Available online at www.sciencedirect.com Advances in Space Research 51 (2013) 366–376 www.elsevier.com/locate/asr Seasonal variability of GPS-VTEC and model during low solar activity period (2006–2007) near the equatorial ionization anomaly crest location in Chinese zone Guoqi Liu ⇑, Wengeng Huang, Jiancun Gong, Hua Shen Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100190, PR China Received 9 March 2012; received in revised form 3 September 2012; accepted 4 September 2012 Available online 12 September 2012 Abstract Variability of vertical TEC recorded at Fuzhou (26.1°N, 119.3°E, geomagnetic latitude 14.4°N), Xiamen (24.5°N, 118.1°E, geomag- netic latitude 13.2°N), Nanning (22.8°N, 108.3°E, geomagnetic latitude 11.4°N), China, during the low solar activity in 2006–2007 have been analyzed and discussed. Remarkable seasonal anomaly was found over three stations with the highest value during spring and the lowest value during summer. The relative standard deviation of VTEC is over 20% all the time, with steady and smooth variation during daytime while it has a large fluctuation during nighttime. The biggest correlation coefficient was found in the VTEC-sunspot pair with a value of over 0.5. It seems that solar activity has a better correlation ship than geomagnetic activity with the variation of VTEC and better correlations are found with more long-term data when comparing our previous study. The results of comparing observation with model prediction in three sites reveal again that the SPIM model overestimates the measured VTEC in the low latitude area. Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Vertical TEC; Low solar activity; Equatorial ionospheric anomaly; Seasonal variation; SPIM 1. Introduction long compared to the geometric range between the satellite and the receiver, which induces range error in the position- Acting as a dispersive medium, the ionosphere has great ing and navigation (Bagiya et al., 2009). These range errors influence on the satellite navigation and communication, are relatively obvious in the ionosphere above the EIA and the effect is directly proportional to the free electron regions due to the high background electron density and density, which could change the phase and strength of elec- its rapid, complex variation. The EIA, i.e., “equatorial ion- tromagnetic radio frequency wave. When the satellite sig- ization anomaly” or “Appleton anomaly” was reported in nal propagates through the ionosphere, the carrier 1946. This interesting phenomenon of EIA is like as: The experiences a phase advance and the code experiences a distribution of ionization density at the F layer in the vicin- group delay due to the total number of free electrons along ity of magnetic dip equator is characterized by a trough at the path of the signals from the satellite to the receiver. the equator and two crests on either side of the equator (at Therefore, the carrier phase pseudo ranges are measured about ±15° magnetic latitude) during the day. The electro- too short and the code pseudo ranges are measured too dynamics drift and diffusion theories have been used to explain this interesting phenomenon. During the daytime, the mutual perpendicular eastward electric field and north- ⇑ Corresponding author. Address: Room A1106B, NO. 1 Nanertiao, ward geomagnetic field give rise to an upward E Â B drift. Zhongguancun, Haidian District, Beijing 100190, PR China. Tel.: +86 10 After the plasma is lifted to greater heights, it diffuses along 62586419; fax: +86 10 62586416. E-mail addresses: [email protected] (G. Liu), [email protected] magnetic field lines due to the combined influence of (W. Huang), [email protected] (J. Gong), [email protected] (H. Shen). pressure gradient forces and gravity, which forms a trough 0273-1177/$36.00 Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2012.09.002 G. Liu et al. / Advances in Space Research 51 (2013) 366–376 367 at the equator and two crests at higher latitudes on F2 layer mate the GPS-derived value (Bhuyan et al., 2006; Bhuyan (Appleton, 1946; Martyn, 1955; Duncan, 1960; Hanson and Borah, 2007; Obrou et al., 2009; Galav et al., 2010; and Moffett, 1966). In order to reduce the detrimental Aggarwal, 2011; Liu et al., 2012; Adewale et al., 2012). effects on the GPS based navigation, it is very necessary One reason is lack of consecutive long-term observation to have a prior and precise knowledge of the variation in low latitude, another is the established ionosphere model and distribution of ionosphere parameters in different is based on the data more from high and middle latitude region, especially low latitude regions. (Bilitza, 2001). Therefore, as the part of the global iono- The Total Electron Content (TEC) is one of the most sphere characteristic, these results of our work will be good important parameters to characterize the ionosphere prop- complements and be propitious to study and establish a erty, which is calculated as an integral of electron number more accurate model of ionosphere. density along the line of sight from satellite to receiver and In this work, Observation data recorded in Fuzhou could vary intensely from day to day (Huang et al., 1989; (26.1°N, 119.3°E, geomagnetic latitude 14.4°N), Xiamen Rastogi and Klobuchar, 1990). Since the F region density (24.5°N, 118.1°E, geomagnetic latitude 13.2°N), and has a great weight for total electron content and the main Nanning (22.8°N, 108.3°E, geomagnetic latitude 11.4°N) contribution to TEC would occur around the height of the were used to investigate the characteristics of VTEC. We will maximum ionization in the F layer, the development and give an overall description of temporal variation character- variation of EIA is as seen in TEC. Now it is accepted that istics of VTEC in low solar activity period from year 2006 to the magnitude and variation of TEC relate to local time, 2007; we also examine the effect of solar and geomagnetic solar activity, geomagnetic conditions, region of the earth activity on the VTEC in those three sites. As the extension and sudden space weather events. In the low latitude of of our previous study, The results will make one too well East Asia, some studies of TEC have been reported using understand the variation of TEC around 110–120°E near the data recorded round 120°E. Huang and Cheng (1996) the Tropic of Cancer, and the difference between observa- studied the solar cycle variation of EIA in TEC using tion and output of Standard Plasmasphere–Ionosphere observed data from a single ground station at Lunping Model (SPIM, Gulyaeva and Bilitza, 2012). (25.00°N, 121.17°E), and found no significant solar cycle effect in the occurrence time of the most developed 2. Observation data equatorial ionospheric anomaly and the winter crest appears larger and earlier than the summer crest. Wu To study the characteristic of TEC near the Tropic of et al. (2004) found there is a weak correlation between Cancer in Chinese zone, a few dual frequency GPS receiv- EIA and F10.7, and the seasonal variation of EIA is likely ers were established since 2005. There are 28 GPS satellites influenced by the seasonal variations of geomagnetic activ- orbiting the Earth at an inclination of 55°and at a height of ity (Dst) during the solar minimum by analyzing a short- 20,200 km. They broadcast information on two frequency term data set (September 1996 to August 1997). Wu et al. carrier signals, which are f1 (1575.42 MHz) and f2 (2008) studied ten years GPS data and suggested that the (1227.60 MHz), respectively. Ground-based GPS receiver geomagnetic effects (Kp) on the crest are short-term and could record constantly the pseudo-ranges (P1 and P2) solar effects (F10.7) are long-term. Many studies also have and the phases (L1 and L2) corresponding to the two sig- been carried out by other researchers in different country nals. In this work, we used these data to estimate the slant using the observational data recorded in different locations. total electron content (TECsl) in 30 s interval using the Various variation properties, the effects of solar and technique of calculating the VTEC developed by Ma and geomagnetic activities, and model comparative analysis Maruyama (2003). Since the slant TEC is the total number were reported by those works (Tsai et al., 2001; Rao, of electrons in a column of the unit cross section along the 2006; Bhuyan and Borah, 2007; Bagiya et al., 2009; Kumar ray path, it is desirable to calculate an equivalent vertical and Singh, 2009; Mukherjee et al., 2010). value of TEC, which is independent of the elevation of In our previous study (Liu et al., 2012) we were the first the ray path. To convert the slant TECsl to vertical TEC, time to analyze the temporal characteristic of VTEC using we assumed the ionosphere to be a thin screen shell model one-year data observed in Xiamen. We found the remark- and its center is assumed to be the same as that of the able seasonal anomaly of VTEC, and poor correlation with Earth. It is because the main contribution to TEC geomagnetic and solar activity. In this work, we extend the variations would occur around the height of the maximum data-set for two years and use three observational stations, ionization and this allows us to consider the ionosphere as including Xiamen site. We think the results in this paper a thin layer located at the height of ionosphere F2 layer.