Mapping of S4 Over the Arabian Peninsula During Solar Minimum

In this letter, we study the temporal and spatial variability of ionospheric irregularities by generating high-resolution maps of the observed amplitude scintillation index (S4) using data from a multiconstellation and multifrequency global navigation satellite system (GNSS) receiver. The study is located in the Arabian Peninsula region, which falls under the northern crest of the equatorial ionization anomaly (EIA). Even though the study was conducted during a solar minimum period, considerable pre-sunset scintillation occurrences have been observed between 15 and 17 local time (LT), particularly during the winter solstices. While most scintillation occurrences have been observed at low elevation (20°–25°), a considerable number of scintillation causing ionospheric irregularities have been observed toward the north, east, and southeast of the receiver location, for elevation ranging from 40° to 60°. Out of all the GNSS constellations with medium-earth-orbit (MEO) satellites, GPS was the most impacted by amplitude scintillation, while BeiDou and Galileo satellites were the least affected. It is anticipated that the patches of ionospheric irregularities reported in this work will be further enhanced, as solar activity increases in the coming years. Therefore, this work can serve as a reference for future studies during periods of increased geomagnetic activity.


I. INTRODUCTION
. The occurrence of ionospheric 30 scintillation depends on various spatial and temporal fac-31 tors, including time of day, season, and geographical loca-32 tion [2]. Ionospheric irregularities may be detected either by 33 continuously monitoring GNSS signals using ground-based 34 receivers or by radars, such as the ionosonde or multistatic 35 high-frequency radars [3]- [5]. 36 Jiang et al. [4] used GNSS and ionosonde data to detect 37 large-scale ionospheric irregularities. They generated maps 38 that detected ionospheric irregularities originating from the 39 equator at low-and mid-latitude stations. Wu et al. [5] 40 detected daytime ionospheric irregularities in the E and lower 41 F regions around midday using a multistatic high-frequency 42 radar. This is significant as ionospheric irregularities primarily 43 occur during the post-sunset to midnight period. 44 Manuscript received 4  Harsha et al. [6] generated 5-min amplitude scintillation 45 index (S4) maps for India during a severe geomagnetic storm. 46 The maps were generated using the Kriging method and had 47 a resolution of 2 • by 2 • . These maps successfully captured 48 the features of the equatorial ionization anomaly (EIA) over 49 the Indian region, which is a neighboring region to the one 50 considered in this letter. Following a similar methodology, 51 Geng et al. [7] generated 30-min S4 maps over southern 52 China for a high solar activity period. The Kriging method 53 had been used in both cases to interpolate for missing values. 54 However, in this letter, we rely purely on data retrieved 55 from observations in our work, and no attempt was made to 56 interpolate for missing data. This should allow for a more 57 accurate representation of the local ionosphere, particularly as 58 we generate higher resolution maps as compared with previous 59 work [6], [7]. Additionally, both [6] and [7] generated maps 60 for high solar activity periods using only GPS data.

61
In this letter, we study the spatial and temporal vari-62 ations of amplitude scintillation during a solar minimum 63 period. The data were obtained from a multiconstellation and 64 multifrequency GNSS receiver with scintillation monitoring 65 capabilities (Septentrio PolaRx5S). The study region is the 66 Arabian Peninsula (25.2827 • N, 55.4621 • E), which falls under 67 the northern crest of the EIA. The presence of pre-sunset scin-68 tillation for the Arabian Peninsula region has been previously 69 highlighted in [8] and [9]. This is a feature that has previ-70 ously been only observed in low-latitude/equatorial regions 71 [10], [11]. In this letter, we expand on the analysis performed 72 in previous work by generating maps with a resolution of 0.2 • 73 latitude and 0.5 • longitude to study the spatial distribution 74 of the observed S4. Thus, our main objective is to identify 75 the regions around the Arabian Peninsula that are consistently 76 influenced by the presence of amplitude scintillation on GNSS 77 signals. We also compare the S4 observed on different orbits 78 of the BeiDou GNSS constellation and observations from the 79 GPS, GLONASS, and Galileo constellations. As this study 80 was conducted during a solar minimum, this work can serve 81 as a reference for future studies during periods of increased 82 geomagnetic activity.

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The outline of this work is as follows. First, in Section II, the 84 methodology and techniques used in this letter are presented. 85 Then, the results and discussion are given in Section III. 86 Finally, the conclusions and future work are in Section IV.

88
Amplitude scintillation is typically observed through the 89 S4 index. The total S4 index (S4 total ) can be defined as the 90 1558-0571 © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See https://www.ieee.org/publications/rights/index.html for more information.  Galileo, and BeiDou constellations, while only two signals 108 are observed from the GLONASS constellation (see Table I).  The study period includes the summer, winter, and equinox 121 seasons of 2019 and 2020, as well as the winter solstice of 122 2018 and the vernal equinox and summer solstice of 2021. 123 Each season is taken as 90 days centered around their respec-124 tive start dates, and the data for the equinoxes of 2019 and 125 2020 have been combined.

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To map the observed S4 value to a particular geographical 127 location, we use the thin shell ionospheric model (TSM) 128 simplification [14]. We consider a perfectly spherical Earth 129 with a radius equal to 6371 km and the TSM to be 350 km 130 above the surface of the Earth [14].

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Once the S4 values have been mapped to a geographical 132 location, we divide the regional map ranging from 22 • to 133 28 • latitude with a resolution of 0.2 • and from 45 • to 65 • 134 longitude with a resolution of 0.5 • . Thus, each pixel would be 135 of size 0.2 • latitude and 0.5 • longitude. Then, the S4 values 136 of 5 min over 90 days centered around the start date of each 137 season are projected onto the map, and the mean S4 value for 138 each pixel is taken by considering all observations that fall 139 within the pixel during the specified time. Data from these 140 5-min maps will be used in the following figures. Considering 141 that the period analyzed is that of a solar minimum, three 142 different categories for amplitude scintillation have been used, 143 weak scintillation corresponding to 0.1 ≤ S4 < 0.2, moderate 144 scintillation corresponding to 0.2 ≤ S4 < 0.3, and high 145 scintillation corresponding to S4 ≥ 0.3. The black, blue, and red lines correspond to signals 1, 2, and 3, 151 respectively (see Table I). The most noticeable feature that can be seen in Fig. 1 is in [8] and [9], and during the last solar cycle minimum 158 at low-latitude/equatorial regions [10], [11]. It is important 159 to note that, unlike previous works, no enhancement of S4 160 was observed during the post-sunset to midnight (17-24 LT) 161 period [7], [11]. This feature may be attributed to the winter During this period, many pixels within the map can be seen 184 to exceed an S4 value of 0.1. This is very significant for a 185 solar minimum period. We note that these maps are a result 186 of averaged S4 over 5 min for 90 days. This means that the 187 S4 activity seen here was persistent for the entire season, i.e., 188 over a total period of 90 days.

189
To better understand the data from the receiver's viewpoint 190 and provide a different perspective on the spatial distribution 191 of S4 values, we present the polar plots in Fig. 3. Similar to 192 Fig. 2, Fig. 3 presents signal 1 S4 data for the winter solstices 193 of the study period from 15 to 17 LT for 90 days. However, 194 the data are not represented in terms of the ionospheric pierce 195 point location; instead, the azimuth and elevation of the line-196 of-sight between the receiver and satellites were considered. 197 Each polar map has been divided into cells of size 30 • azimuth 198 and 5 • elevation. The total number of occurrences of S4 in 199 each cell has been counted and divided by the total number 200 of occurrences for the entire slice of 30 • azimuth to produce 201 the percentage occurrence. This should help us independently 202 analyze each 30 • azimuth segment and understand the rela-203 tionship between among, elevation, and S4 activity.

204
The majority of S4 occurrences can be seen at 20 • -25 • 205 elevation during all three years. A patch of considerable 206 occurrences of weak, moderate, and high scintillation can be 207 seen toward the north (330 • -30 • azimuth), ranging between 208 35 • and 60 • elevation for all years but more prominent in terms 209 of coverage during 2019. This can be attributed to the data gap 210 seen toward the north in Fig. 2. An additional feature that can 211 be seen is the existence of a small patch ranging between 212 45 • and 60 • elevation, toward the south to southeast direction 213 (120 • -180 • azimuth), with scintillation occurrence ranging 214 as the Galileo MEO satellites, displayed similar percentage 240 occurrences, unlike the BeiDou IGSO satellites, which had 241 higher percentage occurrences due to their relatively low ele-242 vation. On the other hand, the GPS satellites had the maximum 243 percentage occurrence for all scintillation categories, followed 244 by GLONASS. Due to the availability of IGSO and GEO 245 BeiDou observations (see the polar plot in Fig. 4), more 246 scintillation occurrences were observed from the regions in 247 the south to southeast of the receiver, as seen in Fig. 3. From 248 the polar plot in Fig. 4, we can deduce that the patches of 249 ionospheric irregularities seen toward the north in Fig. 3     2) The pre-sunset enhancement of S4 during the winter   The patches of ionospheric irregularities reported in this work 294 are anticipated to be further enhanced, as the solar activity 295 increases in the coming years. The interesting observations 296 presented by the BeiDou GEO and IGSO satellites will be 297 further investigated in the future as a potential remote sensing 298 tool to enhance our understanding of ionospheric irregularities. 299 ACKNOWLEDGMENT 300 The authors would like to thank the two anonymous review-301 ers whose comments helped improve this manuscript. They 302 would also like to thank Radhia Fernini, Hebah Tayeh, and 303 Aseel Aljubeh for copy editing the manuscript.