Analysis of the Relation Between Solar Activity and Parameters of the Sporadic E Layer

Abstract

Based on the ionosonde data from stations at different latitudes in high- and low-solar-activity years, the effects of solar activity on the parameters of the Es layer and the foE amplitude spectrum are analyzed. The results show that the influence of solar activity on the intensity of the Es layer at different latitude sites is not consistent, and there is no significant agreement conclusion. And the spectral analysis results show that solar activity has little influence on the amplitude spectrum of foEs. But the incidence of Es layer, the height distribution of Es layer during daytime, and the Es layer traces have a negative correlation with solar activity. The research in the paper has certain significance for the study of influencing factors in the formation of the Es layer.

1. Introduction

The Es layer is a special high-density thin layer structure of the ionosphere, usually 90–130 km in height and varying in thickness from several hundred meters to one or two kilometers. The formation mechanism and spatiotemporal morphological characteristics of the Es layer are different at different latitudes. It is generally believed that the formation of the Es layer is affected by solar activity, geomagnetic activity, electric field, background metal ion density distribution, planetary waves, gravity waves, and tidal wind, etc. [1,2,3,4,5,6,7,8]

A large number of studies have been conducted on the relationship between Es layers and solar activity. Pietrella and Bianchi [9] analyzed thirty-two years of ionosonde data located in Rome (41.8° N, 12.5° E) and found no obvious relation between Es layers and solar activity. But Pezzopane et al. [10], also based on ionosonde data located in Rome, concluded that Es layers are positively correlated with solar activity during the daytime. Based on ionosonde observations at a near-equatorial station and low-latitude station, Fontes et al. [11] suggested that the incidence of Es layers is negatively correlated with solar activity. Based on COSMIC-1 data, Niu [12] found no significant correlation between Es layers and solar activity, while Bergsson and Syndergaard [13] found an anti-correlation between them. Therefore, it can be seen that many studies have reached contradictory conclusions about the influence of solar activity on Es layers, and further studies are needed.

Based on the ionosonde data of three sites at different latitudes, the variations in Es layer intensity, altitude distribution, incidence, the number of Es layer traces, and amplitude spectrum in high- and low-solar-activity years are analyzed, and the analysis results are discussed in the paper.

2. Methods and Data

The Es layer is usually characterized by its intensity, height distribution, time variation, etc. The intensity and height of Es layers can be expressed by the critical frequency of Es layers (foEs) and the virtual height of Es layers (h’Es), respectively. The occurrence rate is usually the ratio of the number of Es layers at a particular frequency or time to the total number of ionograms observed at each station. Es layers observed by ionosondes are discontinuous because of their horizontal movement and sporadic characteristics. In ionograms, an Es layer with a lifetime of more than a few hours can be considered as a trace. Previous statistical results show that the shorter the lifetime, the more traces of Es layers [14].

Since the formation of the Es layer is modulated by atmospheric tide winds, the Es layer has periodic variation (diurnal/semidiurnal/terdiurnal/quarter-diurnal) characteristics. Therefore, to study the effects of solar activity on Es layers, in addition to analyzing the influence on foEs, h’Es, and incidence of Es layers, the effect of solar activity on the traces with a lifetime of more than six hours and foEs greater 3 MHz will be analyzed [15]. And the ionosonde data of three stations located at different latitudes during maximum- (2013–2015) and minimum- (2019–2021) solar-activity years were selected. The three stations were Urumqi (43.8° N, 87.6° E), Lanzhou (36.1° N, 103.9° E), and Kunming (25.5° N, 103.8° E) stations.

Among these three observation stations, the ionosonde collects data once every hour, meaning the data resolution is 1 h. Automatic continuous observation is carried out. The ionogram is interpreted by a professional, and another person reviews it. The hourly averages of foEs and h’Es represent the average of all sample data of foEs and h’Es at each hour throughout the 24 h of a day

Using the Lomb–Scargle spectral method, spectral analyses were conducted on the foEs of the maximum-solar-activity years (2013–2015) and the minimum-solar-activity years (2019–2021) at the Kunming, Lanzhou, and Urumqi stations. The modulation effects of atmospheric tidal winds and planetary waves on the formation of Es were clarified, and then the influence of solar activity on the amplitude spectrum of foEs was analyzed.

3. Results and Discussion

Figure 1 shows the time variation in foEs and h’Es observed at three stations during maximum- and minimum-solar-activity years. It can be seen that the hourly mean foEs of each station are the largest at noon and the smallest in the morning, and the Es layer intensity decreases with the increase in latitude. The hourly mean h’Es of each station has a peak height in the morning and evening.

The value of foEs in low-solar-activity years is greater than that in high-solar-activity years at the Kunming station, indicating that the intensity of Es layers is negatively correlated with solar activity at low–mid latitude, which is consistent with ionosonde observations at low–mid latitude and near-equatorial sites [11]. However, the results at Lanzhou station are contrary to those at Kunming station, which means that the intensity of Es layers is positively correlated with solar activity at Lanzhou station; Pezzopane et al. [10] achieved similar results at the Rome station (41.8° N, 12.5° E). Meanwhile, at Urumqi station, there is no obvious dependence between the intensity of Es layers and solar activity.

Figure 1 also shows that the mean h’Es at night time in the year of maximum solar activity is lower than that in the year of minimum solar activity at the Kunming and Lanzhou stations, which agrees with the results of Bergsson and Syndergaard [13]. However, there is no similar result at the Urumqi station. It is worth noting that the daytime h’Es (9:00 to 17:00 LT) in the year of maximum solar activity is lower than that in the year of minimum solar activity at the three stations, indicating that the height of Es layers during daytime is negatively correlated with the solar activity.

Figure 2 gives the time variation in the incidence of Es layers at three stations. At the Lanzhou and Urumqi stations, the incidence of Es layers has a minimum value in the morning and a maximum value at noon. There is also a minimum value in the morning at Kunming station, while the incidence remains at a consistently high level throughout the day. Different from the results in Figure 1, the incidence of Es layers in low-solar-activity years is higher than that in high-solar-activity years at three stations, indicating an anti-correlation between the occurrence rate of Es layers and solar activity. The conclusion is consistent with the results of Bergsson and Syndergaard [13] and Fontes et al. [11].

The number of the Es layer traces with duration greater than 6 h and foEs more than 3 MHz at three stations is shown in Figure 3. The number of traces is the largest at Kunming station and the smallest at Urumqi station, indicating that the number of traces decreases with increasing latitude, which agrees our previous study [14]. Zhang’s paper analyzed Es data during years of decreasing solar activity, and the observational results show that the number of traces during the day is much greater than that at night; it also shows seasonal variations, with the maximum in summer, followed by autumn, and the minimum in winter, and the simulation indicates that the Es layer mainly descends under the influence of wind shear [14].

In addition, the numbers of traces were 725, 539 and 503, respectively, at the Kunming, Lanzhou, and Urumqi stations in high-solar-activity years, and 776, 610, and 553, respectively, in low-solar-activity years. Therefore, the number of traces in high-solar-activity years is less than that in low-solar-activity years. It can be concluded that the number of the Es layer traces is negatively correlated with solar activity.

The parameter of foEs usually represents the intensity of the Es layer. Figure 4 shows the variations in foEs at Kunming station and Urumqi station over the period of the high-solar-activity years. It can be seen that the Es layer has a higher occurrence rate and greater intensity compared to other months in May to August, and the occurrence rate and intensity of the Es layer at the lower latitude of Kunming station are higher than those at the higher latitude of Urumqi station. Next, spectral analysis of foEs during the maximum- and minimum-solar-activity years was conducted.

The foEs were analyzed using the Lomb–Scargle spectral method with an amplitude spectrum period of 2–40 h at the Kunming, Lanzhou and Urumqi stations. The red line is a 95% confidence level, above which is thought to be related to atmospheric tides. The specific method of spectral analysis can be found in reference [16]. As shown in Figure 5, the foEs amplitude spectrum has mainly 24 h and 12 h periodic oscillations, but there are also weaker 8 h oscillations at the three stations. The diurnal tides dominate at low–mid latitude (Kunming station), and semidiurnal tides increase gradually as the latitude increases. In Figure 5, it is mainly diurnal oscillations of foEs at Kunming station, while as the latitude increases, the semidiurnal oscillations increase. It can be seen that from the lower to the higher–mid latitudes, the formation of the Es layer undergoes a transition from diurnal tides to semidiurnal tides modulation. Comparing the amplitude spectra during the high solar activity years and the low solar activity years, there is no certain regularity in the amplitude spectra, indicating that it is less affected by solar activity. The amplitude spectrum of semidiurnal oscillation is larger during maximum solar activity years than that during minimum years at three stations. Since the mid-latitude Es layers are mainly formed under the action of wind shear (diurnal/semidiurnal/terdiurnal tidal waves) [1,17], combined with the results of Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5, it can be concluded that the amplitude spectrum periodic oscillation of foEs does not have obvious solar activity dependence.

The foEs were analyzed using the Lomb–Scargle spectral method with an amplitude spectrum period of 2–20 days in maximum- and minimum-solar-activity years at the Kunming, Lanzhou and Urumqi stations, as shown in Figure 6. The red line is a 95% confidence level. The narrow peak spectrum greater than the 95% confidence line is plausibly related to the modulation of the Es layer by planetary wave windshear.

The periods were mostly located around 4 days, 8 days, and 14 days during maximum-solar-activity years and 4 days, 9 days, and 14 days during the minimum-solar-activity years, with the narrow peak spectrum exceeding the 95% confidence line at Kunming station. At Lanzhou station, the peak spectrum values exceeding the 95% confidence line were located around 3 days, 7 days, and 8 days during the high-solar-activity years, while they were located around 4 days, 5 days, 10 days, 13 days, and 14 days during the low-solar-activity years. The amplitude spectrum at the Urumqi station was the smallest during the minimum-solar-activity years among the three stations, with the periods being around 4 days, 5 days, 15 days, and 16-days; the periods were around 5 days, 8 days, 13 days, and 20 days during the maximum-solar-activity years. The main PW periods were 2, 5, 10, and 16 days. In the paper, ~4, ~8, and ~14 days are usually considered as 3–5, 7–9, and 13–15 days. The ~14 and >20 day periods are probably associated with solar rotation.

It is generally assumed that the influence of planetary waves on the formation of Es layers is achieved through their nonlinear action on tidal winds [10,18,19], which is also illustrated by the fact that the amplitude spectral period is not the exact integer days in Figure 6. The results in Figure 6 do not show that the amplitude spectrum of Es layers modulated by planetary waves varies significantly between high- and low-solar-activity years, unlike the results of Pezzopane et al. [10], which suggest that the amplitude spectrum of high-solar-activity years is greater than that of low-solar-activity years. Of course, the time period of the samples used in the paper is relatively short. To confirm this conclusion, further analysis with Es layer data from a longer solar activity cycle is necessary.

The dynamic process by which the sun heats the upper atmosphere mainly involves the absorption and reflection of solar radiation by the atmosphere, as well as the heat conduction and convection movements within the atmosphere layer. The heat conduction and convection movements within the atmosphere generate atmospheric tides. Sun et al. [20] suggested that atmospheric tides of 120 km at 10–40° in the northern and southern hemispheres have a strong negative correlation with solar activity. Based on meteor radar observations, Andrioli et al. [21] found that diurnal and semidiurnal tides are greater in low-solar-activity years than in high-solar-activity years, and the observations also show that tidal wind amplitude is related to High-Speed Stream (HSS), while HSS reaches its maximum in the lowest-solar-activity years. Thus, as solar activity increases, tidal dissipation increases, and tidal structures become more difficult to form and reach the lower thermosphere (the region of Es layers formed by wind shear), thereby reducing the probability of the Es layer formation. Therefore, the above analysis can explain why the incidence of Es layer is negatively correlated with solar activity. Although Es layers are modulated by diurnal, semidiurnal and terdiurnal tides, the spectral analysis results of Figure 5 and Figure 6 show that the amplitude spectrum of foEs has no obvious relationship with solar activity. Moreover, the results of Figure 1 show that there is no uniform conclusion on the relationship between the intensity of the Es layer and solar activity for different latitudes. It is because the formation of the Es layer is affected not only by solar activity but also by atmospheric winds (tidal wind, gravity wave and planetary wave), location, time, electric field, geomagnetic field, and distribution of background metallic ions [4,5,22,23,24].

4. Conclusions

Based on the ionosonde data of three sites at different latitudes in high- and low-solar-activity years, the influence of solar activity on the parameters of Es layers and the amplitude spectrum of foEs is analyzed, and the following conclusions are obtained:

(1)

The influence of solar activity on the intensity of Es layers at different latitude is inconsistent, and there is no obvious uniform conclusion.

(2)

The height distribution of Es layers is negatively correlated with solar activity during the daytime.

(3)

Both occurrence rate and the traces number of Es layers show an anti-correlation with the solar activity.

(4)

No obvious effect was identified between the amplitude spectrum of foEs and the solar activity. However, this conclusion still requires further verification using at least one solar activity cycle’s observation data of the Es layer.