Extracting fair-weather data from atmospheric electric-field observations at Syowa Station, Antarctica

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1 Available online at Polar Science 5 (2011) 313e318 Extracting fair-weather data from atmospheric electric-field observations at Syowa Station, Antarctica Yasuhiro Minamoto a, *, Akira Kadokura b a Kakioka Magnetic Observatory, Japan Meteorological Agency, 595 Kakioka, Ishioka, Ibaraki , Japan b National Institute of Polar Research, 10-3 Midoricho, Tachikawa, Tokyo, Japan Received 7 December 2010; revised 10 June 2011; accepted 8 July 2011 Available online 23 July 2011 Abstract At Syowa Station (69.0 S, 39.6 E), located on East Ongul Island near the continent of Antarctica, atmospheric electric-field observations started in 1968 and had been carried out intermittently. An improved electric-field mill at Syowa Station had and obtained better-quality atmospheric electric-field data from February 2005 to January After a 1-year interruption, the observations resumed in January The atmospheric electric-field data from Syowa Station are often contaminated due to local disturbances caused by near-ground meteorological phenomena. We examined correlations between the atmospheric electric field and near-ground weather from February 2005 to January 2006 and from February 2007 to January 2008, and proposed a criterion to extract fair-weather electricfield data based on wind speed and cloud coverage data. The diurnal variation of fair-weather data in January followed the shape of the so-called Carnegie curve. Fair-weather data obtained during a substorm showed some correspondence between the atmospheric electric field and variations in the geomagnetic field. This newly developed extraction method may enable the use of atmospheric electric-field data for studying the solar terrestrial environment. Ó 2011 Elsevier B.V. and NIPR. All rights reserved. Keywords: Atmospheric electric field; Fair-weather data; Diurnal variation; Carnegie curve 1. Introduction Various observations of the atmospheric electric field have been carried out in polar regions to study the distribution of the global circuit and the connection processes between outer space and the upper atmosphere (e.g., Troshichev et al., 2004; Rycroft et al., 2008). Such observations have been performed at Syowa Station (69.0 S, 39.6 E), which is located in the * Corresponding author. Tel.: þ ; fax: þ address: minamoto@kakioka-jma.go.jp (Y. Minamoto). auroral zone, where auroral activity is expected to affect the atmospheric electric field. The observations at Syowa Station started in 1968 during the 3rd Japanese Antarctic Research Expedition (JARE-3), and continued in subsequent years during JARE-4, -5, -10, and -13. Kondo (1971) reported hourly values of the atmospheric electric field obtained from electricity charts at Syowa Station. The average atmospheric electric-field value from February 1969 to January 1970 was 66 V/m. Observations of the atmospheric electric field at Syowa were resumed in February 2002 as part of JARE-44, but the data quality was poor due to /$ - see front matter Ó 2011 Elsevier B.V. and NIPR. All rights reserved. doi: /j.polar

2 314 Y. Minamoto, A. Kadokura / Polar Science 5 (2011) 313e318 contamination by metal dust produced by wear of the rotating axis of the field mill. JARE-46 established an improved electric-field mill at Syowa Station and obtained better-quality atmospheric electric-field data from February 2005 to January After a 1-year interruption, observations resumed in January 2007 as part of JARE-48. Because the atmospheric electric field near the ground is disturbed by local weather, it is necessary to identify and exclude such contaminated data in order to study the relationship between the atmospheric electric field and upper-atmospheric phenomena (e.g., aurora breakups, ionospheric disturbances, and substorms). Observations of the atmospheric electric field have been carried out at several stations in polar regions, including at Amundsen-Scott (Byrne et al., 1993; Reddell et al., 2004), Davis (Burns et al., 1995), Vostok (Frank-Kamenetsky et al., 1999, 2001; Corney et al., 2003), and Hornsund in Spitsbergen (Kleimenova et al., 1996, 2010). Frank-Kamenetsky et al. (2001) extracted 134 fairweather days from data obtained in 1998 at Vostok (78.5 S, E) and discussed the relationship between the interplanetary magnetic field and the atmospheric electric field. Reddell et al. (2004) reported seasonal variations in the atmospheric electric field at Amundsen-Scott (South Pole) and studied the relationship between the Kp index, which is commonly used to characterize planetary geomagnetic activity and the atmospheric electric field. Although they determined fair-weather based on meteorological data, the procedure was not explained in detail. Both Vostok and Amundsen-Scott are located in the Antarctic highland, where winds are generally weak; consequently, details of the extraction procedure may be relatively unimportant. In contrast, Davis and Syowa are located near the coast of Antarctica, where winds are relatively strong (van Lipzig et al., 2004). Burns et al. (1995) showed that atmospheric electricfield data collected at Davis are strongly affected by relative humidity and wind speed, and in their analysis the authors identified and discarded data affected by these meteorological parameters. Minamoto (2008) examined short-period fluctuations in the atmospheric electric field at Syowa, but did not present any criteria to identify fair-weather data based on statistical methods. Kleimenova et al. (2010) analyzed the effects of morning magnetosphere substorms on variations in atmospheric electricity at Hornsund Polar Observatory, Spitsbergen, in the Arctic Circle. The authors analyzed atmospheric electricity data obtained in the absence of strong winds, precipitation, and fog. In this paper, we present a criterion for extracting fair-weather data acquired from atmospheric electricfield observations at Syowa as part of JARE-46, -48, and -49. Furthermore, we show a correspondence between the geomagnetic field and variations in the atmospheric electric field when a substorm occurred during fair-weather period. 2. Instrument description and observations Fig. 1 shows a map of the central part of Syowa Station. The site of the atmospheric electric-field observations is located more than 100 m from diesel generators, and more than 30 m (upwind) from observation buildings. Therefore, artificial disturbances to atmospheric electric-field data are expected to be minor at this site. The observation sites of atmospheric electricity and meteorology are separated by about 300 m (Fig. 1), and the anemometer is located at an altitude of about 10 m. The instrument used for observations is an electricfield mill (EF308T) made by Tierra Technica Co. Ltd., Inagi, Tokyo, Japan. The sensor is raised 1.5 m above the ground, and the output signals are digitized and recorded every second with a resolution of 0.01 V/m within the measurement range of V/m. Calibration of the instrument was carried out several times per month by setting a metal plate at 25 mm from the bottom of the sensor and supplying it with a fixed voltage of 15 V. The downward direction of the atmospheric electric field is assigned positive values. Fig. 2 shows a histogram of atmospheric electricfield values at Syowa Station from February 2007 to January Considering the atmospheric electric- Fig. 1. Map showing the location of the observation site at Syowa Station (from Minamoto, 2008). The blue and brown areas on the map indicate land and sea respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3 Y. Minamoto, A. Kadokura / Polar Science 5 (2011) 313e Fig. 2. Histogram of atmospheric electric-field values recorded at Syowa Station from January 2007 to February field observations at Davis (Burns et al., 1995), we assumed that measured values between 0 and 300 V/m are fair-weather data. The average frequency of values between 0 and 300 V/m during this period was 29%. Fig. 3 shows the daily percentage of atmospheric electric-field values between 0 and 300 V/m from February 2007 to February The plot shows the seasonal pattern of the frequency of fair-weather electric-field data. Atmospheric electric-field data were seldom recorded from April to October, possibly related to the frequent occurrence of blizzard conditions at Syowa Station from late March to late October (Sato and Hirasawa, 2007). The seasonal trend shown in Fig. 3 indicates the effect of surface meteorology on the atmospheric electric field. 3. Criterion for extracting fair-weather electricfield data particles of powder snow are raised by the wind and charged by frictional electricity. The atmospheric electric field is then disturbed by the charged particles. Fig. 3 shows that most of the atmospheric electric-field values in wintertime were larger than 300 V/m or negative, which may reflect the wind and snow conditions at Syowa in winter. The shielding effect of cloud cover also affects atmospheric electric-field measurements. Therefore, we attempted to establish a criterion for fair-weather data based on wind speed and cloud volume. As the representative wind speed for each hour, we used the median value of wind speed measured each minute during the hour. The cloud coverage level (defined as the apparent proportion of cloud cover over the entire sky) was obtained every 3 h at Syowa Station. The cloud coverage level is represented by values from 0 to 10; when the cloud cover is less than 10%, the level is 0þ (Japan Meteorological Agency, 2002). In the present study, a period of clear sky is defined as a period in which the cloud coverage level is 0 or 0þ at both the beginning and end of the period. As the hourly atmospheric electric-field data, we used the mean value of measurements taken every 1 s between 0 and 1000 V/ m during the hour of interest. From the 2 years of data from February 2005 to January 2006 and from February 2007 to January 2008, we identified 777 h when the atmospheric electric-field values were between 0 and 300 V/m and the cloud conditions were judged to be clear sky according to the above definition. Minamoto (2008) showed that negative or large values of the atmospheric electric field are obtained during blizzards. When the wind speed is high, Fig. 3. Daily percentages of atmospheric electric-field values at Syowa Station between 0 and 300 V/m from February 2007 to February Fig. 4. Cumulative frequency of wind speed for 777 data points from 2 years (February 2005 to January 2006 and February 2007 to January 2008) when atmospheric electric-field values were between 0 and 300 V/m and under clear sky.

4 316 Y. Minamoto, A. Kadokura / Polar Science 5 (2011) 313e318 Fig. 5. Diurnal pattern of extracted hourly values of the atmospheric electric field at Syowa Station in January for the years 2005, 2007, and Small dots and squares correspond to data obtained during fine weather and the average value for each time period, respectively. To establish a criterion for fair-weather based on wind speed, we plotted the cumulative frequency of the 777 data points mentioned above (Fig. 4) as a rough visual measure, and adopted 6 m/s as a threshold of wind speed to identify the fair-weather condition. Forty-eight of the 777 data points exceed the threshold. In summary, we present the following criterion for extracting fair-weather atmospheric electric-field data: wind speed <6 m/s and clear sky, corresponding to fair-weather at Hornsund, Spitsbergen (Kleimenova et al., 1996, 2010). To examine the performance of this criterion, Fig. 5 shows the diurnal pattern of the extracted fair-weather electric field for January 2005, 2007, and Only January data are plotted to avoid the effect of seasonal changes in the diurnal curve, and outlier values (>400 V/m) were not plotted. The atmospheric electric Fig. 6. Components X, Y, and Z of geomagnetic field values (aec, respectively), atmospheric electric-field (AE) values (d), and wind speed (e) at Syowa Station from 1400 UT on 6 September 2007 to 1759 UT on 7 September 2007.

5 Y. Minamoto, A. Kadokura / Polar Science 5 (2011) 313e field is relatively weak (strong) during 0200e0700 UT (1300e2000 UT), similar to the so-called Carnegie curve and the diurnal pattern observed at Vostok in January 1998 (Frank-Kamenetsky et al., 2001). The lack of data for the period 1800e2000 UT means that the expected peak of the Carnegie curve (at around 1900 UT) cannot be seen clearly in Fig Discussion We showed that fair-weather atmospheric electricfield data could be extracted using a criterion based on wind speed and cloud cover. The fair-weather data extracted according to this criterion make up just 7.5% of the total data for a 2-year period (February 2005 to January 2006 and February 2007 to January 2008). During 6e7 September 2007, the atmospheric electric field was in the fair-weather condition when a substorm occurred. Fig. 6 shows (from top to bottom) the geomagnetic field, the atmospheric electric field, and wind speed at Syowa Station from 6 to 7 September The atmospheric electric field was disturbed by an increase in wind speed from 0600 to 1000 UT on 7 September. The wind speed was below 6 m/s during the other periods shown in Fig. 6, when the correspondence between the atmospheric electric field and fluctuations in the magnetic field could be traced. The atmospheric electric field exceeded 1000 V/m at around 2140 UT on 6 September, when the substorm started. After lowering to 500 V/m, it exceeded 1000 V/m again during the expansion phase of the substorm. From 0300 to 0400 UT on 7 September, during the recovery phase of the substorm, some correspondence is apparent between fluctuations in the atmospheric electric field and variations in the geomagnetic field. 5. Conclusions Atmospheric electric-field data collected at Syowa Station are usually contaminated by local disturbances, making it difficult to investigate the relationship between the atmospheric electric field near-ground level and upper-atmospheric phenomena. We examined correlations between the atmospheric electric-field data and ground-based meteorological parameters, and identified a criterion with which to extract fair-weather electric field data: wind speed of less than 6 m/s and a cloud coverage level of 0 or 0þ. The diurnal variation observed in the extracted data demonstrates the satisfactory performance of this criterion. In a special case when a substorm occurred during a fair-weather period, a degree of correspondence was observed between the atmospheric electric field and magnetic field variations. Although the extracted fair-weather data at Syowa Station represent less than 10% of the total data, the accumulation of such fair-weather data will enable us to study the relationship between variations in the atmospheric electric field and various upperatmospheric phenomena. Acknowledgments The wind speed and cloud coverage data used in this study were provided by the Japan Meteorological Agency. We would like to thank members of JARE, whose cooperation was essential for this study. References Burns, G.B., Hesse, M.H., Parcell, S.K., Malachowski, S., Cole, K.D., The geoelectric field at Davis station, Antarctica. J. Atmos. Terr. Phys. 57, 1783e1797. Byrne, G.J., Benbrook, J.R., Bering, E.A., Few, A.A., Morris, G.A., Trabucco, W.J., Paschal, E.W., Ground-based instrumentation for measurement of atmospheric conduction current and electric field at the South Pole. J. Geophys. Res. 98 (D2), 2611e2618. 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