Variability of Ionospheric Total Electron Content over some American, Asian and African Equatorial Stations

doi:10.21203/rs.3.rs-3644648/v1

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Variability of Ionospheric Total Electron Content over some American, Asian and African Equatorial Stations

https://doi.org/10.21203/rs.3.rs-3644648/v1

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The Earth’s ionosphere is a highly dynamic region, which interferes with radio wave propagation in the shortwave and L-band frequency. In this study, total electron content (TEC) data derived from the International Global Navigation Satellite System (GNSS) Service (IGS) over four stations were used to investigate the diurnal, seasonal and solar activity effects on ionospheric TEC variability. The stations with their respective geomagnetic latitudes are: DGAR (-16.89^o), MBAR (-10.22^o), BOGT (-3.80^o) and AHUP (12.23^o). Analysis of the diurnal variation showed that, across all the stations for all seasons, the nighttime values (ranging between 13 and 143%) are higher than the daytime values (ranging between 7 and 60%). The nighttime values showed two peaks in the range 35–143% during the post-midnight hours and 36–74% during the post-sunset hours. The diurnal-seasonal variation does not exhibit a regular pattern in any station. At three of the four stations considered, highest TEC variability was observed during low solar activity and lowest during high solar activity, particularly during the nighttime. On the average, considering all the seasons together, minimum TEC variability (7%) was observed at MBAR during March Equinox of the year of moderate solar activity and maximum (143%) was observed at AHUP during December Solstice of the year of low solar activity.

ionosphere

Total Electron Content

Variability

solar cycle

The presence and dynamic nature of the free charged particles in the upper atmosphere (that is, the ionosphere) can severely perturb and degrade ground-to-satellite and satellite-to-ground communications, and adversely affect satellite tracking and navigational control applications (Doherty, 2010;

Bolaji et al, 2023). This is with resultant implication for the society and economy (NRC, 2008), hence the need for improved understanding of this highly variable medium through concerted investigative efforts. The magnitude of the ionospheric impact on communication and navigation systems are directly related to the total electron content (TEC), defined as a measure of the quantity of electrons in a unit area along the signal transmission path from the GPS satellite to a receiver on the earth’s surface (Bagiya, 2009; Bhuyan, 2007). Among the most important parameters of the ionosphere that majorly influence GPS signal and radio-wave propagations is TEC (Bolaji et al, 2023; Moses et al, 2020) due to its direct relation to the ionospheric electron density, which is the principal cause of degradation of trans-ionospheric radio-wave communications signals (Olwendo et al, 2013; Bolaji et al, 2019; Aghogho, 2020). Ionospheric TEC values vary significantly with respect to the solar cycle, season, local time, altitude, latitude, longitude, and geomagnetic activity (Hamzah and Homam, 2015; Yan et al, 2022), because the main source of ionization (solar UV and x-ray intensity) depends on the position of the sun in the sky at a particular location on earth and on the sun’s absolute output. Because of the increased reliance of the modern society on satellite technology and the significant impact of the ionosphere on it, adequate knowledge of the level of deviation of the various parameters of the ionosphere (at different times and different vicinities) from their climatological means, remains paramount to the satellite designers and operators.

Many prior researchers have used various approaches such as the upper/lower Quartiles, the Median Quartile, deciles, standard deviation etc. (Kouris and Fotiadis, 2002; Fotiadis and Baziakos, 2004; Ezquer et al, 2004; Maja and Ljiljana 2008) in the study of ionospheric variability. Standard deviation approach has been reported to be a good measure for describing variability because with it, a Gaussian distribution might be used to match the observational data, making it appropriate for a probabilistic interpretation of geomagnetic variability (Bhuyan and Borah, 2007; Aravindan and Iyer, 2020; Bilitza et al., 2004; Araujo-Pradere et al, 2007; Adewale et al, 2012). According to Adeniyi et al., (2007), studies on variability include those that analyze specific parameters on a large geographical area and those that are limited to a few or a particular location.

Many scientists have studied the relative variability of the ionosphere at various latitudes and during various solar cycles using various ionospheric parameters. For instance, Zhang and Holt (2008) characterized and quantified the ionospheric variability in electron density and plasma temperatures at two mid-latitude sites. The study revealed that the electron density variability is low in the bottom-side in summer and high in the top-side in winter and spring and that the plasma temperature variability increases with height, reaching a minimum in summer. Zhang (2004) used data for four ionospheric parameters: the F2-layer critical frequency FoF2, the F2-layer peak height hmF2, the thickness parameter B0 and the shape parameter B1, to investigate the variability of the ionosphere over a low latitude station, Hainan in China. Among other results, they observed that: variability for foF2 parameter is much lower during daytime hours than during the nighttime hours; variability for hmF2 parameter has a minimum value at hours after sunrise and sunset but a maximum value around 4:00 LT; For B0 parameter, variability has a minimum value at hours after sunrise during the winter and equinox seasons, whereas, during the summer season, maximum variability values were recorded between 2 and 3:00 LT and post noon hours and for B1 parameter. Mosert and Radicella (1995) studied the variability of the ionospheric electron density at fixed heights with South Africa data. In the F2 zone, the study revealed increasing variability with height both at night and during the day, with larger variability at night than during the day at fixed heights.

Jayachandran et al., (1995) studied the variabilities of ionospheric electron content (IEC) and peak electron density (NP) during solar minimum and solar maximum periods for a low latitude station. Their findings showed that the mean daytime variations of IEC and NP do not show a seasonal dependence during the solar maximum while their mean nighttime variations exhibit seasonal dependence with values highest in winter during the solar maximum. However, during the solar minimum, while the mean daytime IEC variations are lowest in summer and highest in winter, the mean nighttime variabilities of both parameters are highest in equinox. They also observed higher variability of the daytime ionospheric F2-region during solar minimum than during solar maximum.

Other studies on ionospheric variability include: (Aravindan and Iyer, 1990; Bilitza et al., 2004; Rishbeth and Mendillo, 2001; Akinyemi, et al., 2020) to mention a few. Common conclusions from these studies include: higher variability during low solar activity than during high solar activity; higher variability at high/sub-auroral and low latitudes than at mid-latitudes; variability increasing with decreasing solar activity; variability higher during the night than during the day with two characteristic peaks regardless of the seasons.

Data from four equatorial stations located in America, Asia, and Africa are used in the current study to examine the diurnal, seasonal, and solar activity variations of TEC. We anticipate that the results of the present study could be beneficial in enhancing the current predicting models.

Total electron content (TEC) data derived from the IGS (GNSS) service over four stations within Equatorial Ionization Anomaly of the American, Asian and African longitudes were used in this study. The version 2.9.5 of the GPS-TEC retrieval software developed by Dr. Seemela, described by Akinyemi et al., (2021), was used to convert the slant TEC (sTEC) measurements made at one-minute intervals to vertical TEC measurements (vTEC).

Local Time (LT) has been chosen for this study instead of the time convention for vTEC, which is in Universal Time (UT). For the purpose of seasonal variability study, the months of year were grouped into four as follows: December solstice (November, December, January and February), March equinox (March and April), June solstice (May, June, July and August) and September equinox (September and October). In order to study the effect of solar activity in variability, data for one year period each were used for the various solar activity levels. In other words, data for year 2014, 2016 and 2019 were used for high solar activity (HSA), moderate solar activity (MSA) and low solar activity (LSA) respectively.

Table 1

Location and coordinates of the stations used for the study.
S/N	Location	Country	Station code	Geogr. Lat. (^ON)	Geogr. Long. (^OE)	Geomag. Lat. (^ON)	Geomag. Long. (^OE)	Local Time (LT)
1	North America	USA	AHUP	19.38	-155.27	20.17	-87.31	LT = UT-10h
2	South America	Colombia	BOGT	4.64	-74.08	16.93	1.93	LT = UT-5h
3	Asia	Diego Garcia Island	DGAR	-7.27	72.37	-16.89	142.84	LT = UT + 6h
4	Africa	Uganda	MBAR	-0.6	30.74	-10.22	102.36	LT = UT + 3h

The relative variability index,$RV$ used in this study is statistically represented as:

$$RV\left(\%\right) = \left(\frac{\sigma }{\mu }\right)\times 100 \left(1\right)$$

where $\sigma$ is the TEC values’ standard deviation and $\mu$ is the mean. Variability index describes the degree of spread or how each data point deviates from the calculated average for the total data set. This variability index, recommended by Aravindan and Iyer (1990) and similar variability indices had been used in many past researches in investigating ionospheric variability (e.g, Bilitza, et al., 2004; Akala et al., 2010a; Akala et al., 2011; Oladipo et al., 2008; Akinyemi et al., 2021). For this analysis, descriptive data analysis and a measure of dispersion were utilized, therefore the monthly hourly means and corresponding standard deviations were obtained. RV, also known as the coefficient of variation, is a measurement of the dispersion of data points from the population mean, according to Bilitza et al., (2004) and Araujo-Pradere et al., (35). The Gaussian distribution provides the likelihood that a data point in a population set with the identical data would disperse within the limits of µ ± σ.

3.1 Diurnal Observations

During the various solar epochs, it is typically observed that the average diurnal characteristics of all the plots are similar.

For all the stations, variability is smaller from 07:00 to 19:00 LT (daytime) than from 2000 to 6:00 LT (nighttime).

In general, nighttime values are relatively high, ranging from around 14–76%, 13–80%, and 16–143% respectively for the HSA, MSA, and LSA years, whereas daytime values are very close across all stations, ranging from about 9–44%, 7–60%, and 11–60% respectively for the HSA, MSA, and LSA years. During all the seasons of the three different solar epoch periods, the nighttime values across all the stations are characterized by two peaks. The first peak (35–143%), which is higher in most cases is called ‘post-midnight peak’ while the second (36–74%) is called ‘post-sunset peak’. From about midnight until the first peak between 0300 and 0600 LT, diurnal variation rises rapidly before decreasing rapidly till daybreak. Following the abrupt drop, there is a rather flat region (daytime hours) between around 0700 and 1900 LT, which is followed by a gradual increase to the second peak between approximately 2100 and 2200 LT.

Over AHUP, BOGT, DGAR, and MBAR, respectively, the maximum nighttime values during the HSA activity year are roughly 55, 70, 76, and 54%; the highest (76%) value was recorded over DGAR at around 0600 LT during the March Equinox. Over AHUP, BOGT, DGAR, and MBAR, respectively, the maximum nighttime values during MSA are roughly 63, 80, 75, and 53%; the highest (80%) was recorded over BOGT at around 0400 LT during December Solstice.

The maximum nighttime readings during LSA are roughly 143, 71, 93, and 74% over AHUP, BOGT, DGAR, and MBAR, respectively. The highest value (143%) being recorded over AHUP at around 0500 LT during December Solstice.

The maximum daytime values across all the stations during HSA activity year are approximately 44, 40, 41, and 37% respectively over AHUP, BOGT, DGAR and MBAR respectively, the highest (44%) being observed over AHUP at about 1900 LT during December Solstice. Over AHUP, BOGT, DGAR, and MBAR, the maximum daylight values during MSA are roughly 54, 60, 42, and 32%, with the highest value (60%) being recorded over BOGT at around 1900 LT during December Solstice. All the stations' maximum daytime values during LSA are roughly 49, 60, 35, and 28% over AHUP, BOGT, DGAR, and MBAR, with BOGT recording the highest value (60%) at around 1900 LT during December Solstice.

3.2 Seasonal Observations

In general, there is an obvious inconsistency in the pattern of the diurnal-seasonal variation of TEC variability across all the stations during the respective solar epochs. In other words, no particular season maintains highest variability values when compared to other seasons consistently throughout. Considering all the seasons together during the HSA, the nighttime RV values ranging between ~ 27 and ~ 55%, ~ 25 and ~ 70%, ~ 18 and ~ 76% and ~ 14 and ~ 54% are observed respectively at AHUP, BOGT, DGAR and MBAR. The daytime values range between ~ 13 to ~ 44%, ~ 14 to ~ 40%, ~ 11 to ~ 41% and ~ 9 to ~ 37% are observed respectively at AHUP, BOGT, DGAR and MBAR. The minimum nighttime value ~ 14% at 2000 LT is observed at MBAR station during March Equinox, while the maximum value ~ 76% at 0600 LT, is observed at DGAR station during March Equinox. The minimum daytime value ~ 9% at 1300 LT is observed at MBAR station during September Equinox, while the maximum value ~ 44% at 1900 LT, is observed at AHUP station during December Solstice. During the MSA, the nighttime RV values ranging from 29 to ~ 63%, ~ 29 to ~ 80%, ~ 23 to ~ 75% and ~ 13 to ~ 53% are observed respectively at AHUP, BOGT, DGAR and MBAR, while the daytime values ranging from 19 to ~ 54%, ~ 16 to ~ 60%, ~ 15 to ~ 48%, and ~ 7 to ~ 32% respectively are observed at AHUP, BOGT, DGAR and MBAR. The minimum nighttime value ~ 13 at 2000 LT is observed at MBAR station during March Equinox, while maximum value ~ 80% at 0300 LT, is observed at BOGT station during December Solstice. The minimum daytime value ~ 7% at 1300 LT is observed at MBAR station during March equinox, while the maximum value ~ 60% at 1900 LT, is observed at BOGT station during December solstice.

During the LSA, the nighttime RV values ranging from (~ 27% to ~ 143%), (~ 29% to ~ 71%), (~ 19% to ~ 93%) and (~ 19% to ~ 74%) are observed respectively at AHUP, BOGT, DGAR and MBAR, while the daytime values ranging from (15% to ~ 49%), (~ 17% to ~ 60%), (~ 17% ~35%) and (~ 11% ~28%) respectively are observed at AHUP, BOGT, DGAR and MBAR. The minimum nighttime value (~ 19% at 20:00 LT) is observed at DGAR and MBAR stations during December and July solstices respectively, while the maximum value (~ 143% at 05:00 LT), is observed at AHUP station during December solstice. The minimum daytime value (~ 11% at 13:00 LT) is observed at MBAR station during March equinox, while the maximum value (~ 60% at 19:00 LT), is observed at BOGT station during December solstice.

During the HSA year, at AHUP station, a December solstice post-midnight RV peak (~ 55% at 3:00 LT) and a September equinox post-sunset peak (~ 47% at 21:00 LT) is observed. For the other seasons, the post-midnight RV peak values range between 36% and 48% while the post-sunset RV peak values range between 33% and 44%. At BOGT station, a December solstice post-midnight RV peak (~ 70% at 3:00 LT) and a December solstice post-sunset peak (~ 40% at 19:00 LT) are observed. For the other seasons, the post-midnight RV peak values range between 41% and 54% while the post-sunset RV peak values range between 26% and 34%.

At DGAR station, a March equinox post-midnight RV peak (~ 76% at 06:00 LT) and a June solstice post-sunset peak (~ 49% at 22:00 LT) are observed. For the other seasons the post-midnight RV peak values range between 40% and 67% while the post-sunset RV peak values range between 29% and 40%. At MBAR station, a December solstice post-midnight RV peak (~ 54% at 06:00 LT) and a June solstice post-sunset peak (~ 36% at 23:00 LT) are observed. For the other seasons the post-midnight RV peak values range between 43% and 45% while the post-sunset RV peak values range between 23% and 30%.

During the MSA year, at AHUP station, a June solstice post-midnight RV peak (~ 63% at 5:00 LT) and a September equinox and December solstice post-sunset peak (~ 54%) are observed. For the other seasons, the post-midnight RV peak values range between 40% and 50% while the post-sunset RV peak values range between 35% and 50%. At BOGT station, a December solstice post-midnight RV peak (~ 79% at 4:00 LT) and a December solstice post-sunset peak (~ 74% at 22:00 LT) are observed. For the other seasons, the post-midnight RV peak values range between 55% and 60% while the post-sunset RV peak values range between 36% and 46%.

At DGAR station, a June solstice post-midnight RV peak (~ 75% at 05:00 LT) and a March equinox post-sunset peak (~ 70% at 23:00 LT) are observed. For the other seasons, the post-midnight RV peak values range between 52% and 65% while the post-sunset RV peak values range between 30% and 46%. At MBAR station, a June solstice post-midnight RV peak (~ 53% at 03:00 LT) and a June solstice post-sunset peak (~ 42% at 22:00 LT) are observed. For the other seasons, the post-midnight RV peak values range between 35% and 50% while the post-sunset RV peak values range between 25% and 30%. During the LSA year, at AHUP station, a December solstice post-midnight RV peak (~ 143% at 05:00 LT) and a December solstice post-sunset peak (~ 76% at 22:00 LT) are observed. For the other seasons, the post-midnight peak RV range between 48% and 119% while the post-sunset peak range between 32% and 72%. At BOGT station, a September equinox the post-midnight RV peak (~ 71% at 5:00 LT) and a December solstice post-sunset peak (~ 65% at 20:00 LT) are observed. For the other seasons, the post-midnight peak RV range between 46% and 52% while the post-sunset peak range between 51% and 63%.

At DGAR station, a September equinox post-midnight RV peak (~ 93% at 02:00) and a September equinox post sunset peak (45% at 22:00 LT) are observed. For the other seasons the post-midnight peak RV range between 34% and 59%, while between 02:00 and 23 LT, RV remain relatively low without any evident peak during the remaining seasons. At MBAR station, a June solstice post-midnight RV peak (~ 74% at 03:00 LT) and a June solstice and September equinox post-sunset peak (~ 49% at 23:00 LT) are observed. For the other seasons, the post-midnight peak RV range between 45% and 60% while the post-sunset peak range between 28% and 37%.

The daytime and nighttime peak RV values for the seasons at all the stations during the three different solar activity years are summarized in Fig. 2.

From the Figure, it was observed on the average, that, during the HSA, the highest variability occurred during the solstices at AHUP, BOGT and MBAR, but during the March equinox at DGAR station. On the other hand, the least variability occurred during the equinoxes, in all the stations. During the MSA, the highest variability occurred during the solstices at BOGT and MBAR, but during the equinoxes at AHUP and DGAR stations. On the other hand, the least variability occurred during the equinoxes at DGAR and MBAR and during the solstices at AHUP and BOGT stations. During the LSA, the highest variability occurred during the solstices at AHUP and MBAR, but during the equinoxes at BOGT and DGAR stations. On the other hand, the least variability occurred during the equinoxes at DGAR and MBAR and during the solstices at AHUP and BOGT stations. It was observed further from the Figure that during the HSA, the December solstice recorded its highest RV at BOGT, while June solstice, March equinox and September equinox had their highest RV values at DGAR station. During the MSA, December solstice and September equinox peaked at BOGT, while both June solstice and March equinox peaked at DGAR. During the LSA, December and June solstices had their peak values at AHUP, while both and March and September equinoxes peaked respectively at MBAR and DGAR. On the average, for all the seasons, during the HSA, DGAR has the highest TEC variability, followed by BOGT, while MBAR has the least TEC variability. During the MSA, BOGT has the highest variability, followed by DGAR, while MBAR has the least variability. During the LSA, AHUP exhibits the largest TEC variability, followed by DGAR, while BOGT records the smallest TEC variability.

Figure 3 depicts the relative diurnal variation of TEC during the three (3) solar epochs, namely the HSA, MSA, and LSA.

At AHUP, during the daytime, the values of the RV of TEC during the HSA, MSA and LSA are close, particularly at sunrise (0500–0700LT). While the closeness in RV values during LSA and MSA persist till around 0700 LT, starting from around 0800 LT, a significant reduction is noticeable during the HSA. At BOGT and MBAR, the plots exhibit consistent trend during the day, with no significant difference in RV during HSA, MSA and LSA years. However, at DGAR, a marked difference between TEC variability values during the various solar activity levels is observed during the day (particularly MSA and LSA). During the nighttime, TEC variability increases with decreasing solar activity at all stations except at DGAR, where RV has higher value during the MSA than during the LSA. In this work, observations, which are in agreement with those that have been previously published have been made ([26, 29, 31, 32, 36 etc.]). The observed higher nighttime variability could be attributed to the lack of intense solar activity during the nighttime (Rishbeth and Mendillo 2001; Araujo-Pradere et al., 2005). The daytime ionosphere is stabilized by the effect of photoionization, resulting in lower recombination and greater production rate (Woodman et al., 1976), however, the nighttime ionosphere is being sustained by transport and recombination processes. Since there is less severe solar activity influence at night, higher TEC variability at night could be attributed to the lower electron density.

Aravindan and Iyer, (1990) had earlier reported that the major factors affecting TEC variability at equatorial stations is solar ionizing flux. According to Akala et al., (2011), gravity waves could also be contributing factors to the higher nighttime TEC variability. The seasonal variations do not exhibit any regular pattern. The post nighttime and post sunset peaks observed across all the seasons during the various solar activity years in this study are attributed to steep electron density gradients which are caused by the on-set and turn-off of solar ionization (Bilitza et al., 2004; Akala et al., 2011; Chou and Lee, 2008) and superimposition ionospheric F-region irregularities (Spread-F) on the background electron density (Woodman et al., 1976). Akinyemi et al., (2020) reported post-midnight and post-sunset peaks in March equinox and only the post-sunset peak during the September equinox, while Akala et al., (2010a) reported the post-sunset and post-midnight peaks in June solstice and September equinox and only the post-midnight peak in March equinox and December solstice. Adebesin et al., (2014) observed the post-sunset peak only, across all seasons. The pattern of the diurnal variations observed in this study could be attributed to ambipolar diffusion, neutral wind systems and electric fields (Rishbeth et al., 1963; Titheridge, 1995; Rishbeth, 1998). Considering the solar activity dependence, at three out of the four stations, the variability of TEC increases as solar activity decreases. This may be due to higher TEC values as a result of higher solar flux during HSA, resulting in smaller deviations of individual values from mean TEC values, which is not the case during the LSA year, when larger deviations from the mean TEC value as a result of lesser solar flux are commonly observed. However, at DGAR station, nighttime variability has been observed to be greater during MSA than both HSA and LSA.

The variability of TEC at four different stations during three distinct solar epochs was investigated using data from the years 2014 (HSA), 2016 (MSA), and 2019 (LSA). The results such as the inconsistencies in the daytime seasonal pattern irrespective of the solar epoch, as well as the higher nighttime variability compared with daytime, agreed largely with documented observations in literature. Dependence of the degree of nighttime variability on solar activity is also observed. Variability decreases with increasing solar activity except at DGAR, where the value is highest during the MSA year and the lowest in the HSA year. The observation at DGAR reflects the dynamics of the solar activity dependence of ionospheric variability, also reported by Ikubanni et al., (2019) for an equatorial station. Although, Ikubanni et al., (2019) reported highest nighttime variability for a MSA year in the ascending phase whereas it was noted in a MSA year in the descending phase. This observation suggests the need for further extensive investigation of this dynamics.

Author Contribution

''G.A. conceived the idea. G. A. wrote the main manuscript text. ''S.O. prepared the figures. ''M.E. analyzed the data. ''A.B. provided supervision and guidance. All authors reviewed the manuscript.

Acknowledgements

GNSS data used in this work was obtained from the intermission IGS (GNSS) services.

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Variability of Ionospheric Total Electron Content over some American, Asian and African Equatorial Stations

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Abstract

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1.0 Introduction

2.0 Data and Methodology

3.0 Results

3.1 Diurnal Observations

3.2 Seasonal Observations

4.0 Conclusion

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Author Contribution

Acknowledgements

References

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