Temporal Variations of GPS Irregularities at Conjugate Points under Storm Conditions ()
1. Introduction
The ionosphere is prone to significant disturbances, which are considerably worse during periods of high solar activity [1]. Large number of observations indicates that the primary cause of geomagnetic storms is the long duration of southward Interplanetary Magnetic Field (IMF). The southward IMF Bz plays an important role in determination the amount of solar wind energy to be transferred into the magnetosphere. In the Earth’s magnetosphere, the charged particles are trapped on the same field line and therefore conjugate points could be affected by the same population [2]. Any two points on Earth’s surface are geomagnetically conjugate if they are on opposite ends of the same geomagnetic field line.
Several studies were made to investigate the similarities of the ionospheric total electron content (TEC) and compare the response between northern and southern hemispheres [3-5]. Most of these studies are highly concentrated on middle and subauroral latitudes but poorly investigated at high latitude regions especially in Antarctica. This paper examines the temporal variations of GPS TEC and scintillation measurements at conjugate points under storm conditions. The measurements have been conducted during the October 2003 geomagnetic storm at approximately conjugate stations in the polar region, Scott Base station, Antarctica (SBA) (GC: 77.85˚S, 166.76˚E; CGM: –79.94˚S, 327.23˚E; LT = UT + 12) and Resolute Cornwallis Island station (RESO) at the high Arctic region (GC: 74.69˚N, 265.12˚E; CGM: 83.17˚N, 320.95˚E; LT = UT – 5). In the analysis, the diurnal variations of ionospheric TEC and scintillation and the percentage of TEC deviation (ΔTEC%) obtained from ground-based GPS receivers at the conjugate polar stations will be compared. The paper is organized as follows; Section 1 introduced the paper. Section 2 briefly discusses the magnetic storms conditions, Section 3 describes the measurement system and data processing, Section 4 presents the results and discussion followed by the conclusion in Section 5.
2. Measurement Setup
The measurement setup at SBA station consists of a Trimble TS5700 24-channel, high precision, dual-frequency GPS receiver, Trimble Zephyr Geodetic antenna and a Dell notebook computer for data logging. The Trimble Zephyr Geodetic antenna has improved accuracy, resulting from sub-millimeter phase centre repeatability, enhanced multi path resistance, and superior satellite tracking at all elevation angles and in difficult environments. The receiver was set to tracks GPS signals at 1 second sampling rate and the cutoff elevation angles was set to 13˚ to maintained the quality of the data. These data are further processed to provide 30 second sampling rate. The measurement system at RESO station consists of Ashtech Z-X113 dual frequency GPS receiver. The Ashtech Z-XII3 receiver, uses the carrier phase and averaging to smooth the pseudorange observations, but with the Ashtech translator the smoothing rate can be selected at the data translation step by the user. In the analysis, the GPS data obtained from both stations were at 30 second sampling rate and above 30˚ cutoff elevation. These data are utilized in determination the absolute vertical TEC (VTEC) and scintillation measurements. The calculation procedure for the GPS VTEC, Percentage deviation of TEC and rate of change of TEC measurements are discussed in the next section.
3. Calculation Procedure
The absolute GPS TEC can be obtained from differential time delay (P1-P2) or from differential phase advance (L1-L2) where P1, P2 are the GPS pseudorange observables (in meter), and L1 and L2 are the GPS carrier phase measurements (in cycles). However, the TEC obtained from differential time delay gives the absolute TEC level but it is highly exposed to multipath effect and contains delays inherent in satellite and receiver hardwares [6] while the TEC obtained from differential phase advance gives high precision TEC but a level of TEC is unknown because of unknown initialization constant in phase data (i.e. ambiguity term). Therefore, the level of TEC is adjusted to the TEC derived from the corresponding pseudorange difference (P2-P1) for each satellite-receiver pair [6]. To get absolute TEC values, the time delay measurements are used to determine the ambiguity term then by combination the GPS carrier phase with the code measurements for each satellite receiver pair, the absolute TEC value can be obtained with high precision [7]. The equivalent vertical TEC for each satellite receiver pair is determined by multiplying the slant TEC with zenith angle of the line of sight at the subionospheric point [8]. Beside the VTEC measurements, the percentage deviation of the absolute TEC measurements (ΔTEC%) was used to describe the perturbation components of the TEC measurements and to determine the fluctuation of the positive and negative storm phases around the zero level. This index is derived by subtracting the TEC values under quiet storm conditions from the TEC values under disturbed conditions where the quite day is the day where the 3-hour kp index is ≤2. The rate of change of GPS TEC (ROT) (in TECU/min) was also used in the analysis to describe the high frequency changes in the TEC due to irregular ionospheric phenomena such as the traveling ionospheric disturbances (TIDs) and scintillation occurrence [9]. The method of determination the rate of change of TEC measurements does not require the estimation of real ambiguity as long as no cycle slips occurs, it is directly determined from the geometry free combination of the phase measurements [9]. The total scintillation index (Σ scintillation) was used to characterize the scintillation activity during storm periods. For each satellite path over the GPS station, the index integrates the scintillation readings that exceed 0.3 TECU/min.
4. Results and Discussion
The October 2003 storm is regarded as the greatest storm during the 23rd solar cycle and it is one of the fastest traveling solar storms in the last two decades. The period from 28th October to 1st November of 2003 was characterized by extreme solar activity that resulted in a series of intense geomagnetic storms. The extreme interplanetary and geomagnetic disturbances during the October 2003 storm were related to the eruptive activity of the sun. The event was preceded by high solar X-ray energy for one week before the onset of the storm [10]. The maximum X-ray energy for this storm was recorded at 09:51 UT on 28th October with a series of X17.2/4B flares accompanied by bursts of radio emissions and ejections of solar mass were observed. The most intense storm activity was observed on 29th and 30th October 2003 with two high speed streams of solar wind originated from the coronal hole on the sun disk.
Figure 1 presents the Dst and kp magnetic indices obtained from the world data centre and GPS-TEC and ΔTEC% measurements at SBA and RESO conjugate points during the October 2003 storm. As shown in Figure 1(a), three Dst minima with maximum values of –180 nT, –363 nT and –401 nT are observed at around 12:00 UT on 29th October, 01:00 UT and 23:00 UT on 30th October 2003. The kp index reached its maximum value of 9 two times on 29th October mainly at 09:00 UT and 21:00 UT, and once on 30th October at round 21:00 UT. The daily GPS-TEC and ΔTEC% measurements at both stations are shown in Figure 1(c) and Figure 1(d). The figures show that the absolute TEC measurements at conjugate points follow the magnetic field variations where the maximum TEC activity is usually seen at maximum kp and Dst indices. Prior to the midday time