Response of Ionospheric Total Electron Content on

International Journal of Applied Physics. ISSN 2249-3174 Volume 3, Number 1 (2013), pp. 23-41 © Research India Publications http://www.ripublication.com/ijap.htm

Response of Ionospheric Total Electron Content on various Geomagnetic and Interplanetary Field Parameters over Indian Sub-Continent B. M. Vyas1* and Baiju Dayanandan2 1

Department of Physics, M. L. Sukhadia University, Udaipur – 313 001, Rajasthan. India 2 Department of Mathematical and Physical Sciences, University of Nizwa, Sultanate of Oman e-mail: [email protected] *Corresponding author: E- Mail address: [email protected]

Abstract Ionospheric Total Electron Content (TEC) data retrieved from ATS-6 Experiment over Indian Stations during October, 1975 to July, 1976 ( Low Solar Activity Period) has been examined to assess the impacts of several Geomagnetic and Interplanetary Field parameters like Dst, Bi, Bz, AE and Interplanetary Electric Field (IEF) on TEC in special reference to understanding the extent of variation in TEC during space weather phenomenon in different seasons of low solar activity period as low reference level over four Indian stations i.e., Thumba, Bombay, Ahmedabad and New Delhi. Out of these four Indian stations, two stations i.e., Ahmedabad and Delhi are quiet near the equatorial Appleton anomaly crest site and remaining other two stations i.e., Thumba and Bombay are located over equatorial and the south of the equatorial anomaly sector, respectively. The behaviour of TEC with geomagnetic and interplanetary field parameters has shown consistent seasonal and local time dependence. The influence on TEC with the above specified parameters is more pronounced during noon- hours than in midnight hours. Overall trend of dependence of Geomagnetic and Interplanetary Field parameters with TEC is found to be in opposite nature in equinoxes months as compared to other seasons. The effect of Dst, IEF, Bi and, AE index on noon time TEC are more significant in winter and summer but the least in equinoxes. The possible explanations due to changing net ionpspheric electric fields such as Prompt Penetration of transient electric field and longer lasting ionospheric

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B. M. Vyas and Baiju Dayanandan Disturbance Dynamo Electric field in such space weather disturbance are discussed in terms of solar wind magnetosphere ionosphere interaction to interpret the present findings. The range error is also discussed in the present study.

1. Introduction Number of free electrons in a unit vertical column cross section extending from earth surface to top of the earth’s ionosphere i.e. Total Electron Content ( TEC in electrons/m2 ) is now becoming one of most important and leading key parameter in the mitigation of ionospheric effects on VHF to L- band radio waves communication systems as well as Space Weather Science Studies (Dabas et al., 2006; Leonard et al., 2004; Beniguel et al., 2004; Rao et al., 2006(a, b); Uberoi, 2001). The space weather disturbances, like Coronal Mass Ejection ( CME ), Geomagnetic Storms, Solar Flares etc., lead to the several serious outages problem on electric power system as well as on satellite communication and navigation systems operating over high, low and equatorial zone (Chen et al, 2008; Basu et al, 2010, Jain et al, 2010). Due to these deleterious impacts like the satellite signal loss some time below 20 dB from normal level in satellite communication links are reported by several investigators over the high, low and equatorial stations. Further, the losses of signals for GPS satellite are also observed to further reduce the availability of satellites for GPS navigation results the perturb the GPS positional accuracy, navigational accuracy ( range error change in TEC 1016 electrons/m2= .16 meter ) and their tracking performances effects (Chen Wu et al, 2008; Dubey et al, 2006).Therefore, the response of TEC, electron density distribution and fluctuation in ionospheric electron density i.e., ionospheric irregularities present at several altitudes of the earth’s ionosphere have been now becoming important research area in predicting the Space Weather effect on telecommunications, communication and navigation systems that use radio signals transmitted to and from satellites (Dabas et al., 2006). These investigations can also be useful in preparing the Ionospheric TEC and Ionospheric Scintillation model as well as the estimation and correction of propagation delays in the Global Positioning System (GPS), improving the accuracy of satellite navigation parameters which are perturbed by the ionospheric scintillation and TEC during several space weather events like ionospheric storms thermospheric storms, geomagnetic storms, solar flares, coronal mass ejection etc., (Aarons & Das Gupta, 1984., Aarons et al., 1997., Basu et al., 2001., Beniguel et al., 2004., Biqiang et al., 2007., Das Gupta et al., 1989., Dashora & Pandey, 2007, Mala et al., 2009, Prasad et al., 2005., Rama Rao(a, b), 2006., Vyas & Pandey., 2002, 2003). During the above wide varieties of space weather events, solar winds or high energy speed of protons and neutrons ejected from the coronal holes or the coronal mass ejection (CME), which may hit and give additional energy and electric field to magnetosphere and ionosphere of high latitude ionosphere

Response of Ionospheric Total Electron Content on various Geomagnetic

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of the Earth, and induce many more complex subsequent phenomena including, solar flares, geomagnetic storms (Kelley, 1989; Blanc & Richmond, 1980; Fejer, 1997; Gonzalez et al., 1994, 1999). The results of huge injected energy accompanied with the geomagnetic storms will be accompanied by the abnormal perturbation in several Interplanetary Magnetic Field (IMF) parameters like Bi, Bz and Geomagnetic Field indices like Dst, AE- index and also in solar wind parameters i.e., proton velocity, interplanetary electric field (IEF) as an indicator of strength and type of several space weather events (Uberoi, 2001). During such space weather phenomena, the interaction between solar wind and magnetosphere under southward IMF or negative value of Bz cause change in region -1 under shielding condition leading to increase to sudden increase in dawn- dusk polar convection east ward electric field in day time and westward in night time ( Hunag et al., 2005, Basu et al, 2010). But, during northward IMF value or over– shielding condition in region-2, the nature of the convection electric field is in reverse trend during dusk-down electric field that is directed in westward in day hours and eastward in night hours (Kelley et al., 1979, 2003, Sastri et al., 1992). This results in nature of prompt penetration of electric field from high latitude to the middle and equatorial ionosphere(Kelley et al., 2002 & 2003; Fejer et al., 2007). These transient electric field or Prompt Penetration Electric (PPE) field exhibits typical rise and decay times, shorter than about 15 minutes to an hour (Huang et al., 2005; Huang et al., 2008). Under such high latitude or polar electric field, the ion convection can cause acceleration of neutral leading to equator ward disturbance winds while the continuing thermal and electromagnetic energy input causes heating of the high- latitude thermosphere- ionosphere system whereby atmosphere disturbance propagate to lower latitude in the form of travelling ionospheric disturbances (TAD’s) including disturbances in themospheric winds (Fuller et al., 2002, Sastri et al., 1992; Sastri, 1988). Due to these, longer– lasting disturbance dynamo electric field (DDE) starts to dominate the low latitude electrodynamics processes of duration interval from several hours to days in geomagnetic storm. However, these shorter lived PPE and longer duration ionospheric DDE both have in general opposite polarity nature ( Fejer et al., 1979; Kelly et al., 2003, Fejer & Scherliess, 1995). Thus, equatorial and low latitude F-region ionosphere modified drastically in space weather events are primarily due to the mixed effect of short duration PPE, longer lasting ionospheric DDE, ionopheric winds in the form of TAD and thermospheric composition (O/N2) changes in these events. Thus, these can perturb the net ionospheric electric field depending upon the local time, type of electric field and consequence of these are observed as enhancement in TEC during eastward PPE or DDE and depletion in TEC during westward PPE or DDE conditions. Since every space weather phenomena exhibit the unique character and peculiar influence. Hence, their prediction of response to ionospheric parameters like TEC has now became the one of the major current scientific leading issue

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B. M. Vyas and Baiju Dayanandan

of space weather science. In this context, the impact of space weather disturbance during the equatorial and low latitude ionospheric phenomena such as ionospheric scintillations, Equatorial Appleton Anomaly, Spread-F etc., have been extensively studied the reported enormous variability in several ionospheric parameters like TEC, foF2, h’F, hmF2, vertical as well as E-W components of drift velocity, electric field etc., during the geomagnetic storms and solar flares over equatorial and low latitude stations (Fejer et al., 1995, 1997, 1999; Kelley., 1989; Basu et al., 2001; Vyas & Pandey 2002, 2003; Biktash, 2004; de Paula et al, 2004; Dashora & Pandey, 2007, Jain et al., 2010). These studies have established a well established fact that equatorial and low latitude F- region ionospheric electric field modified drastically during space weather phenomena as compared to normal days which are mostly due to solar–wind- magnetosphere interaction that results combined effects of relatively short lived PPE, longer lasting ionospheric DDE, ionopheric winds in the form of TAD and thermospheric composition (O/N2) changes (Alfraimovich et al., 2002) In order to further more evaluate the complex solar wind – magnetospheric - ionospheric coupling phenomena, an attempt has been made in the present work to study the dependence of Bi, Bz, Dst, AE and Interplanetary Electric Field (IEF) on TEC over four Indian Stations i.e. Thumba, Bombay, Ahmedabad and Delhi which lies in the equatorial as well as off side of equator i.e., near the crest of equatorial Appleton anomaly region. As earlier works of TEC of the ionosphere shows the local time, seasonal, solar activity, magnetic activity dependence with only Ap, Kp and Dst and anomalous enhancements under a wide range of solar geophysical conditions( Chen Wu et al., 2008; Rao et al., 2006(a, b); Mala et al., 2009; Gupta & Singh, 2001), therefore, the control of geomagnetic activity and interplanetary field parameters on TEC of midnight and noon hours of different seasons by using various IMF parameters i.e. Bi, Bz, IEF, AE and Dst index parameters have been chosen in the present studied specifically for low solar activity period to infer the low level reference of change in the same. Such studies are expected to give a qualitative estimation of the consequent impact of geomagnetic and interplanetary field indices and also rough assessment of expected induced ionospheric electric effect on the TEC variations during space weather events during high solar activity condition. The present investigations would also be helpful in understanding to find out an interaction between magnetosphere and ionospheric processes through the effect of geomagnetic and interplanetary field parameters.

2. Data and method of analysis The hourly mean values of various geomagnetic and interplanetary field parameters such as Dst, AE, Bi, Bz and Interplanetary Electric Field (IEF) for the period Oct., 1975 to July, 1976 are down loaded from the web site of the World Data Centre for Geomagnetism, Kyoto, Japan( http://swdc.kugi.kyoto-


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u.ac.ip/wdc/Sec3.html). These indices have been commonly used as a measure of several space weather phenomenon. Dst is standard storm time monitoring index and more sensitive index of variation in horizontal components of geomagnetic field specifically over low latitude and therefore it is quite convenient and suitable index for the effect of geomagnetic disturbance over the low latitude ionospheric phenomenon. Further, Dst variations are largely affected by the symmetric current in inner magnetosphere( DR current), cross trail current ( DT current) and magnetopause current (DCF current). Thus, Dst = DR+ DT+ DCF ( Kelly, 1989). The AE index is a measure of global electrojet activity, total current or magnetic disturbance energy in Aurora Zone. It is positively correlated with the maximal westward current density and negatively correlated with maximal eastward current density. Thus, Dst index is directly concerned with low latitude geomagnetic disturbance variation where as AE index is related to aurora sub storm activity to study the impact of transferring the electromagnetic energy from high latitude to equatorial station through auroras geomagnetic disturbance over equatorial to low latitude ionospheric parameters. The hourly mean value of TEC data of Thumba, Bombay, Ahmedabad and New Delhi are taken from the Scientific Report on TEC data collected of Indian Stations during ATS-6 Experiment the ISRO, Bangalore, India, 1978 (Scientific Note ISRO-SN-07-78). For the present study, the average midnight hours ( 23:00 to 01:00 Hrs) and noon hours (11:00 to 13:00 Hrs) TEC data of each station are grouped according to the Bi values ranges from 0 to 2 nT, 2 to 4 nT … etc., whereas for Bz values the groups are from the range -3 to 2 nT, -2 to -1 nT, -1to 0 nT, 0 to 1 nT, 1 to 2 nT, 2 to 3 nT and 3 to 4nT. Similarly the grouping of entire TEC data are made for the Dst values ranges from -60 to -40 nT, 40 to -20 nT, -20 to 0 nT, 0 to 20 nT, 20 to 40 nT, whereas for AE values the groups ranges from 100 to 200 nT, 200 to 300 nT, 300 to 400 nT, 400 to 500 nT, 500 to 600 nT. The TEC data are grouped according to the IEF values ranges from -4 to -3mV/m, -3 to -24 mV/m, -2 to -1 mV/m, -1 to 0 mV/m, 0 to 1mV/m, 1 to 2mV/m, 2 to 3mV/m. The mean and standard deviation of TEC value for each group were computed; the mean values have been represented by the some specific symbol whereas the bars shows the standard deviation( Figs. 1 to 5). In order to remove the effect of seasonal variation of TEC aspect, the respective group values of the geomagnetic and interplanetary field parameters with TEC for each season are also analysed separately. Furthermore, the correlation coefficient between TEC and various geomagnetic and interplanetary field parameters are computed for each noon and midnight hours of each season and presented in tabular form Table 1. Due to limitation of data, the result of the particular station and season are discussed in the present paper in which significant correlation coefficient value occurs above.80 i.e., test of confidence level become 95% level.

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3. Results and discussion From the analysis of Figures 1 to 5 and Table-1, the following points are emerged as follows: (i) Figures 1 (a, b) represent the variation of noon and midnight hours average TEC against Bi for summer, winter and equinoxes over different stations. It is observed from figures and Table-1 that on an average, noon hours TEC variation bears a positive relationship with Bi during summer as well as winter months and the corresponding negative relationship in equinoxes months over equatorial and away from equatorial station. However, the significant positive correlation coefficient values above.8 occurs for Ahmedabad station only in summer and over Thumba, Delhi in winter, while in equinoxes, the negative value of correlation coefficient above.8 is observed for Ahmedabad only. It is seen that the extent of variation in summer and winter noon hour average TEC values increases in the magnitude of 3 to 8TEC unit with increasing the Bi values from 2 to 10 over offside of equatorial station only. In contrary, during equinoxes, effect of Bi on the noon time TEC values shows decreasing trend of the order of 5 to 15TEC with increasing in similar change in value of Bi over the different stations. It gave the evidence about existence of plasma enhancement or TEC enhancement phenomena in both summer and winter, but, in equinoxes, it show the TEC depletion phenomena. As far as the midnight TEC values is concerned, the quiet similar trend with marginal change in the increasing TEC values of the order 2 TEC with increasing the Bi values from 2 to 10 is observed in summer and winter over different stations, whereas, in equinoxes, midnight hours TEC does not manifest any appreciable change with similar changing the value of Bi in minimum solar cycle. Thus the relationship between Bi with noon hours TEC values depicts the more pronounced positive trend in winter than in summer. But in equinoxes months, relationship with Bi and TEC does show the opposite trend in comparison to other seasons. Although, the midnight hour results show the weak as well as similar trend in both winter and summer months. (ii) The mean variations of TEC values in mid night and mid day hours during all seasons over specified stations with North- South component of IMF Bz are illustrated in the Figs.2(a, b) which reveal that influence of Bz on both noon and midnight hours does not seem appreciable change as compared to observed effect in other considered parameters. It is noted at here that magnitude of change in Bz is very low in low solar activity year. Due to constraint of low variation in amplitude of Bz, no definite conclusion can be seen drawn from the present work. (iii) The average variations of TEC of midnight and noon hours value in each season for different stations with Dst are presented in Figures 3(a & b). From the figures, summer and winter noon hours TEC values are found to be decreased from 10 to 3 TEC values with change in Dst values from -30 to


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+30 nT. But in equinoxes, marginal increase had been seen from 2 to 5 TEC for the same change in the values of Dst. It leads to change the rang errors of the order of at least.5 meter to 1.7 meters even in low solar activity condition. Thus, similar trends have been emerged from their respective correlation coefficient values given in Table-1 that noon hours TEC values over equatorial and away from equatorial stations decrease with increasing Dst during winter and summer months and hence show the negative correlation coefficient values. While, in equinoxes month, mean TEC values are found to occur slight increase with Dst during equinoxes months and therefore exhibit the positive correlation coefficient value. For the midnight hour group, on an average, significant negative correlation coefficient values are found between mid night hours TEC value with Dst in each season for all the stations except, in equinoxes month over Delhi. The reduction of mean mid night TEC values with Dst is rather so small order of 2.5 TEC in comparison to observed change values in noon hours and therefore do not exhibit any significant range errors values in minimum solar activity level. Thus, it is seen that the control of geomagnetic indices i.e., Dst on TEC values seems to be more in noon hours and the least in mid night hours. As far as concerned to the seasonal dependence, magnitude of noon and mid night hours TEC values both in summer and winter months shows the decreasing trend with corresponding increasing of Dst value over all the stations. However, in equinoxes months, variation of average TEC with Dst does not show any appreciable variation. (iv) Plots of noon time TEC values in winter and equinoxes season with Interplanetary Electric Field (IEF) are shown over different stations in Fig. 4 (a). It is quite clear from the figures that the magnitude of noon time TEC increases order of 11TEC unit in equinoxes and 7 TEC unit in winter with the turning of IEF from west to east i.e., -2 to + 3 mV/m. From Fig. 4(b), it is clear that mean TEC of mid night hours is nearly independent of IEF in summer and equinoxes. But, in winter season, marginal increase in mid night hour TEC values with turning of west to east has been noticed over offside of equatorial stations like Bombay and Ahmedabad. (v) The average variation of noon and midnight hours TEC values with Auroral Electrojet (AE) index are illustrated for equatorial and off side of equatorial site in Figures 5(a, b). During winter month, average value of TEC values of all the stations is found to be increased from 20 to 30 TEC unit with increasing the values of AE from 100 to 900, while in midnight hours, enhancement in TEC values is rather small order of few TEC i.e., 3 to 4 with the similar change in AE values. However, in equinoxes, there is no consistent variation as evident from the negative values of correlation coefficient in the similar studies.

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Figure 1(a, b ): Average Variation of noon and mid night hours TEC values with Bi over different stations during each season (1975-1976).


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Figure 2(a, b ): Average Variation of noon and mid night hours TEC values with Bz over different stations during each season (1975-1976).

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Figure 3(a, b ): Average Variation of noon and mid night hours TEC values with Dst over different stations during each season (1975-1976).


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Figure 4(a, b ): Average Variation of noon and mid night hours TEC values with IEF over different stations during each season (1975-1976).

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Figure 5(a, b ): Average Variation of noon and mid night hours TEC values with AEindex over different stations during each season (1975-1976).


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Table-1: Correlation Coefficients between TEC and Geomagnetic and Interplanetary Field Parameters Season

Station

AE Bi Bz Dst EF Noon Mid Noon Mid Noon Mid Noon Mid Noon Mid night night night night night EquinoxAhmedabad -0.6 -0.41 -0.94 0.54 0.34 0.24 0.34 -0.22 0.98 0.81 Bombay No data -0.41 -0.44 -0.44 0.73 0.85 0.73 -0.93 -0.67 -0.38 Delhi -0.69 No data -0.58 -0.58 -0.84 0.78 0.84 0.83 0.8 0.84 Thumba No dataNo dataNo data No dataNo data 0.95 No dataNo data No data-0.54 SummerAhmedabad No dataNo data 0.9 0.91 -0.64 -0.18 -0.52 -0.99 No data-0.88 Bombay No dataNo data 0.69 0.34 -0.55 -0.21 -0.91 -0.94 No data-0.05 Delhi No dataNo data -0.15 0.82 -0.45 -0.18 -0.54 -0.19 No data-0.63 Thumba No dataNo data 0.091 0.41 0.88 -0.95 -0.95 No data No data 0.23 Winter Ahmedabad 0.76 0.76 0.42 0.89 -0.31 -0.65 -0.85 -0.95 0.88 0.67 Bombay 0.038 0.63 0.53 0.66 0.35 -0.86 -0.55 -0.14 0.58 0.78 Delhi 0.66 0.66 0.83 0.47 -0.42 -0.74 -0.99 -0.49 0.83 0.36 Thumba Nodata 0.66 0.88 0.85 0.36 -0.55 -0.93 -0.98 0.63 0.46

4. Discussion and conclusions From the above results, it may be quite evident that there is close association between TEC over both Equatorial as well as Anomaly Crest region with the interplanetary field and geomagnetic indices which shows the seasonal as well as local time dependence. In the present investigation, the relative effect of all the IMF and geo magnetic activity parameters on TEC variation is seen more prominent in noon hours as compared to midnight hours values of TEC variations. It may be due to the fact that the solar wind magnetosphere coupling is enhanced whenever the IMF convicted by the solar wind to the day side magnetosphere (Gonzalez et al, 1994). As a results of this the solar wind both controls the size of magnetic cavity through its momentum flux or dynamic pressure and energy flow into the magnetosphere coupled from the solar wind mechanical energy flux by the reconnection of the IMF with and the geomagnetic field. However, this coupling is strongly influenced the direction of IMF, being most strong when the IMF is southward. During the interaction between solar wind and magnetosphere under such southward condition of the Bz, high value of AE- index and large negative excursion of negative Dst values cause a change in the region -1 current leading to a sudden increase in the dawn- to – dusk convection eastward electric field at high latitudes (Sreeja et al, 2009; Kelley et al, 2002; Hunag et al, 2005). This results in instantaneous penetration of electric field from high latitude to the mid and equatorial ionosphere. These transient electric fields have typical rise and decay times, shorter than about the 15 min and life time of about an hour or more than an hour in some cases (Sastri, 1988; Sastri et al. 1992; Sreeja et al., 2009) These prompt penetrating electric field have eastward polarity in day

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time and westward in night time. In the recovery phase (positive value of Bz, positive value of Dst and higher value of AE index) the electric field penetrates to equatorial latitudes with opposite polarity gets favourable condition to interconnect with earth’s magnetosphere, under such condition, a large amount of electromagnetic energy is dissipated in the form of thermal heat energy over high latitude stations leading to profound change in electrodynamics condition or electrical parameters of the upper atmosphere (Fuller et al., 2002, Kelly et al., 2003). These perturbations are not restricted to higher altitude as well as higher latitude, rather they may extend all the way to lower atmospheric height and up to equatorial stations results of coupled interaction of altering the Global Upper Atmospheric Circulations Wind Pattern as well as electrodynamics conditions( Jain et al., 2010; Basu et al., 2010, Wolf, 2007). Although, the present analysis is based on the TEC data of low solar activity period and due to constraint of observed very low value of southward component of IMF, direction of IMF values or magnitude of ring current does not show appreciable variation, therefore, the present study reports the insignificant influence of Bz on TEC. However, in the present study, the magnitude of IMF (Bi) seems to show the more prominent influence on TEC changes due to the fact that dynamic interaction between solar wind and magnetosphere causes electrical current between the magnetosphere and high latitude ionosphere along the geomagnetic field lines. As well as due to the dynamical coupling between high and low latitudes or large electromagnetic energy transfer which are deposited in magnetospheric height during high magnitude of IMF (Bi) from high latitude to equatorial latitude stations, results the large scale redistribution of TEC, hence, support the present findings of enhancement in TEC of the peak order of 8 TEC units in winter and summer and reduction in TEC magnitude of the order of 15TEC in equinoxes. These perturbation in magnitude of TEC further lead to errors in the object position measurement by GPS is of order of at least ± 2 meter during such minimum space weather disturbance period ( Jain et al., 2010; Huang et al., 2005). Regarding from the results of IEF, significant enhancement in noon time TEC magnitude are clearly visible during the increasing eastward IEF, maximum negative value of Dst– index, high value of AE index in winter and equinoxes. But, in midnight hours, its variation are less effective while turning the direction of IEF from west to east and positive value of Dst- index. Furthermore, present investigations also support the view of large changes in values of eastward IEF, maximum magnitude of negative Dst-values and AEindex seems to be one of plausible causes of perturb the net ionospheric electric field. In the same lines, previous workers have demonstrated the perturbation in the zonal electric field in the equatorial low latitude ionosphere brought forth by prompt penetration of east ward IEF, negative excursion time period of Dst magnitude from the magnetosphere in low and equatorial


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ionosphere from magnetospheric electric field origin as PPE and complete opposite trend due to DDE ( ionospheric–disturbance dynamo) during such space weather disturbances (Kelly et al, 2003 ). They were also reported the coincidence of enhancement in ionospheric eastward electric field in day hours due to prompt penetration of IEF along with increasing of upward vertical drift, and therefore uplifting of ionospheric F- region height over equatorial and low latitude stations which were primary possible cause of observed ionospheric TEC enhancements during mid day hours. In contrary to this, enhancement in ionospheric zonal electric field in the reverse direction i.e., westward direction in night hours during such specified occasions lead to produce the downward vertical drift velocity and reduction of ionospheric Fregion height in night hours as the cause of plasma depletion reduction of midnight time TEC. which created the reduction in TEC as observed in midnight hours. Thus, in the present studies, increase of IEF in eastward direction give the inference about the views of the enhancement in TEC during midday hours, but in midnight hours, the enhancement westward electric field component reveals the reduction of TEC value as result of reversing the downward drift. Here, it has been quite interesting to note that the relationship between Dst and midday and night time TEC are more clear in comparison with other above mentioned parameters. The TEC values systematically decrease with the increase of Dst values in summer and winter. But, in equinoxes, above trends are in opposite nature. It is also expected that Dst variation is also the one of suitable index to study the dependence of TEC variation or low and equatorial ionospheric irregularities as it does not include the auroral sources (Aarons et al, 1997, Biktash, 2004). As present work is carried out using the limited period set of data for low solar activity period, if similar studies would be carried out by using longer period of TEC data of different stations for mid and high solar activities then definite conclusion can be made., However, the present study has given the clear indication that there is relationship of TEC with the geomagnetic and interplanetary field parameters like Dst, AE, Bi and IEF. So further detailed studies in these direction are essential in the present context of estimation and correction of propagation delays in the Global Positioning System (GPS); improving the accuracy of satellite navigation; predicting changes due to ionospheric storms; predicting space weather effects on telecommunications.

Acknowledgements Authors are grateful to the ISRO, Bangalore, India for providing the Scientific Report on TEC data collected of Indian Stations during ATS-6 Experiment. Thanks are also due for World Data Centre for Geomagnetism, Kyoto, Japan for downloading the geomagnetic and interplanetary field data.

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