1. Introduction
The atmospheric electric field (AEF) is an important physical phenomenon in the field of atmospheric physics and space physics. As the ground carries negative charges and the atmosphere contains net positive charges, there is an electric field in the atmosphere at all times, which is known as the AEF. On a global average, the AEF value is approximately 100~200 V/m on a fair day, and the direction of the AEF on a fair day is vertically downward toward the ground. A total of 1800 simultaneous thunderstorms worldwide can generate an upward current of approximately 1 A to the ionosphere, and the upward current maintains a potential difference between the ionosphere and the ground, and the ionospheric potential has daily variations [
1].
The AEF has a sensitive response to changes in various meteorological elements, local geological conditions, solar activities, human activities, and so on. When there are lightning activities [
2,
3,
4,
5,
6] or clouds [
7,
8], AEF will show values on the order of kV/m. The AEF can be easily changed by precipitation [
9], air pollutants [
10,
11,
12,
13], solar activities [
14,
15,
16], geomagnetic activities [
17,
18], and so on. AEFs have been studied for many years, and researchers have summarized many methods for measuring AEFs, such as flat plate antenna measurements, sounding observations, ground-based rotating electric field meters, sphere-borne double sphere electric field meters, and microrocket electric field meters. Currently, the most popular method is an electric field mill (EFM), which provides the opportunity for faster measurements with multiple options for meteorological conditions. The EFM measures the electric field using the principle that the induced charges in the environmental electric field have been generated when a metal sheet rotates, then the AEF values can be obtained by measuring the weak electrical signals caused by the induced charges. In addition, it is installed outdoors and exposed to the atmosphere. The sensor plates are electrodes that are alternately shielded and exposed to the AEF [
19].
The diurnal variation of the global ionospheric potential overhead drives a current density downward which creates the observed AEF at any location. In the absence of diurnal variation in convection over the ocean, the diurnal variation in AEF is the same as that of the ionospheric potential, with a maximum near 20 UT and a minimum near 4 UT, which is the shape of the “Carnegie curve” [
20]. Shaista Afreen et al. [
21] studied AEF data for the period from June 2019 to May 2020 at Gulmarg station (34°05′ N; 74°42′ E), Kashmir, a research area with high thunderstorm activity and very high radon concentrations. They plotted the fair-weather average AEF variation curve for 58 fair-weather days and compared them with other regions. Zhou et al. [
22] conducted clear-sky atmospheric electric field observations on a high mountain at a distance of 170 m from the Gaisburg Tower in Austria, and the observations were analyzed comparatively for different terrains. The correction factors due to topographic factors were discussed. Wu et al. [
9] discussed and analyzed the characteristics of near-surface AEFs under different weather conditions using the data observed on the roof of the Physics Building of Peking University from August 2004 to November 2005. They surmised that the fair-weather average AEF variation curves under fair-weather conditions in Beijing were “double peaks, double valleys”, with valleys occurring at 05:00 and 12:00 (BJT, UT + 8) and peaks at 07:00 and 23:00 (BJT, UT + 8). Similarly, Burns et al. [
23] conducted a long-term observational study of the AEF at Vostok, Antarctica (78.5° S, 107° E), selecting 720 days of AEF data collected under clear-sky conditions during 1998–2002 and analyzing the daily, seasonal, and annual characteristics of the clear-sky AEF in this region in detail.
A comparison of the characteristics of AEFs in several regions under fair-weather conditions can help researchers analyze different local effects and the distribution of space charges, which is important in near-surface environment exploration. In this paper, AEF observations from three different global stations and their fair-weather average AEF variation curves are compared, and their relationship with PM 2.5 concentrations and other possible physical reasons for the different results are also discussed, as these results are important for atmospheric electrostatic environment detection.
3. Method
To compare the fair-weather average AEF variation curves of different regions worldwide and the electric field meters of different organizations, we chose three AEF stations: the Xanthi station (XAN), the Zhuanghe station (ZH), and the University of Reading station (RDG). The geographical locations of these three stations are shown in
Figure 1, and the red pentagons represent the locations of stations XAN, ZH, and RDG, which are located in Europe and Asia. More detailed station information is shown in
Table 1. They belong to different organizations, different countries, and different observatories with different electric field meters, but they are all located at mid-latitudes, and their altitudes are between 0 and 100 m. In addition, the longitudes of these three stations are different and far away from each other, and in terms of latitudes, the ZH Station and the XAN station have basically the same latitudes, while the RDG station differs greatly from these two.
The electric field instruments used at these three stations vary. The JCI 131 Chilworth instrument used at the RDG station has high precision (<1.5%), low noise, and a stable zero, and its accuracy is within ±1.5% full-scale deflection (FSD). For a measurement range of 0–21,000 V/m of the Campbell CS110 used at the XAN station, the resolution is 3 V/m, the sensitivity is 12 μV/m, and the noise is 4 V/m. For the measurement range of 21,000–212,000 V/m of Campbell CS110, the resolution is 30 V/m, the sensitivity is 118 μV/m, and the noise is 18 V/m. The accuracy of the Campbell CS110 is ±1% of the reading. Unlike these instruments, the ZH station used the EFM 100 with a measurement range of ±50 kV/m, measurement accuracy < 5%, response time < 1 s, linearity < 1%, and resolution of 10 V/m. By comparing the AEF data from these three stations, it is clear that the JCI 131 and CS 110 instruments are more accurate and responsive than the EFM 100, but in weather with abundant lightning activity, the AEF values measured by the JCI 131 and CS 110 instruments are smoother, while the EFM 100 instrument is more responsive and sensitive than they are.
It is well known that aerosol concentration is important for AEF values [
27,
28] and that its daily variation is influenced by sunrise and sunset times. The average sunrise time (SRT) and sunset time (SST) in January–March, April–June, July–September, and October–December for these three stations are shown in
Table 2. In January–March, SRT (RDG) = SRT (ZH) + 7.9 h = SRT (XAN) + 2.6 h, SST (RDG) = SST (ZH) + 7.1 h = SST (XAN) + 2 h. In April–June, SRT (RDG) = SRT (ZH) + 6.9 h = SRT (XAN) + 2.8 h, SST (RDG) = SST (ZH) + 7.9 h = SST (XAN) + 3.8 h. In July–September, SRT (RDG) = SRT (ZH) + 7.1 h = SRT (XAN) + 2.9 h, SST (RDG) = SST (ZH) + 7.9 h = SST (XAN) + 3.6 h. In October–December, SRT (RDG) = SRT (ZH) + 7.7 h = SRT (XAN) + 2.7 h, SST (RDG) = SST (ZH) + 7 h = SST (XAN) + 2.1 h. On average, SRT (RDG) = SRT (ZH) + 7.4 h = SRT (XAN) + 2.7 h, SST (RDG) = SST (ZH) + 7.5 h = SST (XAN) + 2.9 h. In terms of daytime duration, the RDG station has the longest daytime, followed by the XAN station and finally the ZH station, and their differences are minimal. Then, the duration of daytime in April–June and July–September is 3.2–5.9 h longer than that in other seasons.
Data from January to December 2021 for the three stations were counted. The data from the RDG and XAN stations are one data point per minute, and the data from the ZH station are one data point per second. To perform a comparative study, we divided this time period into four parts: January–March, April–June, July–September, and October–December. According to the definition of fair-weather conditions above and the validity of the data, the statistical results are shown in
Table 3. An effective day is a day in which there is no data loss for more than 1 h in a continuous period. For these three stations, the XAN station had more data loss, and the ZH station had the least loss. At the RDG station, the highest number of fair-weather days occurred during April–June, which was 49 days, accounting for 53.8% of the effective days. For the ZH station, the greatest number of fair days was during July–September, which was 50 days, accounting for 64.1%. The largest percentage of fair days at the XAN station was July–September, with 16 fair days accounting for 42.1% of the effective days.
4. Results
The data from these three stations from January to December 2021, excluding data anomalies and non-clear days, will be plotted as a fair-weather average AEF variation curve for each three-month average of clear days for these three stations, as shown in
Figure 2.
Figure 2a–c corresponds to the RDG station, ZH station, and XAN station, respectively. The vertical coordinate corresponds to the value of the AEF, and the horizontal coordinate indicates UT time, with red indicating January–March, green representing April–June, dark blue being July–September, and light blue indicating October–December.
In
Figure 2a, signifying strong convection at 8 UT to 20 UT, and the four different colored curves are all characterized by “double peaks, double valleys”, with the January–March, October–December, and July–September curves being nearly identical, and they are higher than the April–June curve before 12 UT. Between 12 UT and 24 UT, the October–December curve is the highest, followed by the January–March curve, the July–September curve, and the April–June curve. In
Figure 2b, the values in Jan.-Mar. and October–December are much higher than expected from the Carnegie curve, while the other two curves’ values are similar to the Carnegie curve, signifying strong convection at 5 UT, and the four different colored curves are all characterized by a “single peak, single valley”. At most moments, similar to those between 12 UT and 24 UT in
Figure 2a, the January–March and October–December curves are the highest. A submaximal value appears at 15 UT of the January–March curve, which may be caused by artificial aerosols. While
Figure 2c is quite different from
Figure 2a,b, the four different colored curves are different: the January–March and July–September curves are “double peaks, double valleys”, while the April–June and October–December curves are a “single peak, single valley”. The January–March curve is the highest, followed by the April–June curve, and the July–September curve is nearly the same as the October–December curve. By comparing their curves with the meteorological conditions, for the RDG station, the results of the ranking of the percentage of fair days are April–June > July–September > January–March > October–December, which is the opposite trend from that shown in
Figure 2a. For the ZH station, the ranking of the percentage of fair days is October–December > January–March > July–September > April–June, and this is the same as the results in
Figure 2b. For the XAN station, there is signifying strong convection from 8 UT to 14 UT, again in the middle of the day local time. In addition, the ranking of the percentage of fair days is July–September > January–March > October–December > April–June, which is different from the results in
Figure 2c. In
Figure 2a, the January–March, April–June, July–September, and October–December peaks and valleys are consistent with each other. The same is true for
Figure 2b,c except for some minor differences due to unavoidable systematic errors. The measurement data will be jittered with large values for a short time sometimes, caused by the measurement instrument; this can lead to minor errors in the results. Therefore, it can be concluded that the fair-weather average AEF variation curves for different regions on Earth vary, their annual variation characteristics vary, and the relationship between annual variation and meteorological conditions also varies. For a certain region, the AEF values vary for different seasons, but their curves are the same shape.
By comparing
Figure 2a and
Table 2, the peaks of the January–March, April–June, July–September, and October–December curves all correspond to 8 UT and 20 UT, and their valleys correspond to 4 UT and 12 UT, which have little correlation with the sunrise and sunset times of the RDG station, as do the results of the other two stations for comparison. This finding indicates that the shape of the fair-weather average AEF variation curves (including peak and valley corresponding times and values) in a defined area is not entirely determined by sunrise and sunset times but by local content and varied compositions of underground radioactive matter.
To reveal the reason for the differences in the average fair-weather AEF daily variation for various seasons and various locations, the average PM 2.5 concentration for every month has been counted and plotted, as shown in
Figure 3. In
Figure 3, the average concentrations for different seasons have been shown as the short horizontal lines and digits beside them. Based on
Figure 3, it can be clearly seen that the variability of PM 2.5 concentration in different seasons for the RDG station was only 2.4 μg/m
3, which was stable compared to that for the other two stations. In addition, the variability of fair-weather AEF in different seasons in
Figure 2a was no more than 32 V/m, which was also stable. For the average PM 2.5 concentration at the ZH Station, the ranking was January–March > October–December > April–June > July–September, which was consistent with the fair-weather AEF diurnal variation curves of ZH station. For the XAN station, the highest AEF Jan.-Mar. curve corresponded the maximum PM 2.5 concentration with 32.2 μg/m
3. Although April–June curve and July–September curve were close with their close PM 2.5 concentrations, which made a significant difference for the October–December curve and April–June curve. Only at 18 UT–21 UT were the AEF curves of different seasons at the XAN station consistent with the curves of PM 2.5 concentration. This discrepancy can be attributed to a data error, which was caused by the less sunny days at the XAN station.
The daily variations at the RDG, ZH, and XAN stations were averaged to obtain the total fair-weather average AEF variation curve, which makes the mean value of the AEF 100% for the three stations, as shown in
Figure 4. In
Figure 4, the horizontal axis represents the LT time, and the vertical axis represents the percentage of atmospheric electric field/mean atmospheric electric field. The brown curve represents the percentage curve of average fair-weather AEF daily variation of the ZH station, the blue curve represents the curve of the XAN station, and the green curve represents the curve of RDG station, and the purple dashed line indicates the mean value (100%).
First, these three curves have different characteristics: the curve of the ZH station is a “single peak, single valley”, the curve of the RDG station is “double peaks, double valleys”, and the fair-weather average AEF variation curve of XAN station is between them; it resembles both a “single peak, single valley” curve and a “double peaks, double valleys” curve. Then, the values of these three curves are quite different; the maximum average AEF from January to December 2021 was at the XAN station, which was 139.5 V/m. The average AEF at the ZH station was 116.0 V/m and that at the RDG station was 89.2 V/m. Comparing it with the results in
Figure 3, the researchers can find that the average PM 2.5 concentrations were 9 μg/m
3, 27 μg/m
3, 22.7 μg/m
3 for the RDG, ZH, and XAN stations, respectively. The average PM 2.5 concentration at the RDG station was less than that at the other two stations, and the PM 2.5 concentration at the ZH station was close to the XAN station. This was consistent with the difference in the average fair-weather AEF values. Third, the fair-weather average AEF variation curve at the RDG station is smooth, with small fluctuations, but the curves for the ZH station and the XAN station fluctuate widely. In
Figure 4, considering the mean value of the AEF as 100% for the three stations, the variation amount at the ZH station and the XAN station were the same with 64.8% of the mean AEF, which was only 34.4% for the RDG station. The ZH and XAN stations had a large range of variation and strong fluctuations; however, the RDG station has the weakest fluctuations. Similarly, it can be compared with
Figure 3 and it can be found that the average PM 2.5 concentrations were also consistent with the fluctuations in the fair-weather AEF diurnal variation.
For the ZH station, from 0 LT to 14 LT in the morning, the AEF values increased to the peak value at 14 UT in the noon with 156.0 V/m with the increased aerosol concentration. Then, from 14 LT to 24 LT in the evening, the atmosphere stabilized, the aerosols gathered near the ground, the aerosol concentration increased, and the AEF increased. Then, for the XAN station, its curve characteristics were similar to the ZH station, from 2 LT to 10 LT and the AEF increased to the peak value of 197.3 V/m at 10 LT as the aerosol concentration increased. Because of active thermal convection and turbulent vertical transport from 10 LT to 22 LT, the near-surface aerosol concentration decreased, and the AEF decreased to the valley value of 108.2 V/m. Between 22 LT and 2 LT the next morning, the atmosphere stabilized, and the AEF stabilized and slowly decreased. At the RDG station, from 4 LT to 8 LT, the aerosols gathered near the ground. Additionally, the aerosol concentration and AEF increased, with the maximum peak of 91.1 V/m appearing at approximately 8 LT. From 8 LT to 13 LT, due to strong radiation from the sun, the vertical transport of thermal convection and turbulence was active, aerosols were continuously transported upward from near the ground, the near-surface aerosol concentration continuously decreased, and the near-surface AEF reached the minimum valley of 84.6 V/m at 13 LT. Then, from 13 LT to 20 LT, the upper aerosol concentration was greater than that near the ground. Due to atmospheric pressure, aerosols were vertically transported, and the aerosol concentration near the ground again increased. Then, the AEF also increased, reaching another peak at 20 LT. From 20 LT to 4 LT the following day, the atmosphere gradually stabilized, and the aerosol concentration decreased. In addition, the AEF decreased to another valley at 4 LT.
6. Conclusions
By comparing the different curves in
Figure 2,
Figure 3 and
Figure 4 the conclusion can be summarized as follows. (1) The fair-weather average AEF variation curves annual variation characteristics for the RDG station, the XAN station, and the ZH station are diverse, and for the three global stations, the relationship between annual variation and meteorological conditions is also varied. The ZH and XAN stations have strong fluctuations, while the RDG station has the weakest fluctuations. Then, their curves are represented by two different types: the curves at the ZH station and the XAN station are a “single peak, single valley”, while the curve at the RDG station is “double peaks, double valleys,” and the differences in these curves showed an approximate positive correlation with PM 2.5 concentrations. Their different average AEF values are not only determined mainly by the local PM 2.5 concentration but are also affected by the local content and varied compositions of underground radioactive matter, such as radon gas. For a certain region, radon nuclei decay to emit alpha particles which ionize the local atmosphere and positively and negatively charged particles are injected close to the surface. This process changes the distribution of spatial charge density and disturbs the AEF value. (2) In different seasons at a certain station, the AEF fair-weather average AEF variation curves have the same shape even for different sunrise and sunset times. This finding means that the shapes of different fair-weather average AEF variation curves in different seasons in a certain area depend on local PM 2.5 concentration and the varied content and the compositions of underground radioactive matter, including the peak and valley corresponding times and values. (3) The AEF peak and valley values of the mean fair-weather AEF variation curves for the three stations correspond to significantly different times, which is partly due to the differences in sunrise and sunset times caused by different latitudes and longitudes and partly due to the pollution sources and artificial aerosols around the stations, which have an impact on the changes in PM 2.5 concentrations and thus on the daily changes in the AEF. (4) The lowest average value of the AEF is 89.2 V/m for the RDG station, followed by 116.0 V/m for the ZH station, and the maximum average value of the AEF is 139.5 V/m for the XAN station. In addition to the apparent positive correlation with PM 2.5, it can be also affected as follows: i. because of the latitudinal difference among the RDG, ZH, and XAN stations, there is a certain horizontal potential difference between the bottom of the ionosphere at different latitudes; ii. these three stations are in different countries and different regions, and their compositions and contents of radioactive substances underground differ significantly, which leads to differences in the ionization degree in the atmosphere; in turn, this affects the charge density distribution and AEF values; and iii. the current density is given by Ohm’s Law, J
z = V
i/R, where R is the whole column resistance from the surface to the ionosphere at that location and time. The conduction current (J
z) in the atmosphere is approximately constant in altitude at a particular time and location but varies with time during each day, as the overhead ionospheric potential (V
i) varies. There are also differences in the ionospheric potential over these three stations, which can affect the average values of their AEFs.
To summarize, to study the atmospheric electrostatic features in different regions worldwide, researchers collected AEF data, instrument data, meteorological data, geomagnetic data, PM 2.5 data, and sunrise and sunset times from three stations (the RDG station, the XAN station, and the ZH station) worldwide for 2021, filtered the AEF data based on the criteria of clear days and magnetic storms, and finally obtained their average daily variation in clear days in different seasonal curves. The differences in these curves were compared with meteorological characteristics, sunrise and sunset time differences, and PM 2.5 concentration, the differences in the mean fair-weather AEF curves of these three stations and the difference between their curves and Carnegie curve were summarized, and the reasons were discussed. Furthermore, for a given station, the shapes of the Carnegie curves in different seasons were almost the same, and only the peak and trough values differed. In the future, the researchers will study the relationship between the electrical properties of the atmosphere and space weather activity in more depth, including the correlation between changes in high-energy particle precipitation during magnetic storms and changes in the AEF.