Interannual consistency of bi-monthly differences in diurnal variations of the ground-level, vertical electric field

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004jd005469, 2005 Interannual consistency of bi-monthly differences in diurnal variations of the ground-level, vertical electric field G. B. Burns Australian Antarctic Division, Australian Government, Kingston, ACT, Australia A. V. Frank-Kamenetsky and O. A. Troshichev Arctic and Antarctic Research Institute, St. Petersburg, Russia E. A. Bering and B. D. Reddell Department of Physics, University of Houston, Houston, Texas, USA Received 27 September 2004; revised 22 January 2005; accepted 3 March 2005; published 25 May [1] Interyear consistency is demonstrated in the bi-monthly average diurnal vertical electric field measured over 720 fair-weather days collected during a 5-year interval (1998 to 2002) at Vostok (78.5 S, 107 E; magnetic latitude 83.6 S), Antarctica. After correcting for the influence of polar-cap convection, seasonal peaks in the average electric field values occur around July-August with a diurnal maximum at 2050 UT while lowest average magnitudes are measured near November-December when the associated diurnal maximum occurs at 1830 UT. These variations are consistent with expected seasonal changes in global thunderstorm activity. Comparisons of ground-level vertical electric field measurements (Vm 1 ) with Weimer-1996 model cross-polar cap potentials above Vostok (kv) for individual UT hours yield significant correlations over all hours but with reduced standard errors around local magnetic noon (1300 UT). This implies a more direct linkage between solar wind parameters and the cross-polar cap potential near magnetic noon, for this site (magnetic latitude: 83.6 S). An all hours all seasons linkage factor of 0.76 ± 0.06 Vm 1 per kv is determined, broadly consistent with an average ionosphere-ground potential difference of 250 kv and a measured average vertical electric field of 185 Vm 1. Evidence is presented supporting a seasonal variation in this linkage factor, with generally lower magnitudes in the austral winter (May to August). Citation: Burns, G. B., A. V. Frank-Kamenetsky, O. A. Troshichev, E. A. Bering, and B. D. Reddell (2005), Interannual consistency of bi-monthly differences in diurnal variations of the ground-level, vertical electric field, J. Geophys. Res., 110,, doi: /2004jd Introduction [2] Thunderstorms and electrified clouds are the principal generators maintaining a time-varying electric potential of 250 kv, directed downward, between the ionosphere and the ground. In fair-weather regions this potential drives an air-earth current of 3 pam 2 and can be most readily measured as a vertical electric field of 100 V m 1 near ground level. Both these values are approximately doubled on the high (3000 m) Antarctic polar plateau due to the combined influence of the decrease with altitude of atmospheric density and the increase with altitude of ionization due to cosmic rays. The time-constant of this global atmospheric electric circuit is 20 min [Bering et al., 1998; Rycroft et al., 2000]. Thus, in regions where the diurnal-seasonal variation in the conductivity of the atmosphere is minimized Copyright 2005 by the American Geophysical Union /05/2004JD (above the oceans, some mountain tops, ice caps) and at times when local meteorological driven electrical activity is negligible (fair weather), the vertical electric field can be a measure of the time-varying global thunderstorm activity. [3] Atmospheric convective processes which generate thunderstorm activity occur principally over warmed land and maximize in the local afternoon hours. Combined with the global distribution of landmasses, this is believed to be the reason the average, fair-weather diurnal variation in the ground-level, vertical electric field at suitable sites has a diurnal maximum at 2000 UT, a diurnal minimum at 0400 UT, and a diurnal range 37% of the mean. The reference standard for the diurnal variation in the fair-weather field remains the average determined from cruises of the Carnegie in the first half of the twentieth century [see Reiter, 1992]. The more extensive landmasses of the Northern Hemisphere compared with the Southern Hemisphere lead to an expectation of larger magnitude electric field values during the boreal summer. 1of14

2 Table 1. Average Magnitude of the Discrepancy Between Successive Calibrations Within Each Year Year V m Re-analysis of the original Carnegie records [Adlerman and Williams, 1996] has revealed this expected annual variation. [4] The high, dry Antarctic plateau has long been recognized as a desirable location for ground-level measurements of the atmospheric electric field [Park, 1976a; Cobb, 1977]. However, in polar regions an additional generator is active. The interaction of the solar wind and the Earth s magnetic field, the process that results in the generation of the aurora, maintains a variable, generally dawn-to-dusk directed, potential difference across the polar cap. This manifests as a vertical electric field at ground level for scale sizes greater than 200 km. Park [1976b] provides a theoretical presentation of this process. Careful statistical analysis of single-site electric field data has provided experimental proof that the influence of this solar wind generator can be measured at ground level [Tinsley et al., 1998; Frank-Kamenetsky et al., 1999]. Ground-level electric field measurements, which respond to the broad-scale, overhead, cross-polar-cap potential difference independent of the level of ionization or structure of the ionosphere (needed to infer the horizontal electric field from magnetic field arrays or HF radar techniques) have subsequently been used to investigate models which infer polar convection patterns from solar wind parameters [Corney et al., 2003; Reddell et al., 2004]. Above Vostok (magnetic latitude: 83.6 S), which is generally located well inside the peaks of the dawndusk potential difference, the diurnal range of the average potential difference amounts to 30% of the average diurnal range from the global meteorological generator. [5] The apparent sensitive response of thunderstorm activity to summer temperatures [Williams, 1992, 1994, 1999] and the suggestion that the meteorological generators most likely include a significant contribution from processes other than thunderstorms [Williams and Heckman, 1993] has led to the hypothesis that measurements of the meteorological driven component of the global circuit may provide a sensitive measure of global rainfall when Schumann resonance measurements are used to estimate the direct thunderstorm contribution [Markson and Kendra, 1992; Bering et al., 1998]. [6] In this paper we analyze an Antarctic plateau electric field data set considerably larger (720 fair-weather days; 691 days with associated solar wind parameters which can be used to subtract the influence of the polar-cap convection) than published modern data sets (Frank-Kamenetsky et al. [2001] and Corney et al. [2003]: 134 complete days, 104 complete days with solar wind parameters; Reddell et al. [2004]: 145 days, 42 days with solar wind parameters). This provides sufficient data to determine bi-monthly diurnal averages with 20-min resolution, estimate the contribution of polar-cap convection to the measurements, remove that contribution and obtain a measure of the bimonthly meteorological contribution to the global circuit, and examine inter-year consistency in these splits. Monthly averages are also presented to facilitate comparisons with other data sets, and the results from 4-monthly seasonal averages (NDJF, MA+SO, MJJA) are compared with the reference Carnegie results. We examine the correlations between Weimer model predicted polar cap potential differences above Vostok [Weimer, 1996] and ground-level vertical electric field measurements for individual UT hours using bi-monthly data splitting as a test of the model. We discuss the possible influence of seasonal-diurnal variations in local atmospheric conductivity on measurements of the global meteorological generator. 2. Instrumentation, Data, and the Weimer Model [7] A rotating-dipole electric field mill (EFM) collected data at Vostok station, Antarctica, from 1998 to 2002 under a cooperative agreement between Russian, Australian, and American researchers. The EFM was mounted on a steel pole 1.5 m above the snow surface, upwind of the station s main buildings. Electric field measurements were collected at 10-s resolution, converted to 1-min averages, and then to 20-min and hourly averages for the analysis reported here. At least 80% of samples are required for each average. [8] The electric field mill was calibrated monthly by placing a Faraday-shielded box containing parallel plates to which was applied a stepped range of voltages. No calibration of the form factor of the EFM on the steel pole was undertaken, so these measurements are not absolute values but are determined relative to the field applied across the calibration box. The average of the Vostok data, 185 Vm 1, is not all that dissimilar to the value derived from an absolutely calibrated system at South Pole, 178 Vm 1 [Reddell et al., 2004], although the South Pole mill was located 3 m above the snow surface. Relative variations rather than absolute values are most appropriate when using the Vostok data. [9] Carbon brush commutators are used to transfer the signal from the rotating dipole to the pre-amplifier. These are subject to wear. The average magnitude of the discrepancy between successive calibrations increased over the years, as shown in Table 1. Owing to the large calibration uncertainty in 2002, these data are not used in determining the average bi-monthly curves but assist in demonstrating the consistency of their shape from year to year. [10] Fair-weather data were selected on the basis of the electric field values alone. One-minute-resolution meteorological data, including temperature, pressure, wind speed, and wind direction were available only for the interval 2000 to The most common local meteorological contamination of electric field measurements at Vostok is associated with the wind speed [Frank-Kamenetsky et al., 1999]. At varying initiating wind speed values, likely dependent on the speed required to lift snow and ice particles into the air, a rapid increase in the electric field values is observed. A superposed 2of14

3 data. Corney et al. [2003] discuss the relative advantages of the measurements used to derive these models, and the respective input parameters. Figure 1. Diurnal variation in the percentage of fair weather, presented as bi-monthly averages. JF, January and February; MA, March and April, etc. A significant austral summer decrease is apparent around local noon, 0500 UT. See color version of this figure in the HTML. epoch analysis with key-times selected when the electric field rose above and fell below 350 Vm 1 was used to decide on rejecting data between and within 2 hours of these transitions. Spikes are removed by rejecting data within 30 min of a jump between minute averages, separated by 5 min, exceeding 50 Vm 1. For the 2002 data, which are noisier than earlier years, spike removal was activated if variations exceeded 70 Vm 1 over 5 min. Austral summer data (November through February) regularly show a susceptibility to spikes around local solar noon (0500 UT). This is associated with a diurnal increase in wind speed around this time, in these months. Figure 1 shows the diurnal variation in the percentage of fair weather in bi-monthly intervals, calculated from times when the EFM was operational. Fair-weather coverage generally ranges between 45% and 65%, but reaches as low as 10% around local solar noon in November- December. [11] Hourly averaged solar wind parameters were obtained from the National Space Science Data Center (NSSDC) OMNIWeb database ( Solar wind data were available 99% of the time when electric field measurements were made. [12] The Weimer-1996 model is used to calculate the cross-polar-cap potential difference above Vostok. This model derives the ionospheric potential differences in the polar regions from measurements of the interplanetary magnetic field (IMF) B z and B y components, solar wind velocity, and the dipole-tilt angle (the angle between the magnetic dipole axis and the Sun-Earth line; it varies both diurnally and seasonally). It is an empirical spherical harmonic coefficient model, derived using a minimum error fit of satellite ionospheric electric field measurements and coincident solar wind measurements [Weimer, 1996]. Corney et al. [2003] compares the Weimer model with an early version of IZMEM [Papitashvili et al., 1994] and report that the Weimer model is more consistent with selected (104 complete days) Vostok 1998 electric field 3. Analysis and Results [13] Bi-monthly diurnal averages of 20-min-resolution, fair-weather, Vostok vertical electric field measurements are presented in Figure 2. Averages derived after combining the 1998 to 2001 bi-monthly data are also presented. The 2002 data are excluded from these averages due to the previously noted greater variability in the calibration data from this year. Table 2 lists values derived from the bi-monthly diurnal curves; the mean electric field value, the range expressed as a percentage of the mean, the time of the minimum, the time of the maximum, and the amount of data contributing to each curve. Indicative standard errors-in-themean are marked on the multi-year diurnal curve. The mean of the standard errors is 3.4 Vm 1, with a maximum of 6 Vm 1 around local noon for the austral summer months when few fair-weather measurements are obtained (see Figure 1). Significant variation is apparent in average magnitude and shape between the bi-monthly diurnal curves; however, a consistency of shape between years is apparent within each bi-monthly interval. [14] As previously noted, the interaction of the solar wind and the Earth s magnetic field provides an additional electric field generator in the polar regions. To estimate the contribution of the global meteorological generator to the Vostok electric field measurements, we need to subtract this influence, as best we may. Figure 3a shows the diurnal variation of the cross-polar-cap potential differences above Vostok calculated using the Weimer model [Weimer, 1996] when fair-weather electric field measurements were available during the bi-monthly interval of March-April. Figure 3b shows the average diurnal variations of the cross-polar-cap potential differences above Vostok when electric field measurements are available, for each bi-monthly interval. These values vary depending on parameters of the solar wind and due to the seasonal dependence of the diurnal variation in the dipole-tilt angle. A seasonal variation is apparent, but typically the cross-polar-cap potential difference above Vostok is high around the time of the diurnal minimum in the electric field values and conversely for the respective diurnal minimum and maximum. This association is location specific and varies with the displacement of local magnetic time with respect to universal time. [15] Vertical electric field measurements depend inversely on the conductivity of the air at the altitude of the measurement. If this near-ground-level air conductivity and the integrated column conductivity of the atmosphere between the ionosphere and Vostok does not vary, then a change in the broad-scale, cross-polar-cap potential difference above Vostok will be related by a constant value to the associated fluctuation in the measured electric field. For an average ionosphere-ground potential difference of 250 kv and a measured, average vertical electric field of 185 Vm 1,this constant should be 0.74 Vm 1 per kv. By comparing the calculated Weimer model potential differences with the fairweather electric field measurements for individual hours, and using bi-monthly data splits, we can examine the 3of14

4 Figure 2. Diurnal averages of Vostok electric field measurements for bi-monthly : yearly intervals. Averaged data (Ave) excludes measurements from 2002 (see text). Indicative standard errors-in-the-mean are plotted on the bi-monthly average curves. See color version of this figure in the HTML. validity of our constant conductivity assumptions and/or the accuracy of the model. In order to remove the average diurnal variation imposed by the global meteorological generator, variations from the hourly means of both variables, for each bi-monthly interval, are determined (DE and DF) and linear regressions are calculated. DEðhÞ ¼ Eh ð Þ E mean ðþ h DFðhÞ ¼ FðhÞ F mean ðþ; h where E(h) and F(h) are the hourly averaged Vostok vertical electric field value and the Weimer model cross-polar-cap potential difference above Vostok, respectively, and E mean (h) and F mean (h) are the bi-monthly averages for that hour. [16] Figure 4 shows the linear regression obtained for the hour between 1600 and 1700 UT, for all years combined. Considerable noise results from the day-to-day variability in the global meteorological generator, and perhaps from day-to-day variability in either of the air conductivity values discussed previously. Hourly values of the gradient of the linear regression (or ratio) for all the data and for bi-monthly data splits (all years combined) are presented in Figure 5. Two negative value ratios are not plotted. Standard errors are plotted for the all-months-combined 4of14

5 Table 2. Values Derived From The Bi-Monthly Diurnal Curves: the Mean Electric Field Value, the Range Expressed as a Percentage of the Mean, the Time of the Minimum, the Time of the Maximum and the Amount of Data Contributing to Each Curve Specifications January/February Mean, V/m Range, % average 42% 44% 39% 49% 49% 44% UT of minimum UT of maximum Data, days March/April Mean, V/m Range, % average 29% 30% 31% 33% 35% 29% UT of minimum UT of maximum Data, days May/June Mean, V/m Range, % average 16% 29% 27% 21% 22% 22% UT of minimum UT of maximum Data, days July/August Mean, V/m Range, % average 31% 26% 28% 28% 27% UT of minimum UT of maximum Data, days September/October Mean, V/m Range, % average 29% 41% 44% 35% 29% 35% UT of minimum UT of maximum Data, days November/December Mean, V/m Range, % average 46% 49% 57% 49% UT of minimum UT of maximum Data, days values. Although considerable variability is apparent in the individual bi-monthly values, only for two hourly intervals ( UT and UT) are the values more than twice the standard error-in-the-mean from the average ratio. These data therefore do not provide strong evidence for a diurnal variation in the ratio. [17] Combining all the data across all the hours, a ratio of 0.76 ± 0.06 Vm 1 per kv is obtained. Given the uncertainty in the average ionosphere-ground potential difference of 250 kv used to calculate the 0.74 Vm 1 per kv value previously introduced, these ratios are consistent. The standard error-in-the-mean is estimated using the assumption that an independent data point is obtained for each day contributing to the regression. This makes a reasonable allowance for the self-correlation of both data sets between consecutive hours. The UT interval displayed in Figure 4 was chosen because its ratio (0.77 Vm 1 ) was the closest to the overall average. Figure 4 shows the typical variability within an individual hour, while the all-yearscombined curve in Figure 5 provides an indication of the variability across the day. [18] The error-in-the-mean for the overall ratio noted in the paragraph above is an estimate of the uncertainty in the annually averaged value and does not preclude seasonal or diurnal variations in this ratio, as will now be considered. Diurnal variations in the standard errors are plotted in Figure 6 for times when at least 90 samples are available. Local magnetic noon at Vostok occurs at 1300 UT. Minimum standard errors are obtained around this time. To reduce the noise and to consider the possibility of a seasonal variation in the linkage between the electric field measurements and the imposed cross-polar-cap potential, hourly data are combined across the interval between 1000 and 1700 UT when the variation in the ratios determined and in the standard errors are minimized. Figure 7 shows the seasonal variation in the ratios obtained. Standard errors-in-the-mean shown in Figure 7 are again calculated using the assumption that only one independent data point is obtained for each day contributing to the regressions. We will return to issues associated with the linkage between the solar wind and the vertical electric field measurements in section 4. [19] We have estimated the influence of the cross-polarcap potential on the Vostok vertical electric field measurements as the average of the associated Weimer-model calculated potential differences above Vostok for each bi-monthly interval, scaled by the factor 0.74 Vm 1 per kv. Hourly averages determined using the Weimer model were linearly interpolated to yield the desired 20-min resolution. 5of14

6 Figure 3. Weimer model predictions of the cross-polarcap potential differences above Vostok. (a) March-April values for individual hours when fair-weather electric field measurements were available, and the diurnal average. (b) Diurnal averages for each bi-monthly interval. See color version of this figure in the HTML. component of the atmospheric global circuit. Seasonal splitting of the Carnegie data is provided by Reiter [1992, p. 130] for the boreal winter (NDJF), equinoxes (MA+SO), and the boreal summer (MJJA). Table 4 compares parameters derived from the Carnegie seasonally averaged diurnal curves with the appropriate 4-monthly Vostok averages from which the model-estimated influence of the cross-polar-cap convection has been removed, in the manner described previously. The Carnegie curve seasonal means have been calculated from the monthly values determined by Adlerman and Williams [1996]. Yearly average values are also listed in Table 4. [21] An estimation of the uncertainties in the parameters derived from the Vostok data are provided in Table 4. The uncertainty in the mean values is estimated as the weighted average of the annual calibration discrepancies listed in Table 1. The error in the range is derived from the standard error-in-the-mean of the minimum and maximum diurnal values, combined in quadrature. An estimation of the uncertainty in the time of the diurnal minimum (maximum) is provided by listing the times when values are less than the minimum value plus one standard error-in-the-mean (maximum value standard error). The UT times of the seasonal and yearly maxima and minima of the Carnegie and Vostok diurnal curves agree to within an hour, which is also the temporal resolution of the Carnegie values. An earlier UT time for the diurnal maximum for the November- December-January-February (NDJF) interval, and a later time of maximum for the MJJA interval, with respect to the time of the maximum for the yearly average, are consistent between the data sets. The yearly range values (expressed as a percentage of the mean) are in good agreement, but within the seasonal splits the Vostok measurements have a significantly greater range than the Carnegie data for the NDJF and MA+SO intervals. Vostok seasonal and annual diurnal means are larger than the respective Carnegie values. The Carnegie means for NDJF These are subtracted from the measured Vostok vertical electric field averages to obtain an estimate of the average diurnal variation of the global meteorological generator for each bi-monthly interval. The resultant curves are plotted in Figure 8. Parameters derived from these curves are listed in Table 3 and plotted in Figure 9. Considerable variation is apparent between the mean electric field values obtained for the bi-monthly intervals, but generally an annual maximum is obtained around July-August and an annual minimum near November-December (see Figure 9a). A more consistent annual variation is apparent in the diurnal range (see Figure 9b). An annual variation in the time of the diurnal range, averaging 59% of the mean, is obtained in November-December with a minimum, averaging 29%, recorded for May-June. An annual variation in the time of the diurnal maximum is apparent, while the diurnal minimum values are more variable but show no significant annual trend (see Figure 9c). [20] The Carnegie measurements provide the generally accepted reference standard for the meteorologically driven Figure 4. Linear regression analysis of the variation from the mean of the vertical electric field values and the Weimer model inferred cross-polar-cap potential difference above Vostok (means calculated separately for each bi-monthly interval) for all data from the hour centered on 1630 UT. 6of14

7 Figure 5. Gradients (ratios) of the linear regression analyses for each UT hour, for each bi-monthly interval (JF, January-February, etc.) and for all the data combined. The error bars plotted on the combined data are ±1 standard error-in-the-mean. See color version of this figure in the HTML. and MA+SO are equal, while the Vostok MA+SO diurnal mean is significantly larger than the Vostok NDJF value. [22] Sufficient data are available in the interval to provide monthly averages of the corrected data for ease of comparison with other data sets. Figure 10 displays these monthly diurnal averages, and Table 5 lists parameters derived from these curves. Uncertainty estimates are provided, calculated in a similar manner to that provided in Table 4. Parameters derived from the monthly curves are also plotted in Figure 9. The monthly averages contain fewer samples than the bi-monthly and 4-monthly averages, and are thus less well determined. To quantify the magnitude of this, consider the number of values within one standard error of the diurnal minimum (maximum). The average range of the uncertainty for the monthly curves is 120 min (110 min). As an extreme example, the monthly diurnal minimum for May occurs at 1030 UT, an unusually late time compared to other months. There are Figure 6. Errors-in-the-mean for gradients determined from the linear regression analyses of DE and DF, when at least 90 data points were included in the analysis. See color version of this figure in the HTML. Figure 7. Linear regression gradients (ratios) determined after combining the values for each hour within the interval between 1000 and 1700 UT. The error bars are ±1 standard error-in-the-mean calculated on the basis that only one independent data point is obtained for each day of data. 12 values covering the time range from 0250 UT to 1110 UT that fall within one standard error-in-the-mean of the minimum value for this month. Similarly estimated uncertainties for the 4-monthly averages amount to 60 min (90 min). 4. Discussion [23] It is appropriate to first consider the results of comparing the Weimer-model calculated cross-polar-cap potentials above Vostok with the measured vertical electric field values, and the implications upon either the model or the assumptions made in subtracting the influence of the solar wind generator. The dominantly positive correlation of DE and DF (only two of 27 independent regression analyses yielded a negative slope) provides further strong proof of the influence of the solar wind on ground-level, polar-cap electric field measurements [Burns et al., 1998; Tinsley et al., 1998; Frank-Kamenetsky et al., 1999; Corney et al., 2003; Reddell et al., 2004]. [24] The significantly larger data set available for the present analysis compared to those previously reported allows further insight. While the slopes of the linear regressions obtained for individual UT hours over bi-monthly intervals exhibit substantial variability (see Figure 5), that variability is reduced near local magnetic noon (1300 UT). Standard errors-in-the-mean are also reduced around this time (Figure 6). The Weimer model predicts the polar-cap convection directly from solar wind parameters. The interaction of the solar wind and the Earth s magnetic field is a more direct interaction process on the dayside. On the nightside, the storing and dumping of energy extracted from the solar wind (auroral substorms) is likely to make the prediction of the cross-polar-cap convection more variable with respect to the instantaneous solar wind parameters. The processes by which the solar wind interacts with the Earth s magnetic field generating the crosspolar-cap convection most likely account for the noted results of the analysis. That the variations are generally symmetrical with respect to local magnetic noon is consistent with this interpretation. [25] Within the limits imposed by the standard errors-inthe-mean, a uniform diurnal value linking the cross-polar- 7of14

8 Figure 8. Diurnal averages of Vostok electric field measurements for bi-monthly : yearly intervals, corrected for the influence of polar-cap convection. Averaged data (Ave) excludes measurements from See color version of this figure in the HTML. cap potential difference above Vostok and the near-groundlevel variation in the vertical electric field cannot be excluded, when the all-bi-monthly intervals-combined results are considered. Corney et al. [2003] analyzed 104 complete days from the 1998 Vostok data set and obtained results implying that the Weimer model underestimated the dawn-to-dusk potential difference at Vostok (magnetic latitude 83.6 S). Reddell et al. [2004] reach a similar conclusion using 42 days of data from South Pole (magnetic latitude 74.2 S). The January-February diurnal variations in the ratio obtained in our present analysis, with high-magnitude slopes symmetrically located about magnetic noon, are similar to the general results obtained by Corney et al. [2003] but are not sustained when all the bi-monthly intervals are combined. [26] When the data around local magnetic noon are considered separately (the interval between 1000 and 1700 UT), an annual variation in slope is implied (see Figure 7). The November-December slope is least consistent with this interpretation, but also has the largest uncertainty as only 3 years contribute to these data. The ratios obtained are smallest for the austral winter months (MJJA). One possibility is that seasonal-diurnal variations in local conductivity, or more specifically in the ratio of the conductivity at the height of the measurement (1.5 m) to the column conductivity of the atmosphere between the ground and the ionosphere at the site, may lead to this result. This is an important parameter to determine, but beyond the scope of this paper. Conjecture is possible. [27] The wind speed may play an intermittent role in local conductivity variations. Positive associations of electric field variations and local wind speed have been reported for Antarctic sites [Burns et al., 1995; Frank-Kamenetsky et 8of14

9 Table 3. Values Derived From the Bi-Monthly Diurnal Curves After Subtraction of the Influence of Polar-Cap Convection Specifications January/February Mean, V/m Range, % average 49% 51% 44% 56% 57% 51% UT of minimum UT of maximum Data, days March/April Mean, V/m Range, % average 36% 36% 37% 41% 43% 37% UT of minimum UT of maximum Data, days May/June Mean, V/m Range, % average 25% 34% 33% 28% 26% 29% UT of minimum UT of maximum Data, days July/August Mean, V/m Range, % average 38% 32% 33% 33% 33% UT of minimum UT of maximum Data, days September/October Mean, V/m Range, % average 38% 45% 52% 45% 36% 43% UT of minimum UT of maximum Data, days November/December Mean, V/m Range, % average 54% 59% 64% 59% UT of minimum UT of maximum Data, days al., 1999], but this association is not consistent. There are times when a particular wind speed will lead to a high electric field value, and on other occasions it will not [see, e.g., Frank-Kamenetsky et al., 1999, Figure 1d]. This may be due to variations in the ease with which the varying forms of snow and ice may be lifted by the wind into the atmosphere. Other atmospheric parameters, including temperature and pressure, may also influence local conductivity, although the manner of their relative influence on the ratio of local conductivity to the total column integrated value cannot be readily inferred. [28] Polar plateau sites undergo a dramatic seasonal variation in solar irradiance. However, this only has a significant influence on the conductivity of the upper altitudes, above 60 km, of the ionosphere-ground conductivity column. These high altitudes have an insignificant influence on the total ionosphere-to-ground resistance; thus the seasonal variation in solar irradiance cannot explain the measurements. Natural radioactivity of the Earth, which is known to influence the conductivity of the lower atmosphere, may at some sites be subject to seasonal variation via changes in local snow and ice cover. No such influence is possible at Vostok, which is situated on an ice sheet of thickness greater than 3 km. Ionization by cosmic rays controls the conductivity of the middle and lower atmosphere above the polar plateau, and average diurnalseasonal variations in cosmic ray intensity are less than 1% [Pomerantz and Duggal, 1974], insufficient to significantly influence our results. [29] The presence, in the Southern Hemisphere winter, of polar stratospheric clouds and ice particles in layers of thin cirrus clouds near the tropopause will decrease the conductivity of this region of the atmosphere and will alter the resistance ratio between the near-ground-level region and the total atmospheric column in a manner which may explain the measurements [Tinsley, 2000; Brian Tinsley, personal communication, 2004]. The general interval covered by the occurrence of polar stratospheric cloud above Vostok is from late May to a more variable end time ranging between September and November (Andrew Klekociuk, personal communication, 2004). This is not incompatible with the interval when the low ratios are recorded. If this hypothesis were correct, the boreal winter ground-level electric field response in the Northern Hemisphere polar regions may be measurably different from Antarctic polar plateau results. Stratospheric clouds, which are associated with enhanced ozone depletion in Antarctica, are much less prevalent in the Northern Hemisphere. 9of14

10 Figure 9. Annual variations in the (a) diurnal mean, (b) diurnal range (expressed as a percentage of the diurnal mean), and (c) the time of the diurnal minima and maxima for individual years, for the data from 1998 through 2001 combined (Ave) and for the monthly averages (Mnth). [30] Theoretical analysis [Park, 1976b] indicates that large-scale features (>200 km) of the cross-polar cap potential should map directly into the vertical electric field measurements at the ground. The experimental analysis presented above shows that the Weimer model potential differences above Vostok influence the ground-based electric field measurements at approximately the expected ratio, across all UT hours. Corney et al. [2003] obtained a similar result for 19 individual hours across the day using a subset of the Vostok data (1998: 104 complete days). The extra data that we have utilized indicates that on average, the magnetic nightside of the model at Vostok magnetic latitudes is also consistent with ground-level electric field measurements. As the Weimer model is related to the Vostok data in the manner demonstrated, then it can be used to remove the cross-polar cap convection with confidence that we are improving the determination of the global meteorological signal. [31] In order to estimate and remove the influence of the solar wind generator on the electric field measurements at Vostok, we have assumed that the Weimer model accurately infers the cross-polar-cap potential difference above the site and that the factor linking these values to the near-ground-level electric field measurements is a constant 0.74 Vm 1 per kv. As discussed previously, the all-bi-monthly intervals-combined diurnal variation in the ratio is not inconsistent with this interpretation. The possible seasonal variation in the ratio is an influence which we presently cannot confidently allow for. Our approach makes a significant allowance for the influence of the solar wind generator, and is the best option presently possible. [32] Although our analysis confirms that we are improving our estimation of the global meteorological signal by this approach, it is sensible to compare the diurnal curves obtained with and without correction, and with the Carnegie diurnal curves which provide the reference data set for the global atmospheric circuit. Removing the influence of the cross-polar-cap potential difference in the manner indicated increases the bi-monthly diurnal ranges by an average of 25% (compare Figures 2 and 8 and Tables 2 and 3). This results from the general association of the diurnal peak in the cross-polar-cap potential above Vostok with the minimum in activity of the global meteorological generators, and the converse association. Corney et al. [2003] report that correction using the Weimer model increased the diurnal range for their all-year analysis from 28% to 37%, with the final figure being equal to the Carnegie all-year value. Our analysis confirms this result for the corrected data (see Table 4). A seasonal split (Table 4) shows that the Vostok austral winter (MJJA) range is also consistent with the Carnegie value, while the Vostok austral summer (NDJF) and equinox (MA+SO) range values are higher than the Carnegie values, with the austral summer (NDJF) discrepancy being greatest. The diurnal minimum in electric field values occurs at 0400 UT while local Vostok geographic noon is 0500 UT. A seasonal-diurnal variation in conductivity associated with local noon and/or remnant influences of the diurnal wind variation which limits the availability of data in November and December (see Figure 1) may contribute to this discrepancy. [33] Correcting the diurnal curves using the Weimer model reduces the variation between the bi-monthly minima and maxima for individual years, and the combined data set (see Tables 2 and 3). The average magnitude of the time difference between the combined years and each individual year s minimum values, without Weimer correction, is 100 min. With the Weimer model applied, this reduces to 40 min. The discrepancy between the maxima is generally lower, but reduces from an average of 33 min to 16 min depending on whether the Weimer model correction has been applied. These changes are 10 of 14

11 Table 4. A Comparison of Carnegie and Vostok ( ) 4-Monthly, Seasonal-Diurnal Curves NDJF MA+SO MJJA Year Carnegie Time of minimum, UT Time of maximum, UT Range, % Mean, a Vm Vostok Time of minimum, UT Times (<minimum + standard error) Time of maximum, UT Times (>maximum standard error) Range, % 57 ± 2 42 ± 2 32 ± 2 37 ± 1 Mean, V m ± ± ± ± 5 Amount of data, days a Calculated from Adlerman and Williams [1996]. dominated by the reduction in extreme events. There are four bi-monthly minima for individual years (1998-ND; 1999-JA, 2000-JF, and 2002-MJ) and one bi-monthly maximum (2002-MJ) which differ from the average by at least 3 hours, prior to correction for the influence of the cross-polar-cap convection (see Table 2). After the Weimer model corrections are applied (see Table 3) the bi-monthly minima and maxima for individual years differ from the multi-year average by no more than 2 hours. This implies that the solar influence on the electric field measurements via polar-cap convection is more variable from year to year than the global meteorological influence, when averaged over bi-monthly intervals. [34] The Carnegie 4-monthly seasonal maxima and minima differ from the similarly determined Vostok values by no more than an hour (see Table 4), and both show a similar seasonal drift in the time of the maxima and no more than minor variability in the time of the seasonal minima. Some variations between the monthly diurnal curves (see Table 5) can be associated with expected variations in global thunderstorm activity. The timing of the diurnal maximum drifts from 1750 UT in January to Figure 10. Diurnal averages of Vostok electric field measurements for monthly intervals, combining years 1998 to Indicative standard errors-in-the-mean are also plotted. See color version of this figure in the HTML. 11 of 14

12 Table 5. Parameters Derived From Monthly Diurnal Vostok Electric Field Curves After the Influence of Polar Convection has Been Removed Using the Weimer-1996 Model January February March April May June July August September October November December Time of minimum, UT Times (<minimum + standard error) Time of maximum, UT Times (>maximum standard error) Range, % of mean 57 ± 4 53 ± 4 41 ± 3 33 ± 3 28 ± 4 32 ± 4 33 ± 3 37 ± 3 41 ± 3 50 ± 3 60 ± 4 64 ± 7 Mean, Vm ± ± ± ± ± ± ± ± ± ± ± ± 5 Amount of data, days of UT in June, associated with the more westward location of North America compared with South America and the seasonal variation in thunderstorm activity. July is when the largest individual month diurnal mean is recorded, consistent with the larger land coverage of the Northern Hemisphere. [35] Reddell et al. [2004] published 3-monthly seasonal maxima and minima corrected for cross-polar-cap convection using the Weimer model and determined from a total of 42 days of data collected at South Pole. The 3-monthly splits and the number of accumulated days of data contributing to these seasonal averages are: JFM 10 days; AMJ 10 days; JAS 5 days and OND 17 days. The times of seasonal minima (maxima) reported are: JFM: 0330 UT (1500 UT); AMJ: 0600 UT (2100 UT); JAS 0030 UT (1900 UT), and OND 0600 UT (1900 UT). The differences between the published South Pole values and both the Vostok and Carnegie values is likely to be due to the sparsity of South Pole data with concurrent Interplanetary Magnetic Field observations, as required for Weimer model calculations. Some indication of the expected variability can be gained from Table 5 where we have listed the range of times when values fall within one standard error-of-the-mean for Vostok monthly minima and maxima. The Vostok monthly diurnal curves are determined from an average of 51 days of data (minimum 36 days; maximum 75 days). Specifically, the South Pole JAS minimum at 0030 UT is earlier than any of the Vostok monthly minima (earliest Vostok monthly minimum is 0250 UT for April) and the South Pole JFM maximum at 1500 UT is earlier that any Vostok monthly maximum (earliest Vostok monthly maximum is 1750 UT in January). These values are not in significant disagreement considering the sparsity of the South Pole data. [36] Allowance for the process suggested to explain the possible seasonal variation in the gradient of the regression between DE and DF would imply a larger annual variation in the strength of the meteorological generator than is implied by our measurements. The principal reason for the increased diurnal range during the austral summer (November through February), expressed as a percentage of the global mean, is due to a significant reduction in the electric field values around the time of the diurnal minimum (0430 UT). This may be associated with the larger expanse of the Pacific Ocean (associated with the diurnal minimum in global thunderstorm activity) in the Southern Hemisphere. Some caution is warranted with this interpretation. As noted previously, not many fair-weather electric field measurements are possible at this time (see Figure 1), and a diurnal variation in conductivity (associated with either local temperature or wind speed variations) is possible. [37] We have demonstrated that it is possible with single-site, Antarctic polar-cap measurements of the vertical electric field to obtain bi-monthly diurnal curves which are generally consistent in shape from year to year and are broadly associated with global thunderstorm activity. Similar measurements from other polar plateau stations would enhance our ability to discriminate between local variations and the global signal. It is only to the extent that diurnal averages of the influence of the meteorological generators derived at different sites are similar, that claims

13 of measuring a global signal can be justified. Significant advances can be made, and are planned, in the interpretation of the global circuit and the influence of its various generators if we can obtain concurrent data from multiple Antarctic plateau sites and make simultaneous, accurate long-term measurements of the miniscule air-earth current. The air-earth current is less influenced by local air conductivity than the electric field. Simultaneous measurements of both parameters should enable quantification of the influence of local conductivity. Reddell et al. [2004] include analysis of 84 days of air-earth current measurements at South Pole and report no significant diurnal or seasonal variation in local conductivity, within the accuracy of their available measurements. [38] Simultaneous measurements at multiple sites would allow us to infer the cross-polar-cap potential difference above the sites, independent of the contribution of the global meteorological generators which are equivalent at both sites. This would provide a more direct test of polarcap convection models. Improving or confirming the accuracy of the global convection models would allow a better or more quantifiably accurate estimate to be made of the contribution of the global meteorological generator. 5. Conclusions [39] Bi-monthly diurnal averages of the vertical electric field at Vostok have been obtained with an average relative accuracy of 3.4 Vm 1. Austral summertime measurements around local solar noon (0500 UT) are less well defined (6 Vm 1 ), most likely due to diurnal fluctuations in the local wind speed. Bi-monthly averaged diurnal curves with 20-min resolution have been shown to vary significantly in magnitude and shape, but to be generally consistent in shape for the same bi-monthly interval between years. [40] Linear regression analysis of variations about the mean of Weimer-model calculated cross-polar-cap potential differences above Vostok and near-ground-level measurements of the vertical electric field calculated for each hour over bi-monthly intervals have been used to demonstrate that the Weimer model yields a better prediction of the cross-polar-potential above Vostok around local magnetic noon (1300 UT) than at other times. This is consistent with the linkage between the solar wind and polar-cap convection being a more direct association on the dayside of the magnetosphere. A seasonal variation in the linkage between these parameters is also apparent, with their ratio (in Vm 1 per kv) generally smaller in the austral winter (MJJA). The average, all-data, value of this ratio is 0.76 ± 0.06 Vm 1 per kv. [41] When the best estimate of the influence of the solarwind generator is subtracted from the bi-monthly diurnal averages of the electric field measurements to obtain an estimate of the contribution of the meteorological generators, we find that the annual variation in the diurnal average electric field peaks in July August. This is consistent with thunderstorm activity being a summertime phenomena and the Northern Hemisphere having a greater land mass than the Southern Hemisphere. The annual variation in the timing of the diurnal peak is also consistent with the more westerly location of North America with respect to South America. A dramatic annual variation is measured in the diurnal range expressed as a percentage of the diurnal mean from 59% in November December to 29% in May June. Care is advised in ascribing this variation solely to the global meteorological generator as fair-weather measurements are sparse around the time of the diurnal minimum in November December and diurnal variations in local meteorology may also significantly influence austral summertime measurements via the near-ground-level air conductivity. [42] Acknowledgments. This research was approved by the Australian Antarctic Science Advisory Committee (AAS 974) and supported by the Russian Foundation for Basic Research (project ) and the National Science Foundation (award OPP to the University of Houston). Lloyd Symons, Australian Antarctic Division, developed the electric field mill electronics and data collection software. The Vostok electric field measurements were collected by specialists of the Russian Antarctic program. 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Few (2004), Seasonal variations of atmospheric electricity measured at Amundsen-Scott South Pole station, J. Geophys. Res., 109, A09308, doi: /2004ja Reiter, R. (1992), Phenomena in Atmospheric and Environmental Electricity, 541 pp., Elsevier, New York. 13 of 14