements of the electric field using a reduction technique, which was found to be inadequate. Further studies in this direction are needed.
Measurements at the Indian station Maitri show similar diurnal variations in the Maxwell current and the electric field with a maximum between 1800 UT and 2000 UT, which are attributed to global lightning discharge activity. The small- and large-ion concentrations follow the air-earth current density diurnal variations, whereas the intermediate-ion concentration does not follow the air-Earth current density, but it shows a flat maxima between 0900 UT and 1700 UT. The enhancement in ion concentration correlates with the rise in daytime surface temperature and may be attributed to new particle formation. This flat maxima becomes more prominent in the case of group two days, when the night-time temperature goes below the freezing point and the day time temperature is less than 3°C. The averaged data of group one days show a peak in the air-Earth current at 0500 UT, which is not present in the data of the group two days. However, no such peak is observed in the electric field and Maxwell current density. Further study is required to understand the presence of the peak on the group one days and its disappearance on the colder fair-weather days.
The total ion concentration does not show any correlation with the electric field; however, when the night-time temperature goes below freezing, there is a small positive correlation (correlation coefficient = 0.28). The Maxwell current is well correlated (correlation coefficient = 0.68 and 0.70) with the electric field in both groups. The small, and the large, ions are very well correlated with the air-Earth current density. However, the situation is quite different with the intermediate ions. In the case of the group one days, there is a negative correlation (correlation coefficient = −0.41) whereas there is no correlation in the case of group two days. Thus, most of the contribution to the air-Earth current seems to come from the small and large ions.
The impact of wind velocity on the ion concentration, air-Earth current density, Maxwell current density and electric field varies. On group one days, the intermediate ions are barely negatively correlated (correlation coefficient = −0.20) whereas the small ions show some positive correlation. The large ions do not show any meaningful correlation. On relatively colder days (group two), there is a negative correlation (correlation coefficient ~ −0.31 to −0.46) for all three categories of ions. The electric field, Maxwell current density and air-Earth current density decrease with the increase in wind speed, except that on group one days the air-Earth current density does not show any dependence.
On fair-weather days, the surface temperature varied either between −3°C and 8°C or between −7°C and 3°C. The small-ion concentration in both cases showed a negative correlation with the surface temperature. The intermediate-ion concentration showed a good positive correlation. The large ions show a barely negative correlation on colder days and almost no correlation on the rest of the days. With the increase of surface temperature, the electric field and the Maxwell current density increase whereas the air-Earth current density decreases. Interestingly, it is found that the intermediate ions showed a relatively larger effect as compared with the small and the large ions.
Furthermore, the electric field measured at the Maitri station on the fair-weather days is compared with different available fair-weather measurements. The values presented in this paper and the diurnal variations are quite close to the Carnegie values, having a correlation coefficient of ~0.93 with a very good significance level. The measured values match between 0800 UT and 2200 UT, and outside this time our values are somewhat smaller. Based on the present study, we can safely conclude that the electric fields at the Maitri station, Antarctica, truly represent the global nature of the electric field. However, further study is recommended to find the effect of geomagnetic storms and other solar events on different atmospheric electrical parameters during both fair-weather days and disturbed days.
Devendraa Siingh (DS) gratefully acknowledges the National Centre for Antarctic and Ocean Research (NCAOR), Goa, India, for giving him an opportunity to participate in the 24th Indian Scientific Expedition to Antarctica (ISEA), and to the India Meteorological Department for providing the Meteorological data. Thanks are due to Dr. A. K. Kamra for the support extended during the expedition and Vimlesh Pant (research scholar) for his support during the period of observation. DS and VG are grateful to the Ministry of Earth Sciences, Govt. of India, New Delhi, also express their sincere gratitude to Professor B. N. Goswami, Director, Indian Institute of Tropical Meteorology, Pune, for his continuous encouragement and support. This work is partially supported under the collaboration programme of IITM, Pune and BHU Varanasi. We gratefully acknowledge the constructive remarks of the anonymous reviewers, which has improved the quality of the work.
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