Climatology of total electron content near the dip equator under geomagnetic quiet-conditions

ARTICLE IN PRESS Journal of Atmospheric and Solar-Terrestrial Physics 72 (2010) 207–212

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Climatology of total electron content near the dip equator under geomagnetic quiet-conditions C.C. Lee a,n, Y.J. Chuo b, F.D. Chu c a

General Education Center, Ching Yun University, Jhongli, Taiwan Department of Information Networking and System Administration, Ling Tung University, Taichung, Taiwan c National Standard Time and Frequency Laboratory, Telecommunication Laboratories, Chunghwa Telecom Co., Ltd., Taiwan b

a r t i c l e in fo

abstract

Article history: Received 15 July 2009 Received in revised form 12 November 2009 Accepted 14 November 2009 Available online 24 November 2009

This study analyzes the TEC data during 1998–2007, observed by the AREQ (16.51S, 71.51W) GPS station to investigate the equatorial ionospheric variations under geomagnetic quiet-conditions. The diurnal TEC values generally have a maximum value between 1330 and 1500 LT and a minimum around 0500 LT. For the seasonal variation, the semi-annual variation apparently exists in the daytime TEC with two peaks in equinoctial months. In contrast, this semi-annual variation is not found in the nighttime. Furthermore, the results of the annual variation show that the correlation between the daytime TEC value and the solar activity factor is highly positive. & 2009 Elsevier Ltd. All rights reserved.

Keywords: Ionosphere Equatorial ionosphere Ionospheric dynamics Plasma temperature and density

1. Introduction When satellite signals travel through the ionosphere to ground receivers, the signals would be influenced by the free electrons along the path. Scientists, therefore, can derive the total electron content (TEC) from the changes in the trans-ionospheric signals (e.g. Davies, 1990). TEC is the total number of electrons integrated along a signal path through the ionosphere. Previous studies (e.g. Mannucci et al., 1999) demonstrated that TEC has become an important parameter of the ionosphere at different latitudes. In this study, the focus is given to the variations in TEC near the dip equator. The F2 region is a major contributor of electron to TEC. Electron density in this region is therefore very important to TEC variations. In the equatorial F-region, the electron density variation is controlled by electrodynamics, in addition to production by solar radiation and loss by chemical processes. At the dip equator, the electrodynamics in the F-region is peculiar, because the electric fields result from a complicated interaction between the E- and F-region dynamo fields and the geomagnetic field lines are horizontal. Furthermore, due to the horizontal geomagnetic field lines, the zonal electric field forms the vertical E  B drift (e.g. Kelley, 1989). Generally, the vertical E  B drifts are upward and downward during the daytime and nighttime, respectively (Scherliess and Fejer, 1999). Besides this, during the sunset

n

Corresponding author. E-mail address: [email protected] (C.C. Lee).

1364-6826/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2009.11.011

period, the pre-reversal enhancement (PRE) of the eastward electric field causes a large upward E  B drift velocity (called the PRE upward velocity), because of the development of the F-region dynamo electric field under the post-sunset decay of the E-layer conductivity (Farley et al., 1986). Since 1960s, many studies have examined the characteristics of TEC near the dip equator. In Asia, Rastogi et al. (1973) calculated TEC at Thumba, India (8.51N, 771E, geomagnetic latitude: 0.31S) from the satellite signals of BE-B and BE-C for the period covering 1965–1968. Klobuchar and Rastogi (1988) used the signals of ATS-6 satellite to estimate TEC at Ootacamund, India (10.51N, 72.51E, geomagnetic latitude: 1.41N) between 1975 and 1976. Rufenach et al. (1968) used the data of 1964 from Transit-4A to study TEC over Bangkok, Thailand (13.71N, 100.11E, geomagnetic latitude: 2.41N). In Africa, the satellite signals of BEB, Transit-4A, and Transit-4B were used to investigate the variations in TEC (Skinner, 1966; Olatunji, 1967). Skinner (1966) and Olatunji (1967) studied the TEC variations at Zaria, Nigeria (11.21N, 7.71E, dip latitude: 21N) using the data of 1964–1965 and at Ibadan, Nigeria (7.51N, 3.91E, dip latitude: 61S) with the data of 1962–1963. In the American sector, Ross (1966) analyzed the satellite signals of Transit-4A to study the TEC variation at Huancayo, Peru (12.11S, 75.41W, geomagnetic latitude: 0.61S) between 1961 and 1963. At the same location, Klobuchar and Rastogi (1988) investigate the variations of TEC derived from the ATS-6 signals during 1974–1975. Although many studies on TEC variations in the equatorial ionosphere have been done, a climatological study with long-duration

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TEC data has not yet examined. Recently, Liu and Chen (2009) and Liu et al. (2009a) explored the TEC climatology by analyzing the mean TEC from GPS global ionospheric maps of 11–12 years. They reported the annual, semi-annual, and solar activity variations in the mean TEC at low-, middle-, and high-latitudes of both hemispheres. Nevertheless, those two studies did not focus on the TEC variation at the dip equator. In this work, we study the TEC climatology in the equatorial ionosphere under geomagnetic quiet-conditions. The TEC data of 1998–2007 were observed by the AREQ station (16.51S, 71.51W, geomagnetic latitude: 3.91S), receiving the satellite signals of Global Positioning System (GPS).

2. Data analysis The data, used in this study, is received by the AREQ GPS receiver, which is one of the GPS stations of the International GNSS Service (IGS) (Dow et al., 2005). This station, near the dip equator, is located on the west side of South America. The AREQ receiver, equipped with dual-frequency, can provide the data of carrier phase and pseudo-range measurements at two frequencies (f1 =1575.42 MHz, f2 =1227.60 MHz). The slant TEC, STEC, which is the total number of electrons along the entire line-of-sight (LOS) between receiver and satellite, can be estimated using the following equation (Mannucci et al., 1999): STEC ¼

 1 f12 f22  ðR2 R1 Þ þ Dbr þ Dbs ; 40:3 f12 f22

where R1 and R2 are the pseudo-range of the signal of f1 and f2, respectively; Dbr and Dbs are the different code biases (DCB) of receiver and satellite, respectively. By combining the measurements of pseudo-range and carrier phase, the precise STEC for each tracked satellite at each observation epoch is derived. The STEC value is then converted to the vertical TEC (VTEC) using a simple mapping function, assuming that the ionosphere is a thin shell at the peak height of 350 km (e.g. Komjathy, 1997). It is noted that TECu is the units of TEC, where 1 TECu=1016 electrons/m2. In order to decrease the errors caused by electron density gradient at low-latitudes, the VTEC located in the area inside 16.5731S latitudes and 71.5731W longitudes is chosen for the following analyzes. Therefore, the chosen VTEC can be attributed as the TEC above the AREQ station. In the following sections, the notation, TEC, is used to represent the chosen VTEC. Since the DCB data are provided by the Center for Orbit Determination in Europe (CODE) from October 1997, the data analyzed in the current study begins from January 1998. In order to cover a full solar cycle, the period considered the ends at December 2007. The period of 1998–2007 is the part of the 23rd solar cycle, which has a maximum in April 2000. The yearly values of the solar sunspot number, solar flux (F10.7), and solar activity factor (P) are displayed in Table 1. The P is derived by (F10.7+F10.7A)/2, where F10.7A is the 81-day average of F10.7 (Richard et al., 1994). Because Liu et al. (2006) showed that the P value is a better proxy for representing the solar activity of EUV, than F10.7 and F10.7A, the P is used in this work to indicate the solar activity. To eliminate the effects of geomagnetic disturbances, the TEC values on the days for SKpr24 are analyzed in this study.

3. Results and discussions

Table 1 Yearly sunspot number, yearly solar flux (F10.7), yearly solar activity factor (P) during 1998–2007. Year

Yearly sunspot number

Yearly F10.7

Yearly P

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

64.3 93.3 119.6 111.0 104.0 63.7 40.4 29.8 15.2 7.5

117.9 153.5 178.7 181.1 179.4 128.4 106.5 91.7 80.0 73.1

118.0 153.9 180.1 181.1 179.5 128.5 106.7 91.5 80.1 73.0

solar activities. The solar activities are grouped into low, moderate, and high, when P values are smaller than 100, between 100 and 150, and greater than 150, respectively. The monthly P values are shown at the top of each figure. In Fig. 1a, the TEC values for March increase sharply after 0500 LT, and reach a peak at 1330–1500 LT, then decrease gradually to a minimum around 0500 LT. For March 2007, 2003, and 2002, the maximum TEC values are 38, 78, and 116 TECu, respectively; while the minimum values of TEC are 13, 17, and 22 TECu, respectively. This kind of variation is also found in the results of June (Fig. 1b), September (Fig. 1c), and December (Fig. 1d). In Table 2, the maximum and minimum TEC values for each month are listed. Generally, the TEC values have a simple diurnal variation. The diurnal variations for low solar activity in this current study are close to the results at Huancayo (Ross, 1966; Klobuchar and Rastogi, 1988), which is located at the same longitude sector as the AREQ station. Furthermore, in Africa and India, Olatunji (1967) and Klobuchar and Rastogi (1988) also obtained a similar diurnal variation for low solar activity. The greater TEC values during the daytime are mainly due to the solar EUV production of ionization and the upward E  B drift velocity. The upward velocity can lift the ionosphere to higher altitudes, where the loss rate is smaller. In contrast, because the EUV production ceases during the nighttime, the smaller TEC values are caused by the downward E  B velocity, which lowers the ionosphere to altitudes where the chemical losses are larger (e.g. Davies, 1990; Lee and Reinisch, 2006; Lee et al., 2008). Moreover, it is noted that the diurnal variation of TEC does not have the ‘‘noontime bite-out’’ feature, which usually appears in the diurnal variation of F2-peak density (NmF2) (Lee and Reinisch, 2006; Lee et al., 2008). Because TEC is the total number of electrons integrated along a signal path through the ionosphere, a thicker ionospheric thickness would help TEC to maintain a greater value. The previous studies (Rufenach et al., 1968; Klobuchar and Rastogi, 1988) have found the noontime maxima in the semi-thickness and equivalent slab thickness. In addition, Lee and Reinisch (2006) and Lee et al. (2008) reported that the thickness parameter of the bottomside profile (B0) has a maximum near the noontime. Accordingly, the maximum ionospheric thickness during the noontime would account for this difference between TEC and NmF2. In Fig. 1, the TEC values in March and September are larger than those in June and December. This variation will be shown and discussed in Section 3.2. Furthermore, the daytime TEC values increase with increasing P. These variations will be presented and discussed in Section 3.3.

3.1. Diurnal variation 3.2. Seasonal variation The diurnal variation of TEC is obtained by averaging the data on the days for SKpr24 in a month. Fig. 1 displays the diurnal TEC variations in March, June, September, and December for different

In Fig. 2, the seasonal variations of TEC in 2007, 1998, and 2000 are displayed. The seasonal variation of a year is produced from

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Fig. 1. The diurnal variations of TEC for March (a), June (b), September (c), and December (d). For each month, the diurnal TEC variations under low (dashed line), moderate (dotted line), and high (solid line) solar activities are presented. The monthly P value of a month is noticed at the top of the axes.

Table 2 Maximum and minimum TEC values of the diurnal TEC in Fig. 1. Month

Year

Maximum TEC (TECu)

Minimum TEC (TECu)

March

2007 2003 2002

38 78 116

13 17 22

June

2007 2003 2001

32 46 58

7 8 6

September

2006 1999 2002

35 83 111

11 13 10

December

2006 1998 2000

30 78 83

11 9 16

the diurnal TEC variation, obtained by the same calculation in Section 3.1, of 12 months of that year. The yearly P values are 73.0, 118.0, and 180.1 for 2007, 1998, and 2000, respectively. In Fig. 2a, it is found that a semi-annual variation obviously exists in the daytime TEC during 2007, which is the year of low solar activity. The peak values are 38 and 36 TECu in March and October, respectively. This kind of semi-annual variation also appears in the daytime TEC values for the years of moderate and high solar

activities, as shown in Figs. 2b and 2c. In Fig. 2b, the peak values in 1998 are 65 and 84 TECu in March and September, respectively. For 2000 (Fig. 2c), the peak values are 121 and 114 TECu in March and October, respectively. Generally, during the daytime, the peak TEC values occur in the equinoctial months, while the minima appear in the winter months. On the other hand, the semi-annual variation is not found in the nighttime TEC values. In Ross (1966) and Olatunji (1967), the semi-annual variation has been found in the daytime TEC at Huancayo and Ibadan, respectively. Recently, Liu et al. (2009a, 2009b) reported that the semi-annual variation exists in the low-, middle, and high-latitude ionospheres. And, the semi-annual variation is pronounced at low-latitudes during high solar activity. Furthermore, according to the earlier results (Bailey et al., 2000; Liu et al. 2007, 2009a, 2009b), the low-latitude semiannual variation seems to appear mainly in the ionosphere below 840 km. Olatunji (1967) found that this semi-annual variation might be related to the variation of the noon solar zenith angle, which is an important factor for the production of ionization. However, Ross (1966) suggested that the variation of the intensity of the equatorial electrojet (EEJ), obtained from the magnetometer H component observation, is mainly responsible for the semiannual TEC variation. The noon solar zenith angles for the AREQ station, calculated by the IRI-2007 model (Bilitza and Reinisch, 2008), are 17.51, 39.91, 16.61, and 7.01 for spring equinox, winter solstice, autumn equinox, and summer solstice, respectively. The

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Fig. 2. The seasonal variations of TEC in 2007 (a), 1998 (b), and 2000 (c). The yearly P values are presented at the top of the axes. This seasonal variation of a year is produced from the diurnal TEC variation, obtained by the same calculation in Section 3.1, of 12 months of that year.

cosine of the noon zenith angle is largest at summer solstice, moderate at equinoxes, and smallest at winter solstice Since the cosine of the noon zenith angle does not have the largest values at equinoxes, the variation of the zenith angle would not be directly associated to the TEC seasonal variation. Regarding the EEJ intensity, Anderson et al. (2002) have showed that during the daytime, the vertical E  B drift velocity in the equatorial F-region is positively related to the EEJ intensity. Accordingly, the current

study examines the daytime vertical E  B drift at Jicamarca, which is located near the dip equator and in the same longitude sector as the AREQ station. Scherliess and Fejer (1999) found that the daytime E  B drift velocities are larger in the equinoctial and winter months than in the summer months. Either, this kind of variation in the daytime E  B drift would not be directly associated to the semi-annual TEC variation. Nevertheless, when the noon zenith angle and the daytime E  B drift are considered

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Fig. 3. The annual TEC variation for March (a), June (b), September (c), and December (d). The annual variation for a month is produced from the diurnal TEC variation, obtained by the same calculation in Section 3.1, of that month during 1998–2007.

Table 3 Correlation coefficients of the TEC value in Fig. 3 and the monthly P value at different local times for 4 months.

March June September December

0600 LT

1000 LT

1400 LT

1800 LT

2200 LT

0.24 0.45 0.16 0.44

0.92 0.91 0.97 0.90

0.97 0.95 0.97 0.94

0.94 0.86 0.85 0.93

0.51 0.73 0.72 0.23

simultaneously, the cause of the semi-annual TEC variation might be the combination of these two factors. In the equinoctial months, because both the daytime E  B drift and the cosine of the noon zenith angle are larger, the TEC has the peak value. On the other hand, for the summer (winter) months, the cosine of the noon zenith angle is larger (smaller), but the daytime E  B drift is smaller (larger). Therefore, the TEC values are smaller in these two seasons than in the equinoctial months.

3.3. Annual variation Fig. 3 shows the annual TEC variation for March, June, September, and December. The annual variation for a month is produced from the diurnal TEC variation, obtained by the same calculation in Section 3.1, of that month during 1998–2007. It is noted that the gaps in Figs. 3a and 3b is because the GPS data is not available in March 2006 and June 2002, respectively. In Figs. 3a–d, the daytime TEC values for March start to increase in 1998, and have a peak value in 2000–2001, then begin to decrease in 2002. These daytime TEC values seem to be correlated to the solar activities. On the other hand, there seems no obvious trend in the variation of the nighttime TEC.

To further examine the correlation between the TEC and the solar activity, the diurnal TEC values at different local times are applied on the correlation analysis. Table 3 presents the correlation coefficients of the TEC values and the monthly P values at different local times for 4 months. It is found that the TEC is highly correlated to the P value at 1000, 1400, and 1800 LT, but is moderately or poorly correlated at 0600 and 2200 LT. It is noted that the correlation coefficients at mid-night and presunrise are not showed in Table 3, because the minimum TEC values during this period are usually used to be the assumption of base line in evaluating TEC. These results are similar to that of Liu and Chen (2009), who reported that the correlation between TEC and P is high during the daytime, and decreases in the nighttime.

4. Conclusion and summary This work examines the TEC data during 1998–2007, observed by the AREQ GPS station in order to understand the ionospheric variations near the dip equator. The data on the days for SKp r24 are used to characterize the diurnal, seasonal, and annual variations under geomagnetic quiet-conditions. The diurnal TEC values have a maximum values between 1330 and 1500 LT and a minimum around 0500 LT. This diurnal variation in TEC is primarily caused by those in the solar EUV production and the vertical E  B drift. For the daytime, in addition to the solar production, the upward drift velocity would lift the ionosphere to higher altitudes, where the loss rate is smaller. During the nighttime, the solar production is stopped, and the downward drift would lower the ionosphere to altitudes, where the loss rate is greater. For the seasonal variation, the semi-annual variation apparently exists in the daytime, because two peaks of the daytime TEC

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values are located in the equinoctial months. The equinoctial peaks of the daytime TEC are because both the upward drift and the cosine of solar zenith angle are greater in this season. On the other hand, the nighttime TEC does not have an obvious trend. Regarding the annual variation, the daytime TEC values are larger and smaller in the years of high and low solar activates, respectively. Furthermore, the maximum values in the diurnal TEC are highly positively correlated to the P values.

Acknowledgements We greatly thank two reviewers for their valuable suggestions. This work is supported by the grants of National Space Organization98-NSPO(B)-IC- FA07-01(R), and National Science CouncilNSC 98-2119-M-231-001 and NSC 98-2111-M-275-001. The authors thank IGS for the AREQ data, CODE for DCB files, National Geophysical Data Center (NGSC) for data of sunspot number, F10.7, and Kp, and National Space Science Data Center (NSSDC) for the IRI-2007 model. Reference Anderson, D., Anghel, A., Yumoto, K., Ishitsuka, M., Kudeki, E., 2002. Estimating daytime vertical E  B drift velocities in the equatorial F-region using groundbased magnetometer observations. Geophysical Research Letters 29, 1596. Bailey, G.J., Su, Y.Z., Oyama, K.I., 2000. Yearly variations in the low-latitude top ionosphere. Annales Geophysicae 18, 789. Bilitza, D., Reinisch, B., 2008. International reference ionosphere 2007: improvements and new parameters. Advance in Space Research 42, 599. Davies, K., 1990. In: Ionospheric Radio. Peter Peregrinus Ltd., London, United Kingdom. Dow, J.M, Neilan, R.E., Gendt, G., 2005. The international GPS service (IGS): celebrating the 10th anniversary and looking to the next decade. Advance in Space Research 36, 320. Farley, D.T., Bonelli, E., Fejer, B.G., 1986. The pre-reversal enhancement of the zonal electric field in the equatorial ionosphere. Journal of Geophysical Research 91, 13723. Kelley, M. C., 1989. . The earth’s ionosphere. Int. Geophys. Ser., 43, Academic, San Diego, California.

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