Seasonal Variability of GPS-VTEC and Model During Low Solar Activity Period (2006–2007) Near the Equatorial Ionization Anomaly Crest Location in Chinese Zone
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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 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. are necessary because there are a few observations and According to Davies (1990), the height of the mean layer study of the TEC along the tropic of cancer in the Chinese of the ionosphere could lie between 300 and 450 km. It is zone. On the other hand, as the advent of GPS, which has assumed to be 400 km in this paper and got the calculation served as a valid instrument to study ionosphere, many technique of VTEC as shown: researchers use the observation data to set up a new TEC model or verify the classical ionospheric model. But the RE cos a VTEC ¼ðTECsl bs brÞ cos arcsin ð1Þ results are not very good, which overestimate or underesti- RE þ h 368 G. Liu et al. / Advances in Space Research 51 (2013) 366–376 where bs and br are the satellite and receiver biases, respec- denotes that it is difficult to predict the defining perfor- tively, RE = 6378 km, and the h is the height of the iono- mance of VTEC in the next day. Another fact revealed spheric layer, a is the elevation angle of satellite. Because by those three curves in the figure is that there are some dif- the pseudo-range with low elevation is apt to be affected ferences (including the magnitude and variation trend) by multipath effect and the reliability decrease, thus, in caused by different location, of the stations. Therefore, it order to minimize the time shift and neglect unwanted is necessary to make a study of statistic analysis to under- errors due to multipath and insure the number of data, stand the basic characteristic of TEC variation in this we chose 30° as the cut off elevation angle. region. To investigate the mean diurnal variation and sea- In this paper, we choose the VTEC in an hour interval sonal property in three sites, the observational data were (in the whole point of UT) and then make average for sea- sorted into four seasons, spring (March, April and May), son to investigate the temporal characteristic over three summer (June, July, and August), autumn (September, stations. To analyze the correlation between VTEC and October, and November), winter (December, January, geomagnetic and solar activity, Kp and Dst data were and February). Fig. 3 exhibits the seasonal variation of downloaded from the website (
Fig. 2. Day-to-day variation for VTEC from 18 to 27 May, 2006 recorded in Nanning, Xiamen and Fuzhou.
Fig. 3. The seasonal variations of VTEC during year 2006 (left) and 2007 (right) recorded in Nanning (22.8°N, 108.3°E, geomagnetic latitude 11.4°N), Xiamen (24.5°N, 118.1°E, geomagnetic lat 13.2°N), Fuzhou (26.1°N, 119.3°E, geomagnetic lat 14.4°N), respectively. (LST = UT + LON/15.) stations. It is increasing from 00 UT until reaching the 21 UT. Some researchers (Bagiya et al., 2009; Perevalova maximum in the afternoon (06–07 UT), and then decreas- et al., 2010; Chauhan and Singh, 2010) reported the similar ing gradually to attain the minimum value around results. They also found the maximum of TEC occurs in 370 G. Liu et al. / Advances in Space Research 51 (2013) 366–376
Fig. 4. Variation of VTEC (left) and relative standard deviation (right) during different seasons for two-year data over Nanning, Xiamen and Fuzhou. (LST = UT + LON/15.) the afternoon and minimum occurs in the pre-dawn. On from the summer (hot) to the winter (cold) hemisphere. the other hand, the highest value of peak VTEC is found As a result, an increase of the O/N2 ratio caused by the in Nanning, which is followed by Xiamen and the lowest convection of atomic oxygen is formed in the winter hemi- VTEC occurs in Fuzhou. For the occurrence time of peak sphere. Therefore, the recombination in winter hemisphere value, again, one could find that it is 07 UT in Nanning is weaker than that in the summer hemisphere, which (14.22 LST) while other two stations are still at 06 UT results in the relatively higher electron concentration in (13.87 and 13.95 LST). Considering the similar latitude of winter (Rishbeth, 1961; Johnson, 1963; Torr and Torr, three sites (26.1°N, 24.5°N and 22.8°N, respectively), the 1973). Another possible mechanism for this seasonal result that the seasonal mean peak value of VTEC in Fuz- anomaly is the change of direction of neutral wind. A hou is manifest lower compared to that in Xiamen and meridional component of neutral wind blows from the Nanning is interesting, which also reveal that the complex summer to the winter hemisphere which can reduce the local variation in this region. The main reason of lower crest value during summer solstice as it blows in an oppo- value may not be the latitude, but the especial location of site direction to the plasma diffusion process originating Fuzhou, which lies on the fringe of the northern equatorial from the magnetic equator; at the equinoxes (or spring), ionization anomaly. During high solar activity, the EIA meridional winds from equator blows pole-wards should crest could move higher latitude which makes the corre- result in a high ionization crest value. Based on this sce- sponding high peak TEC. But in low solar activity the nario, a seasonal effect on the crest should be expected with EIA crest may occur in lower latitude, therefore, the the crest maximum at the equinoxes and minimum in the TEC recorded in Fuzhou, where the geomagnetic latitude summer (Bramley and Young, 1968; Stening, 1992; Wu is 14.4°N, could be lower than Xiamen and Nanning. et al., 2004, 2008; Bhuyan and Borah, 2007). The phenomenon of seasonal or semiannual variation (anomaly or winter anomaly) for VTEC over three stations 3.2. The relative standard deviation of VTEC presented in this work is anticipated and reasonably. It has been discussed and explained by many researchers and sci- In order to further understand the variations of VTEC, entists during past years. The behavior of seasonal (winter) we calculate the relative standard deviation (RSD) of variation could be interpreted using the change of compo- hourly VTEC for four seasons. The RSD = (VTEC stan- sition of the atmosphere. Due to the asymmetric heating of dard deviation) 100/(VTEC average value). RSD can the two hemispheres, neutral constituents are transported reveal more information of temporal variation of VTEC, G. Liu et al. / Advances in Space Research 51 (2013) 366–376 371 which is propitious to help people to better understand the VTEC is found in the figure. The monthly trend shows that characteristic in detail. The diurnal distributions of RSD the first maximum occurs in April of 2006 at all observa- for three stations during different seasons are shown in tional stations while it is found in March during 2007 in right panels of Fig. 4. It could be seen that RSD in all Xiamen and Fuzhou (no data in March and April of observation stations has the similar and smooth increasing 2007 for Nanning station due to technical reason). The sec- tendency from 00 UT to 14 UT during all seasons, while ond maximum of mean peak VTEC is found in the October variations have a few differences after 15 UT. In the detail, regardless of year. Except for the basic pattern of variation, RSD in Fuzhou resembles that in Xiamen, with the max to a certain extent, the magnitude and temporal fluctuation value at 15 UT and reaches a second peak value in characteristic of TEC is influenced by solar and geomag- 20 UT during spring. The difference of them in spring is netic activity. In order to examine the effect of these two that the values of RSD in Xiamen are higher than in Fuz- factors on the variation of peak VTEC during the low solar hou after 12 UT, while RSD in Nanning has other trend period from year 2006 to 2007, solar activity index sunspot with the highest value occurring at 17 UT and second high SSN, F10.7, geomagnetic activity index Kp and Dst are value at 22 UT. During summer, RSD in Fuzhou reaches analyzed in this work. The monthly variations of their val- the highest value in 20 UT while 21 UT in Xiamen, ues in these 2 years are shown in top four panels of Fig. 5. 17 UT in Nanning. RSD in Xiamen and Fuzhou display It is clearly found from the figure that the monthly mean same trends during autumn with maximum at 21 UT, but value of the SSN and F10.7 areP both at their high value value in Nanning is increasing after 02 UT. During winter, in April, while the maximum of Kp and the minimum the variation of RSD in three sites is almost the same with of Dst occurs in December during 2006, but four indices high levels at 21–22 UT. The behaviors of RSD for all sea- did not show large fluctuation during 2007.P The solar activ- sons in all sites indicate that the distribution of VTEC has a ity index shows a declining trend while Kp and Dst did large fluctuation, especially after 12 UT. This is night to the not exhibit remarkable annual variation during the chosen breakdown of local solar time, which is corresponding to period. Fig. 6 shows the correlation of monthly peak the low value period of VTEC. From Fig. 4 one also could VTEC with geomagnetic activity indices over three sta- find that the minimum of the RSD is larger than 20%, tions. Correlation coefficients (represented by C.C) are which reveals that the periods of low solar activity may determined and the corresponding fitting is shown by the lead to higher levels of relative standard variation of straight line. We could find thatP the correlations of VTEC. Another similar variation characteristic is that monthly peak VTEC with monthly Kp, Dst, sunspot RSD during spring is higher than the other three seasons SSN and F10.7 are 0.14, 0.15, 0.33 and 0.25, respectively during 12–16 UT. Our result is slightly similar to Lazo and the values of coefficients are 0.23, 0.26, 0.34 and 0.34 et al. (2004). They found that the relative variability (rela- in Xiamen, while they are 0.23, 0.35, 0.53 and 0.52 in tive standard deviation) of TEC is higher during dawn and Nanning. sunset period, mainly in equinoctial months. Adewale et al. Kp index represents planetary magnetic activity on a (2012) also found that the variability of VTEC has large global scale, while Dst records the equatorial ring current value during nighttime, especially post-midnight hours variation (Mayaud, 1980). Wu et al. (2004) found a general and post-sunset. The steep electron density gradients good relationship between Kp as well as Dst index. They caused by the turn-off of solar ionization and onset could suggested that the Dst index is a more suitable parameter explain this variation. However, Mukherjee et al. (2010) for studying long-term ionospheric dynamics around the analyzed VTEC data during the period 2005–2006 at EIA regions. Kumar and Singh (2009) also found a good Bhopal (23.2°N, 77.6°E, geog. 14.29°N, 151.12°E) and relationship between Kp index while there is a bad relation- found that the standard deviation of VTEC is relatively ship between the monthly values of the EIA crest in TEC smooth during nighttime hours and increases after pre- and the monthly Dst-indexP (R = 0.03). The value of cor- dawn with a maximum values in the afternoon hours, while relation coefficient for Kp in our finding is relative small the relative variability index for summer is higher than that compared these two reports, but value for Dst index lies on compared to other seasons. Aggarwal (2011) found the dif- between them. On the other hand, The sunspot number ferent result using data from Rajkot (geog. 22.29°N, SSN indicates the degree of sunspot activity on the sun, 70.74°E), i.e., the standard deviation of TEC from the and the solar radio flux F10.7 cm, which is the proxy for median values is relatively smaller during nighttime than solar EUV radiation responsible for photo-ionization in daytime. These reports further demonstrate that the varia- the ionosphere, is reckoned as one of the factors to affect tion of TEC relates with the local latitudes and longitudes. the magnitude and variation of TEC. The analysis in our work indicates that the best correlation is found between 3.3. Correlation with solar and geomagnetic activity monthly VTEC and SSN among four indices, wherein the largest value of correlation coefficient occurs in Nanning To reveal the seasonal variation of peak VTEC in month with a little over 0.5, while the minimum correlation is details, we calculate the monthly value of the data and the shown in Fuzhou. results are presented in bottom panels of Fig. 5. Again, a For the coefficients in Xiamen station in this paper, we remarkable semiannual variation of monthly average peak notice that the value of correlation coefficient is bigger than 372 G. Liu et al. / Advances in Space Research 51 (2013) 366–376
P Fig. 5. Monthly variations of Dst, Kp, F10.7, sunspot number SSN and averaged peak VTEC in three observational stations.
P Fig. 6. Correlation coefficients comparing monthly averaged peak VTEC with monthly averaged Kp, Dst, SSN and F10.7 over three sites. G. Liu et al. / Advances in Space Research 51 (2013) 366–376 373 that in our previous study (Liu et al., 2012), which was got- relative high value of coefficients in Nanning (22.8°N, ten by analyzing one-year observed data. It was found in 108.3°E) and lowest values in Fuzhou (26.1°N, 119.3°E) that paper the coefficient is 0.26 between peak VTEC and regardless indices. sunspot SSN, while it was 0.14 for F10.7 and 0.11 for In general, electron populations in the ionosphere are Dst. This difference may be due to the longer term data mainly controlled by solar photo-ionization and recombi- used in this paper. The meaning of this increase of effect nation processes. The photo-ionization caused by solar similar to some results reported by other researchers EUV radiation can produce more electrons and therefore though the value of correlation coefficient may be unlike. enhances the background electron density. During the Wu et al. (2008) found a good correlation of EIA and equinoxes, the subsolar point is around the equator, where F10.7 (correlation coefficient = 0.87) through analyzing a the eastward electrojet-associated electric field is often lar- long-term data set (1994–2003). Bagiya et al. (2009) ger. Because of the collocation of the peak photoelectron reported that there is a positive correlation between peak abundance and the most intense eastward electric field TEC and solar radio flux. Galav et al. (2010) showed a region, the fountain effect should be developed the most; good correlation with the F10.7 during the declining phase during the solstices, photoelectrons at the equator decrease of the solar activity. A positive linear relationship between because the subsolar point moves to higher latitudes and the value of EIA and the sunspot number also has been the fountain effect is expected to wane (Wu et al., 2004). found by Huang and Cheng (1996). On the other hand, This EUV effect could be well demonstrated in our results. some researchers found the different result in their works. For example, Kumar and Singh (2009) found that the 3.4. Difference between observed and model SSN has very little effect (r = 0.03) on the variation of EIA crest in TEC and suggested that this may be due to The model outputs of the International Standard the solar minimum period of May 2007 to April 2008. Plasma-sphere Ionosphere Model, SPIM, are used to com- As a whole, the values of correlation coefficients showed pare with the observational result of those three sites. The in Fig. 6 revealed a relatively good correlation between detailed characteristic of SPIM will not to be described in solar activity and monthly Peak VTEC, while low correla- this part, but the comparative results are displayed directly. tions were found between geomagnetic activity and it Fig. 7 shows the predicted VTEC by SPIM and GPS mea- during 2006–2007. One also could see that there is a litter sured results for eight seasons during 2006–2007 at three difference between three observation sites, which is the sites. It is easy to see that.
Fig. 7. Comparison of seasonal average of observed VTEC with predicted values from SPIM in three sites during 2006–2007. 374 G. Liu et al. / Advances in Space Research 51 (2013) 366–376
Fig. 8. Diurnal distribution of the relative difference RD_VTEC and average values vary in three sites regardless seasons.
The model overestimates the magnitude of VTEC during tude region. One reason of difference could be related with all season. Although SPIM predict a good variable tendency, the second part of SPIM, Russian Standard Model of Ion- the absolute value of the difference between measured result osphere (SMI), which based on the data more from high and model output is unsatisfactory. Fig. 8 gives the diurnal latitude, so when the model is used to predict the TEC or distribution of relative difference parameter regardless the other ionospheric parameters, it may not show great agree- seasons, which is calculated as follows: ment with the measurements from the low latitude. Another is could be partly caused the shape of profile VTECðObsÞ RDVTEC ¼ 1 assumed by the model. Bhuyan and Borah (2007) found VTECðSPIMÞ the outputs of IRI may overestimate the GPS TEC at most Where VTEC (Obs) represents the GPS-derived VTEC time during low solar activity both in the Indian and East and VTEC (SPIM) is the output of SPIM. It is noted that Asian longitude sectors, thus, the SPIM, which based IRI RD_VTEC are almost plus, which means the predicted and merged the Russian Standard Model of Ionosphere results of SPIM overestimate the VTEC at all seasons overestimates the TEC is to be expected. and almost all the time. We found that the diurnal distribu- tion of RD_VTEC is similar in three sites, so the average 4. Conclusion values of RD_VTEC for all seasons in three sites were cal- culated and the variation vs. UT was shown in the right In the present paper we investigated the diurnal and bottom of Fig. 8. The average values of relative difference seasonal variations, solar (F10.7 and SSN) and magnetic could reveal the general precision and availability of SPIM (Kp and Dst) activity effects on the characteristics of the to some extent. In brief, the existential difference between northern equatorial ionospheric anomaly by using the practical observation and model results of SPIM, which GPS-derived VTEC at three ground observational stations, is recommended by the International Standardization Fuzhou (26.1°N, 119.3°E, geomagnetic latitude 14.4°N), Organization (ISO), should be consider while it was used Xiamen (24.5°N, 118.1°E, geomagnetic latitude 13.2°N), to study the ionosphere. Nanning (22.8°N, 108.3°E, geomagnetic latitude 11.4°N) Although the height (20,000 km) that SPIM could near the Equatorial Ionization Anomaly crest location in model seems more reasonable for model the TEC, the mor- Chinese zone, from 2006 to 2007 during the low solar activ- phological characteristics of VTEC as shown in this paper ity. Some results confirm our previous study (Liu et al., 2012) indicate again that the shortcoming of SPIM can easily be using the data recorded single station. More data from differ- seen when it is applied at these three sites, in the low lati- ent stations during the period of low solar activity are G. Liu et al. / Advances in Space Research 51 (2013) 366–376 375 analyzed in this work, which are more convincing to repre- www.ndgc.noaa.gov>) for providing SSN data; and World sent the law of variation of TEC in the low latitude of China. Data Center for Geomagnetism, Kyoto ( Mayaud, P.N. Derivation, Meaning, and Use of Geomagnetic Indices. 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