Characterization of Ionospheric Scintillation Index over low latitude: Nepal Region

The ionospheric scintillation is a rapid phase and amplitude fluctuation of satellite signals due to the small-scale irregularity of electron density in the ionosphere. The characterization of the scintillation index in a proper way is a crucial aspect of the Global Positioning System (GPS) satellite signals for the purpose of space-based navigation, satellite communication, space weather as well as earth observation applications. In the current study, we analyzed the ionospheric scintillation index during the year of 2018 to 2019 over the Nepal region which locates itself almost being sandwiched between India and China and in the vicinity of low latitudes. The characteristic variations of scintillation occurrence are studied during the several geomagnetic storm and quiet days. The results show that the S4 indexes are varying from the 0.05 to 0.45 during the whole year. The S4 indexes behave higher variations during the whole day in the starting of the year and start to decrease at end of the day as well as at the ending months of the year 2019. The S4 values become completely less during the sunset time, while they have higher values during the sunrise. Especially, the S4 index during the storm days are larger than during the quiet days. It is worthy to note that the variations of S4 index studied in this current study do not follow the sunset property during the year of 2019. Consequently, the causes should be discovered and discussed additionally in the next research paper.


Introduction
The ionospheric irregularities in the low latitude and equatorial regions are remained a challenging task for the ionospheric investigators in the last decades. The issue of ionospheric irregularities is mainly related to the complex interaction of several physical activities which are responsible for large scale growth and decay in F region are known as plasma bubbles (Srinivasu et al., 2018). The phase and amplitude of the radio waves during the passing through the ionospheric F region fluctuate rapidly and randomly are called scintillations. These scintillations sometimes effect the and creates issue on the well performing of the navigation and communication system (Wernik, Alfonsi, & Materassi, 2007). This type of events generally occurs during the dusk or after the dusk in the equatorial regions, may occur at any daytime at the polar regions. It is noticed mainly at the equatorial areas, least at mid-latitude while lesser at high-latitude regions (Akala et al., 2012). Global Positioning System (GPS) signal travel from satellite to the receiver are used to monitor the atmospheric error such as ionosphere and troposphere as well as crustal deformation Ansari, Althuwaynee, & Corumluoglu, 2016;Ansari, 2014). Ionospheric scintillations are able to reduce the phase and pseudorange measurements accuracy and sometimes may become responsible for significant weakening in accuracy of GPS or even lead to a complete failure of the system (Moraes et al., 2014). The scintillation, in extreme cases can be cause of full disturbance for the operating receiver (Rezende et al., 2007). Hence, the scintillation mitigation is a complex process for dualfrequency GNSS receivers. It is well-known that the scintillation occurrence is related to solar activity, local time, season, geomagnetic activity, and geographical location. The accurate positioning and signal availability of space and ground based augmentation systems (SBAS and GBAS) have been affected during extreme equatorial ionospheric scintillations. Moreover, during severe scintillations, the carrier-to-noise ratio below 30 dB-Hz can result in the satellite signal outages. Several studies on the distribution and morphology of scintillation known as "scintillation climatology" is very important so as to mitigate the above-mentioned problems. Several observations related to climatology and dynamics of irregularities near the equatorial low-latitude zone have been reported earlier. Here, a fading model was also constructed to be beneficial for designing an aviation receiver. Moreover, Srinivasu et al. (2018) reported the scintillation effects on various types of GNSS signals over Indian zone using the multi-GNSS observations. They found that the impact of scintillation intensity also relies on the visible satellites of selected GNSS system in conjunction with solar and seasonal effects. The characteristics of temporal and spatial amplitude scintillations and their distributions of percentage occurrence during year 2013 are also studied over both Indian equatorial station, Bengaluru and EIA region, Lucknow (Sahithi et al., 2019). Here, the impact of geomagnetic quiet and disturbed days in the high solar activity period of the 24th solar cycle has also been investigated on GNSS systems. Although numerous literatures have been proposed to investigate the ionospheric scintillation in different parts of the globe, however, the scintillation has never been studied over Nepal region, an interesting boundary between low latitude and midlatitude. Therefore, we studied and analyzed the ionospheric scintillation index during the year of 2018 to 2019 over Nepal region in the vicinity of low latitudes in this current study. The characteristic variations of scintillation occurrence are studied during the several geomagnetic storm and quiet days. The main objective of this study is to understand the probability of scintillation occurrence over Nepal region.

Data Used
In the present study, we tried to investigate the effects of scintillation over the GPS sites from Nepal during the year of 2018 and 2019. Geographically the boundary of Nepal is presented by trapezoidal (approximately) shape with length and width of 800 and 200 km respectively (area around 147,181 km 2 ).
In terms of geographical coordinates, the country lies between 26° to 31°N latitudinal and 80° to 89°E longitudinal directions (Ansari, 2018;Ansari et al., 2019). The major part of the country is surrounded by India from three sides (except north) while northern part is bounded by Tibet, China (Figure 1). The GPS observation data in rinex formats for five GPS sites all over Nepal has been accessed from UNAVCO archives (ftp://data-out.unavco.org/pub/rinex/) and processed with the GPS-TEC analysis software (Seemala & Valladares, 2011). We chose the maximum elevation height (hmax=350 km), the elevation angle (α=20°), and the radius of the earth (RE = 6378 km) during the processing of GPS data (Sharma, Ansari & Panda, 2018).

Ionospheric Scintillation Index
Ionospheric scintillation is a stochastic (random) phenomenon concerning the phase and amplitude fluctuations of GNSS signals. In severe situations, the scintillation can lead to the loss of signal locking (or cycle slip). It is worthy to note that the effects of scintillation are not removed by dual-frequency observations. Hence, Trimble has formulated a global ionospheric scintillation sounding network to detects scintillation effects and provides up-to-date warning information on scintillation effects in different parts of the globe. The scintillation generally occurs in equatorial regions after sunset for several hours, whereas it can exist at any time over polar regions. A map showing the current ionospheric scintillation activity can be found on the http://www.trimbleionoinfo.com/Images.svc/SCINTI. The intensity of amplitude scintillation is commonly indicated by amplitude scintillation (S4 index), which is the normalized standard deviation of the signal power/intensity, given by (Dierendonck & Arbesser-Rastburg 2004). The scintillations index climatological amplitude for the geomagnetic quiet and disturbed days during the investigation period (2018 -2019) is conversed according to the geomagnetic Dst index. The scintillation amplitude is measured by computing the S4 index. The occurrence percentage of S4 index is calculated by the following expression.

= X (S4>Threshold) Xtotal
(1) where X stand for the S4 index amplitude sample space and Xtotal means the total number of S4 values. Generally, the S4 amplitude is characterized as a strong scintillation when S4 > 0.8, moderate scintillation when 0.5 < S4 < 0.8 and weak scintillation when 0.2 < S4 < 0.5 (Abadi, Saito, & Srigutomo, 2014). The threshold value can be chosen any number, for example, the threshold value 0.2 is chosen by Sahithi et al. (2019) in their investigation. In the current study we took the threshold value of 0.05.

Result and Discussion
The daily variation of S4 values during the year of 2019 is shown in Figure 2. The horizontal axis in given figure shows the daily (or monthly) variation of S4 and the vertical axis presents the local time. The color bar in the figure is the indication of S4 variation. It is clear from the figure that the S4 values during the whole year are varying from the 0.05 to 0.45. The S4 indexes have higher variations during the whole day in the starting of the year. These values start to decrease at end of the day and at the ending months of the year 2019. The S4 values become completely less during the sunset time, while they have higher values during the sunrise. The observed scintillation peak during the noon time possibly related to the precipitation into the daytime (Tiwari et al., 2012). As we already discussed in the introduction section that the ionospheric scintillation is a phenomenon that occurs after sunset, especially in the low-latitude regions, affecting radio signals that propagate through the ionosphere. Here, Nepal is in the upper low latitude region, but the variations of S4 index do not follow the sunset property during the year of 2019. These are the complicated issues which we need to find the causes. Here, we presented the initial results, and then we will try to discuss more analysis in the next research paper.  Figure 4. It is clear from Dst index that lies between around -180 to 20 presenting storm days effect. The value of S4 during the storm days is noted around 0.05 to 0.40 which means that the S4 index during the storm days are larger compared to the quiet days. The same results can be found and discussed additionally in Tiwari et al. (2012). The occurrence of scintillations is increasing as Dst increased indicates the dependency on magnetic activity. This happened due the large-scale ionospheric structure of enhanced electron density during the storm days compared to quiet days. These variations may be associated with the occurrence of the polar patches which increased about 2 TEC units in the duration of about 30 minutes (Basu et al., 1998). The high electron density polar patches generate scintillation in the signal and large enhancement in the TEC values (Basu et al., 1998;Tiwari et al., 2012).

Conclusion
We used the GPS data located over Nepal for the year of 2018 and 2019 and investigated the S4 index variation during the diurnal, quit and storm days. The S4 values become completely less during the sunset time, while they have higher values during the sunrise in the year of 2019. The S4 values are higher during the storm days, while they are low values during the quiet days in the year of 2019. These are the just initial results; we will thoroughly discuss about the results in our next research paper. Finally, we hope that our proposed approach in this current study will help to understand the probability of scintillation occurrence over Nepal region.