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Plasmaspheric electron content (PEC) over low latitude regions around the magnetic equator in the Indian sector during different geophysical conditions
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Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 1066–1073 www.elsevier.com/locate/jastp
Plasmaspheric electron content (PEC) over low latitude regions around the magnetic equator in the Indian sector during different geophysical conditions G. Manju, Sudha Ravindran, C.V. Devasia, Smitha V. Thampi, R. Sridharan Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695 022, Kerala, India Received 24 February 2007; received in revised form 12 December 2007; accepted 9 January 2008 Available online 20 January 2008
Abstract The variation of plasmaspheric electron content (PEC) is an important parameter for studying the effects of space weather events in the low latitude ionosphere. In the present study, the vertical TEC (VTEC) measurements obtained from co-located dual-frequency Global Positioning System (GPS) and Coherent Radio Beacon Experiment (CRABEX) systems have been used. The daytime PEC variations under different geophysical conditions have been estimated (around the magnetic equator) over the Indian sector, for the first time. The first observations of the nighttime PEC variations over the Indian sector are also estimated from the simultaneous measurements of Faraday rotation, differential Doppler and modulation phase delay made using the CRABEX system on-board the Indian geostationary satellite GSAT2. The study shows that the PEC varies over a range of 10–22% (of the total electron content (TEC)) during daytime of magnetically quiet period. There is an increase in PEC with latitude during magnetically quiet period. During a magnetically disturbed period of 9 November 2004, the PEC increased to 30% of the TEC over the magnetic equatorial location of Trivandrum (8.51N, 76.91E, dip 0.51N), while at Bangalore (131N, 781E, dip 101N) it showed a large depletion. The implications of the new observations are discussed. r 2008 Published by Elsevier Ltd. Keywords: Plasmaspheric electron content; Equatorial ionization anomaly; Total electron content
1. Introduction It is well known that the position accuracy achievable from navigation satellites (e.g. Global Positioning System (GPS)) is largely affected by the intervening ionosphere. The range error is directly proportional to the total electron content (TEC) along the ray path. This TEC has an ionospheric electron content (IEC) part (below 1000 km) and a plasmaCorresponding author. Tel.: +91 471 2563563; fax: +91 471 2706535. E-mail address: [email protected] (G. Manju).
1364-6826/$ - see front matter r 2008 Published by Elsevier Ltd. doi:10.1016/j.jastp.2008.01.006
spheric electron content (PEC) part (in the region above 1000 km). Measurements of IEC have been made using satellite beacons (Titheridge, 1973; Davies, 1980; Balan et al., 1993; Ciraola and Spalla, 1997; Thampi et al., 2005). These and similar measurements have been used to develop models of IEC variations. Single frequency GPS receivers account for the ionospheric contribution to the time delay by using certain models like the Klobuchar model (Klobuchar, 1987). But these models do not account for the electron content in the plasmasphere, which is the region above 1000 km. Early PEC estimates were made from the American and European sectors, by taking the
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difference between the electron content measured by the differential Doppler and the Faraday rotation technique using radio beacons on geostationary satellites (Poletti-Liuzzi et al., 1976; Soicher, 1976; Kersley and Klobuchar, 1978; Ogawa et al., 1980). With the advent of navigation systems like GPS, the need to quantify the contribution of the plasmasphere to the time delay of the GPS signal has come to the fore. Accordingly, in recent times certain PEC measurements have been made in the American and European sectors in the mid-latitudes (Doherty et al., 1992; Lunt et al., 1999). PEC measurements in the mid-latitudes of the Asian sector have been made by Balan et al. (2002). The present study, examines the PEC variations over the Indian sector around the magnetic equator for the first time. It highlights the need to quantify the PEC variations at low latitudes especially in the light of the increasing use of GPS for purposes of navigation and positioning. 2. Data and method of analysis The present study has been carried out using TEC values derived from GPS and Coherent Radio Beacon Experiment (CRABEX) data from a number of receiving stations along the same meridian. The time delay of the incoming signal is a function of the TEC in a GPS system. It is given by the expression, 40:3 time delay; Dt ¼ ½TEC, (1) cf 2 range error Ds ¼ ðcDtÞ, (2) R where TEC ¼ N ds N is the electron density along the ray path from the GPS altitude to the ground and ds is an element of length along the path, f is the transmitted frequency and c is the free space velocity of light. As mentioned earlier, the TEC values obtained using GPS system represent the electron content along the line of sight up to the satellite altitude (20,000 km), while those obtained from LEOs segment of CRABEX system (using beacon transmissions at 150 and 400 MHz from low earth orbiting satellites) represent the electron content up to 1000 km. In certain previous studies the transition height from O+ to H+ is taken as the base of the plasmasphere. It is seen that the diurnal mean transition height varies from 950 to 1150 km during different seasons (Balan et al., 2002), while in
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the present study the boundary of the ionosphere is taken as 1000 km. The Absolute Slant GPS-TEC values derived from the carrier phase delays and pseudoranges of the GPS signals (L1 andL2) are then converted to absolute vertical TEC (VTEC) values following the standard procedure using the mapping function as given below. VTEC ¼ STEC cosðwÞ,
(3)
where w is the zenith angle at ionospheric pierce point (IPP) which is estimated from the satellite elevation angle. Here, only those ray paths with elevation angles greater than 501 are used. It has been shown by Ramarao et al. (2006) that an elevation angle cut off of 4501 is ideally suited to represent the TEC over the Indian sector. IPP is the point where the line joining the satellite and the receiver cuts the ionosphere at an altitude where the entire ionization is assumed to be concentrated (single shell model). For the LEOS segment of the CRABEX system, the relative slant TEC values are obtained from the differential Doppler measurements at 400 and 150 MHz. The relative VTEC is estimated from the relative slant TEC using the cos(w) mapping function given in Eq. (3). The relative TEC values are converted to absolute VTEC by removing the 2np ambiguity using the Leitinger two-station method (Leitinger et al., 1975). The daytime PEC estimates have been made, by taking the differences between the GPS (o20,000 km) TEC and the CRABEX (o1000 km) derived ionospheric TEC (IEC) values estimated nearly simultaneously. For the nighttime PEC estimates reported here, the basic electron content measurements were made using the data obtained from the transmissions of CRABEX beacon payload onboard the Indian geostationary satellite GSAT2, at four linearly polarized frequencies 400, 399, 150 and 149 MHz. The differential Doppler and modulation phase delay measurements together yield the line of sight TEC up to the satellite altitude of 36,000 km. The Faraday rotation measurement gives a measure of the rotation of the plane of polarization of a radio wave as it travels through the ionosphere. The data from the Faraday rotation measurements essentially give the IEC part for altitudes below 2000 km beyond which the effect of the geomagnetic field is negligible. The IEC contribution (from Faraday rotation measurement) is removed from the TEC to derive the PEC values.
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It is to be noted that the nighttime GSAT based estimates of PEC reported here correspond to the electron content in the region between 2000 and 36,000 km along the slant path. Hence, they are not directly comparable with the GPS and CRABEX LEOS based measurements, which are made almost overhead. DH values (the deviation of the horizontal component of the earth’s magnetic field H from its mean nighttime level) and ionosonde data at Trivandrum and SHAR have also been used for the study. 3. Results and discussion The variabilities of TEC, IEC and PEC during different geophysical conditions are presented in the following section. Comparisons with the IRI model derived TEC values are also given. 3.1. Plasmaspheric electron content during magnetically quiet period and comparison of GPS and CRABEX derived electron content with IRI model values The PEC has been estimated for selected magnetically quiet days in the solstice period of the low solar activity year of 2004. Fig. 1 shows the GPS-
TEC, CRABEX-TEC, IRI model derived TEC and PEC variations at Trivandrum (TRV), an equatorial station and Bangalore (BLR), a low latitude station, on 8 June 2004 (Ap ¼ 9). The two top panels depict the GPS-TEC (TEC), while the two middle panels depict the CRABEX-TEC (IEC). The bottom panels correspond to the PEC obtained by removing the IEC from the TEC. The IRI-model derived TEC values for altitudes below 2000 and 1000 km are also shown in the top and middle panels, respectively, of Fig. 1. It is clearly seen from the figure that the IRI-model is over estimating the TEC, with the model-derived electron content for altitudes o2000 km itself being much higher than the GPS-TEC (up to 20,000 km). The over estimation is much more for TRV than for BLR. Similarly the IRI model-derived electron content values for altitudes o1000 km are also much higher than the CRABEX-TEC values. This only indicates the limitations of the models like the IRI for application purposes, like positioning and navigation, in view of the substantial overestimation of TEC values, over Indian longitudes. Further, it is seen that the PEC value over TRV varies from 11.7% to 14% of the corresponding TEC from morning (10:00 IST) to evening (16:00 IST) on 8 June 2004. At BLR, the PEC is found to be around
Fig. 1. Variations of GPS-TEC (top panels), CRABEX-TEC (middle panels), IRI model derived TEC and PEC (bottom panels) at TRV and BLR on 8 June 2004.
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24% of the TEC during evening times. For all the quiet days examined it is seen that the PEC values range from 4 to 6 TECU over TRV and from 7 to 9 TECU over BLR. The percentage of PEC is found to range from 10% to 20% of the TEC over TRV and from 15% to 25% over BLR. Thus, one could infer an increase in PEC from TRV to BLR during magnetically quiet times. The cause of the increase in PEC from TRV to BLR is an aspect which needs to be examined in detail with a larger database. The PEC values of 2–4 TECU that have been reported for the European and American sectors during low solar activity (Poletti-Liuzzi et al., 1976; Soicher, 1976; Doherty et al., 1992; Lunt et al., 1999), are consistently lower than the present estimates made over the Indian sector. Estimates made using GPS data and SUPIM (Sheffield University plasmasphere ionosphere model) over the mid-latitude Asian sector by Balan et al. (2002) have shown that PEC is of the order of 8–11 TECU during high solar activity. During low solar activity over the Asian mid-latitude sector, using GPS data and MU radar observations, Otsuka et al. (2002) have reported PEC values of the order of 3–4 TECU. The percentage contribution of PEC during high solar activity as given by Balan et al. (2002) varies from 10% to 20%. Moreover, they have shown that the PEC has a significantly decreasing trend with latitude. The present estimates, however show significantly high PEC values over low latitudes around the magnetic equator (within the EIA crests) even during low solar activity. Further these PEC estimates also register an increase with
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latitude (within the EIA crests). These new aspects of the PEC variabilities over the latitudes around the magnetic equator in the Indian sector, underlines the need for quantifying the same. 3.2. Plasmaspheric electron content during magnetically disturbed period Fig. 2 shows the temporal variations of DH (left panel) at the magnetic equatorial location of TRV and Dst (right panel) from 6 to 12 November 2004. This was a magnetically disturbed period with disturbance effects being seen from the noon of 7 November 2004 onwards. It is seen from the figure that there is a sudden commencement of the storm at 12:00 h and a consequent increase in the magnetic field around that time. Thereafter, the DH values decreased corresponding to the main phase of the storm, which extends up to the noon of 8 November 2004. The ring current development, which causes the main phase decrease of DH, is clearly seen in the Dst variability. After the main phase when the ring current decays and the Dst values start recovering, it gets reflected in DH too. The estimate of PEC is made for the noon time of 9 November 2004, when the DH values had more or less recovered from the storm effects. It is also to be noted that there is a recurrent storm activity in the afternoon of 9 November 2004 as is indicated by the Dst variations and these disturbance effects persist for a couple of days. The upward arrow (left panel) indicates the time for which PEC estimates are made.
Fig. 2. The temporal variations of DH (left panel) and Dst (right panel) from 6 to 12 November 2004.
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Fig. 3 illustrates the latitudinal variations of VTEC from GPS data for the stations of TRV, BLR, Hyderabad (HYD) and Bhopal (BHO) at three different times on 9 November 2004 (storm day) and 4 November 2004 (quiet day). At 09:30 IST on the quiet day as well as on 9 November 2004, the VTEC variations show the development of the EIA, with the trough close to the magnetic equatorial location of TRV (8.51N) and the crest over HYD (17.31N). But the VTEC variations at 12:30 IST shows the inhibition of the EIA on the disturbed day while on the quiet day at the same time it is seen that the anomaly is well developed, i.e., on the quiet day at 12:30 IST, the VTEC variations show the trough over TRV and crest over HYD, whereas on the disturbed day, the anomaly seems to be inhibited. This situation of an inhibited anomaly persists at 15:30 IST also, on the disturbed day, while the variations on the control day show a well-developed anomaly. During magnetic disturbances, the disturbance electric fields could enhance the development or inhibition of the equatorial ionization anomaly, whereas the disturbance winds seem to always inhibit EIA (Abdu, 1997). It is reported that EIA can undergo enhancement (expansion) due to the magnetospheric disturbance electric fields, which penetrate to low latitudes during the growth phase of a storm/substorm,
whereas EIA inhibition occurs more often under a disturbance dynamo (DD) electric field. During very severe magnetic storms the EIA has been shown to undergo dramatic expansion even up to midlatitudes with a very wide low latitude trough (Mannucci et al., 2005). The relative roles of different mechanisms, responsible for the suppression of the equatorial anomaly were studied numerically by Pavlov et al. (2004). In the presence of any such effect during the storms, inhibition of the EIA can occur and this inhibition will be seen in the ionosonde measurements also. In this context, it is interesting to examine the latitudinal variations of peak electron density (Nmax) as depicted by ground based ionograms. Nmax (m3) is given as 1/81[f2], where f is the critical frequency of the F layer in MHz. The latitudinal variations of the foF2 values at 12:30 IST as deduced from the TRV and Shriharikota, SHAR (13.71N, 80.21E; dip 101N) ionograms on the storm day (Fig. 4) reveal, that the EIA has indeed developed at 12:30 IST, while the TEC values show the contrary. This implies that the increased electron content shown at TRV by the GPS data is probably produced by an increase in the topside content extending up to the plasmasphere. In order to cross check the above statement, PEC values were deduced from GPS and CRABEX data. Fig. 5 depicts the variations of GPS-TEC (top left panel), CRABEX-TEC (bottom left panel) and PEC (right panel) on the quiet day (4 November 2004) and the disturbed day (9 November 2004). The development of EIA at 12:30 IST is brought out by the CRABEX data on both the quiet day and the disturbed day, while as mentioned previously, the GPS-TEC shows the inhibition of the anomaly on the disturbed day. Using the
Fig. 3. The latitudinal variations of VTEC from GPS data at three different times on 9 November 2004 (disturbed day) and 4 November 2004 (control quiet day).
Fig. 4. The latitudinal variations of the foF2 values at 12:30 IST as deduced from TRV and SHAR ionograms on 9 November 2004 (disturbed day).
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Fig. 5. The variations of GPS-TEC (top left panel), CRABEX-TEC (bottom left panel) and PEC (right panel) on the control day (4 November 2004) and the disturbed day (9 November 2004).
simultaneous GPS and CRABEX TEC values the PEC is estimated for both the quiet and disturbed days. While the PEC deduced for the quiet day is in agreement with the PEC estimated for the previously presented quiet days both in magnitude and percentage, for the disturbed day at TRV it is seen that the PEC shot up to 23 TECU which is significantly higher than the typical values of 4–6 TECU during quiet times. For BLR, the PEC is seen to be depleted to 4.5 TECU, much lower than the quiet time values of 7–9 TECU. Thus, the percentage contribution of PEC to GPS-TEC at TRV turned out to be very large during this particular phase of the storm. The present analysis shows large TEC enhancements at TRV and depletions at BLR, which is only 51 to the north of Trivandrum indicating the presence of large spatial fluctuations in TEC. This shows that during storm time, there is significant redistribution of plasmaspheric ionization, and this aspect has to be investigated in detail using a larger database. Large spatial fluctuations in topside electron content during the storm of 20 November 2003 as measured by the CHAMP space craft (at 400 km) have been reported by Yizensaw et al. (2006). They have observed very large spatial topside electron content fluctuations on the storm day in relation to
the measurements made on the control day of 19 November 2003. The observations made during the present study are also showing a similar pattern. It therefore seems that it is not possible to completely summarize the scenario during storms merely by discussing about the effect of the different disturbance electric fields and winds on the ionosphere. A full understanding of the behavior of the storm time upper atmosphere requires more quantitative information on the plasmasphere and factors that modulate the PEC. In the context of the increased use of GPS system for positioning and navigation purposes, measurement accuracies of better than 3 TECU are ideally required. With single frequency GPS users resorting to ionospheric models without including PEC variations for removing ionospheric error, the derived position information will be highly erroneous at low latitude regions more so during magnetically disturbed periods as shown in the present analysis. The PEC values even during quiet times are 10–20% of TEC and during storm time they are substantially more at the equator (30%), thus greatly degrading the quality of single frequency GPS based positioning. These observations thus highlight the need to develop the climatology of PEC variations.
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3.3. Nighttime plasmaspheric content on 1 February 2006 Fig. 6 shows the temporal variation of nighttime electron content on 1 February 2006, which is a magnetically quiet day (Ap ¼ 4) obtained from the Indian Geostationary satellite GSAT. Here, TEC corresponds to the TEC in the entire ray path up to 36,000 km, while IEC refers to the Faraday rotation based electron content, which represents the ionospheric part (IEC) of the total content. The IEC is found to be higher (27 TECU) in the evening hours at 18:30 h while it decreased subsequently and stabilized at a level of 23 TECU. The reduction in IEC in the later part of the night is essentially due to the chemical recombination effects. The almost constant level of IEC in the latter part of the night is attributed to increased
Fig. 6. The temporal variation of nighttime total electron content and IEC on 1 February 2006.
lifetime of the ions. The TEC is found to show temporal fluctuations, which are believed to be due to the fluctuations in the plasmaspheric part of the electron content. Fig. 7 illustrates the nighttime variations of PEC (left panel) and percentage PEC (right panel) on 1 February 2006. The PEC value increased in the initial part of the night and attained a maximum value of 15 TECU at 20:00 h. Thereafter, it decreases to 12 TECU at 21:30 h. The PEC variation is over a range of 9–15 TECU during the entire duration for which data were available. The percentage variation of PEC ranged from 25% to 40%. Earlier measurements of nighttime PEC, of the order of 56% of the total content have been reported in the mid-latitude American sector, during low solar activity by Poletti-Liuzzi et al. (1976) and Soicher (1976). In the mid-latitude Asian sector, during high solar activity, Balan et al. (2002) have reported, nighttime PEC variations of the order of 60% of the TEC. The present estimates made for the first time at low latitude regions around the dip equator in the Indian sector, seem to be of the same order as has been obtained by previous workers. These estimates highlight the significant modulation of TEC by the PEC especially at night. It underlines the need to quantify the nighttime PEC variabilities for reliable position estimation using GPS. Balan and Bailey (1995) have shown using the SUPIM model that strong nighttime increase occurs in TEC due to reverse fountain effects. During daytime, when the ExB drift is upward, the fountain rises to about 800 km at the equator and covers a wide range of latitudes; at regions above the fountain there is a plasma flow towards the equator
Fig. 7. The time variations of PEC (left panel) and percentage PEC (right panel) on 1 February 2006.
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from both sides. In the evening as soon as the drift turns downwards, the fountain becomes a reverse fountain with supply of ionization in both hemispheres from regions above the fountain. But the ionization in the region below 1000 km decreases because of rapid recombination while at higher altitudes in the plasmasphere there is no such effect resulting in an increase in PEC during nighttime. 4. Conclusions The first estimates of PEC in the low latitude Indian sector have been presented here for both daytime as well as nighttime. The results bring out the significant contribution of PEC to the TEC (along the ray path from the satellite to the receiver), at these locations at all local times. It brings into focus the inadequacy of present day models being used in single frequency GPS receivers to accurately predict the error induced in GPS positioning by the ionized medium. The present study therefore emphasizes the urgent need to quantify the PEC variations during different geophysical conditions so that the models can be fine tuned accordingly. References Abdu, M.A., 1997. Major phenomena of the equatorial ionosphere–thermosphere system under disturbed conditions. Journal of Atmospheric and Terrestrial Physics 59, 1505–1519. Balan, N., Bailey, G.J., 1995. Equatorial plasma fountain and its effects: possibility of an additional layer. Journal of Geophysical Research 100, 21421–21432. Balan, N., Bailey, G.J., Jayachandran, B., 1993. Ionospheric evidence for a non-linear relationship between the solar EUV and 10.7 cm fluxes during an intense solar cycle. Planetary and Space Science 41, 141–145. Balan, N., Otsuka, Y., Tsugawa, T., Miyazaki, S., Ogawa, T., Shiokawa, K., 2002. Plasmaspheric electron content in the GPS ray paths over Japan under magnetically quiet conditions at high solar activity. Earth Planets and Space 54, 71–79. Ciraola, L., Spalla, P., 1997. Computation of ionospheric total electron content using NNSS and the GPS. Radio Science 32 (3), 1071–1080. Davies, K., 1980. Recent progress in satellite radio beacon studies with particular emphasis on the ATS-6 radio beacon experiment. Space Science Reviews 25, 357. Doherty, P.H., Klobuchar, J.A., Bailey, G.J., Balan, N., Fox, M.W., 1992. Determinations of protonospheric electron content from TEC measurements from GPS and Faraday rotation and comparisons against the Sheffield plasmasphere
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model. In: Lee, M.C. (Ed.), Proceedings of the International Beacon Satellite Symposium. Massachusetts Insttiute of Technology, Cambridge, pp. 118–121. Kersley, L., Klobuchar, J.A., 1978. Comparison of protonospheric electron content measurements from the American and European sectors. Geophysical Research Letters 5, 123–126. Klobuchar, J.A., 1987. Ionospheric time-delay algorithm for single frequency GPS users. IEEE Transactions on Aerospace and Electronic System AES-23, 325–331. Leitinger, R., Schmidt, G., Tauriainen, A., 1975. An evaluation method combining the differential Doppler measurements from two stations that enables the calculation of electron content of the ionosphere. Journal of Geophysics 41, 201–213. Lunt, N., Kersley, L., Bailey, G.J., 1999. The influence of the protonosphere on GPS observations: model simulations. Radio Science 34, 725–732. Mannucci, A.J., Tsurutani, B.T., Iijima, B.A., Komjathy, A., Saito, A., Gonzalez, W.D., Guarnieri, F.L., Kozyra, J.U., Skoug, R., 2005. Dayside global ionospheric response to the major interplanetary events of October 29–30, 2003 ‘‘Halloween storms’’. Geophysical Research Letters 32, L12S02, doi:10.1029/2004GL021467. Ogawa, T., Sinno, K., Fujita, M., Awaka, J., 1980. Severe disturbances of VHF and GHz waves from geostationary satellites during a magnetic storm. Journal of Atmospheric and Terrestrial Physics 42, 637–644. Otsuka, Y., Ogawa, T., Saito, A., Tsugawa, T., Fukao, S., Miyazaki, S., 2002. A new technique for mapping of total electron content using GPS network in Japan. Earth Planets and Space 54, 63–70. Pavlov, A.V., Fukao, S., Kawamura, S., 2004. F-region ionospheric perturbations in the low-latitude ionosphere during the geomagnetic storm of 25–27 August 1987. Annals of Geophysics 22, 3479–3501. Poletti-Liuzzi, K., Yeh, C., Liu, C.H., 1976. Simulation and measurement of plasmaspheric electron content. In: COSPAR Satellite Beacon Symposium, Boston University, June. Ramarao, P.V.S., Gopi Krishna, S., Niranjan, K., Prasad, D.S.V.V.D., 2006. Temporal and spatial variations in TEC using simultaneous measurements from Indian GPS network of receivers during the low solar activity period of 2005–2006. Annals of Geophysics 24, 3279–3292. Soicher, H., 1976. Diurnal, day-to-day and seasonal variability of NF, NT and NP at Fort Monmouth, New Jersey. In: COSPAR Satellite Beacon Symposium, Boston University. Thampi, S.V., Ravindran, S., Devasia, C.V., Pant, T.K., Sreeelatha, P., Sridharan, R., 2005. First observation of topside ionization ledges using radio beacon measurements from low earth orbiting satellites. Geophysical Research Letters 32, L11104. Titheridge, J.E., 1973. The electron content of the southern midlatitude ionosphere 1965–1971. Journal of Atmospheric and Terrestrial Physics 35, 981. Yizensaw, E., Moldwin, M.B., Komjathy, A., Mannucci, A.J., 2006. Unusual topside ionospheric density response to the November 2003, superstorm. Journal of Geophysical Research 111, A2308.
Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 1066–1073 www.elsevier.com/locate/jastp
Plasmaspheric electron content (PEC) over low latitude regions around the magnetic equator in the Indian sector during different geophysical conditions G. Manju, Sudha Ravindran, C.V. Devasia, Smitha V. Thampi, R. Sridharan Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695 022, Kerala, India Received 24 February 2007; received in revised form 12 December 2007; accepted 9 January 2008 Available online 20 January 2008
Abstract The variation of plasmaspheric electron content (PEC) is an important parameter for studying the effects of space weather events in the low latitude ionosphere. In the present study, the vertical TEC (VTEC) measurements obtained from co-located dual-frequency Global Positioning System (GPS) and Coherent Radio Beacon Experiment (CRABEX) systems have been used. The daytime PEC variations under different geophysical conditions have been estimated (around the magnetic equator) over the Indian sector, for the first time. The first observations of the nighttime PEC variations over the Indian sector are also estimated from the simultaneous measurements of Faraday rotation, differential Doppler and modulation phase delay made using the CRABEX system on-board the Indian geostationary satellite GSAT2. The study shows that the PEC varies over a range of 10–22% (of the total electron content (TEC)) during daytime of magnetically quiet period. There is an increase in PEC with latitude during magnetically quiet period. During a magnetically disturbed period of 9 November 2004, the PEC increased to 30% of the TEC over the magnetic equatorial location of Trivandrum (8.51N, 76.91E, dip 0.51N), while at Bangalore (131N, 781E, dip 101N) it showed a large depletion. The implications of the new observations are discussed. r 2008 Published by Elsevier Ltd. Keywords: Plasmaspheric electron content; Equatorial ionization anomaly; Total electron content
1. Introduction It is well known that the position accuracy achievable from navigation satellites (e.g. Global Positioning System (GPS)) is largely affected by the intervening ionosphere. The range error is directly proportional to the total electron content (TEC) along the ray path. This TEC has an ionospheric electron content (IEC) part (below 1000 km) and a plasmaCorresponding author. Tel.: +91 471 2563563; fax: +91 471 2706535. E-mail address: [email protected] (G. Manju).
1364-6826/$ - see front matter r 2008 Published by Elsevier Ltd. doi:10.1016/j.jastp.2008.01.006
spheric electron content (PEC) part (in the region above 1000 km). Measurements of IEC have been made using satellite beacons (Titheridge, 1973; Davies, 1980; Balan et al., 1993; Ciraola and Spalla, 1997; Thampi et al., 2005). These and similar measurements have been used to develop models of IEC variations. Single frequency GPS receivers account for the ionospheric contribution to the time delay by using certain models like the Klobuchar model (Klobuchar, 1987). But these models do not account for the electron content in the plasmasphere, which is the region above 1000 km. Early PEC estimates were made from the American and European sectors, by taking the
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difference between the electron content measured by the differential Doppler and the Faraday rotation technique using radio beacons on geostationary satellites (Poletti-Liuzzi et al., 1976; Soicher, 1976; Kersley and Klobuchar, 1978; Ogawa et al., 1980). With the advent of navigation systems like GPS, the need to quantify the contribution of the plasmasphere to the time delay of the GPS signal has come to the fore. Accordingly, in recent times certain PEC measurements have been made in the American and European sectors in the mid-latitudes (Doherty et al., 1992; Lunt et al., 1999). PEC measurements in the mid-latitudes of the Asian sector have been made by Balan et al. (2002). The present study, examines the PEC variations over the Indian sector around the magnetic equator for the first time. It highlights the need to quantify the PEC variations at low latitudes especially in the light of the increasing use of GPS for purposes of navigation and positioning. 2. Data and method of analysis The present study has been carried out using TEC values derived from GPS and Coherent Radio Beacon Experiment (CRABEX) data from a number of receiving stations along the same meridian. The time delay of the incoming signal is a function of the TEC in a GPS system. It is given by the expression, 40:3 time delay; Dt ¼ ½TEC, (1) cf 2 range error Ds ¼ ðcDtÞ, (2) R where TEC ¼ N ds N is the electron density along the ray path from the GPS altitude to the ground and ds is an element of length along the path, f is the transmitted frequency and c is the free space velocity of light. As mentioned earlier, the TEC values obtained using GPS system represent the electron content along the line of sight up to the satellite altitude (20,000 km), while those obtained from LEOs segment of CRABEX system (using beacon transmissions at 150 and 400 MHz from low earth orbiting satellites) represent the electron content up to 1000 km. In certain previous studies the transition height from O+ to H+ is taken as the base of the plasmasphere. It is seen that the diurnal mean transition height varies from 950 to 1150 km during different seasons (Balan et al., 2002), while in
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the present study the boundary of the ionosphere is taken as 1000 km. The Absolute Slant GPS-TEC values derived from the carrier phase delays and pseudoranges of the GPS signals (L1 andL2) are then converted to absolute vertical TEC (VTEC) values following the standard procedure using the mapping function as given below. VTEC ¼ STEC cosðwÞ,
(3)
where w is the zenith angle at ionospheric pierce point (IPP) which is estimated from the satellite elevation angle. Here, only those ray paths with elevation angles greater than 501 are used. It has been shown by Ramarao et al. (2006) that an elevation angle cut off of 4501 is ideally suited to represent the TEC over the Indian sector. IPP is the point where the line joining the satellite and the receiver cuts the ionosphere at an altitude where the entire ionization is assumed to be concentrated (single shell model). For the LEOS segment of the CRABEX system, the relative slant TEC values are obtained from the differential Doppler measurements at 400 and 150 MHz. The relative VTEC is estimated from the relative slant TEC using the cos(w) mapping function given in Eq. (3). The relative TEC values are converted to absolute VTEC by removing the 2np ambiguity using the Leitinger two-station method (Leitinger et al., 1975). The daytime PEC estimates have been made, by taking the differences between the GPS (o20,000 km) TEC and the CRABEX (o1000 km) derived ionospheric TEC (IEC) values estimated nearly simultaneously. For the nighttime PEC estimates reported here, the basic electron content measurements were made using the data obtained from the transmissions of CRABEX beacon payload onboard the Indian geostationary satellite GSAT2, at four linearly polarized frequencies 400, 399, 150 and 149 MHz. The differential Doppler and modulation phase delay measurements together yield the line of sight TEC up to the satellite altitude of 36,000 km. The Faraday rotation measurement gives a measure of the rotation of the plane of polarization of a radio wave as it travels through the ionosphere. The data from the Faraday rotation measurements essentially give the IEC part for altitudes below 2000 km beyond which the effect of the geomagnetic field is negligible. The IEC contribution (from Faraday rotation measurement) is removed from the TEC to derive the PEC values.
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It is to be noted that the nighttime GSAT based estimates of PEC reported here correspond to the electron content in the region between 2000 and 36,000 km along the slant path. Hence, they are not directly comparable with the GPS and CRABEX LEOS based measurements, which are made almost overhead. DH values (the deviation of the horizontal component of the earth’s magnetic field H from its mean nighttime level) and ionosonde data at Trivandrum and SHAR have also been used for the study. 3. Results and discussion The variabilities of TEC, IEC and PEC during different geophysical conditions are presented in the following section. Comparisons with the IRI model derived TEC values are also given. 3.1. Plasmaspheric electron content during magnetically quiet period and comparison of GPS and CRABEX derived electron content with IRI model values The PEC has been estimated for selected magnetically quiet days in the solstice period of the low solar activity year of 2004. Fig. 1 shows the GPS-
TEC, CRABEX-TEC, IRI model derived TEC and PEC variations at Trivandrum (TRV), an equatorial station and Bangalore (BLR), a low latitude station, on 8 June 2004 (Ap ¼ 9). The two top panels depict the GPS-TEC (TEC), while the two middle panels depict the CRABEX-TEC (IEC). The bottom panels correspond to the PEC obtained by removing the IEC from the TEC. The IRI-model derived TEC values for altitudes below 2000 and 1000 km are also shown in the top and middle panels, respectively, of Fig. 1. It is clearly seen from the figure that the IRI-model is over estimating the TEC, with the model-derived electron content for altitudes o2000 km itself being much higher than the GPS-TEC (up to 20,000 km). The over estimation is much more for TRV than for BLR. Similarly the IRI model-derived electron content values for altitudes o1000 km are also much higher than the CRABEX-TEC values. This only indicates the limitations of the models like the IRI for application purposes, like positioning and navigation, in view of the substantial overestimation of TEC values, over Indian longitudes. Further, it is seen that the PEC value over TRV varies from 11.7% to 14% of the corresponding TEC from morning (10:00 IST) to evening (16:00 IST) on 8 June 2004. At BLR, the PEC is found to be around
Fig. 1. Variations of GPS-TEC (top panels), CRABEX-TEC (middle panels), IRI model derived TEC and PEC (bottom panels) at TRV and BLR on 8 June 2004.
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24% of the TEC during evening times. For all the quiet days examined it is seen that the PEC values range from 4 to 6 TECU over TRV and from 7 to 9 TECU over BLR. The percentage of PEC is found to range from 10% to 20% of the TEC over TRV and from 15% to 25% over BLR. Thus, one could infer an increase in PEC from TRV to BLR during magnetically quiet times. The cause of the increase in PEC from TRV to BLR is an aspect which needs to be examined in detail with a larger database. The PEC values of 2–4 TECU that have been reported for the European and American sectors during low solar activity (Poletti-Liuzzi et al., 1976; Soicher, 1976; Doherty et al., 1992; Lunt et al., 1999), are consistently lower than the present estimates made over the Indian sector. Estimates made using GPS data and SUPIM (Sheffield University plasmasphere ionosphere model) over the mid-latitude Asian sector by Balan et al. (2002) have shown that PEC is of the order of 8–11 TECU during high solar activity. During low solar activity over the Asian mid-latitude sector, using GPS data and MU radar observations, Otsuka et al. (2002) have reported PEC values of the order of 3–4 TECU. The percentage contribution of PEC during high solar activity as given by Balan et al. (2002) varies from 10% to 20%. Moreover, they have shown that the PEC has a significantly decreasing trend with latitude. The present estimates, however show significantly high PEC values over low latitudes around the magnetic equator (within the EIA crests) even during low solar activity. Further these PEC estimates also register an increase with
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latitude (within the EIA crests). These new aspects of the PEC variabilities over the latitudes around the magnetic equator in the Indian sector, underlines the need for quantifying the same. 3.2. Plasmaspheric electron content during magnetically disturbed period Fig. 2 shows the temporal variations of DH (left panel) at the magnetic equatorial location of TRV and Dst (right panel) from 6 to 12 November 2004. This was a magnetically disturbed period with disturbance effects being seen from the noon of 7 November 2004 onwards. It is seen from the figure that there is a sudden commencement of the storm at 12:00 h and a consequent increase in the magnetic field around that time. Thereafter, the DH values decreased corresponding to the main phase of the storm, which extends up to the noon of 8 November 2004. The ring current development, which causes the main phase decrease of DH, is clearly seen in the Dst variability. After the main phase when the ring current decays and the Dst values start recovering, it gets reflected in DH too. The estimate of PEC is made for the noon time of 9 November 2004, when the DH values had more or less recovered from the storm effects. It is also to be noted that there is a recurrent storm activity in the afternoon of 9 November 2004 as is indicated by the Dst variations and these disturbance effects persist for a couple of days. The upward arrow (left panel) indicates the time for which PEC estimates are made.
Fig. 2. The temporal variations of DH (left panel) and Dst (right panel) from 6 to 12 November 2004.
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Fig. 3 illustrates the latitudinal variations of VTEC from GPS data for the stations of TRV, BLR, Hyderabad (HYD) and Bhopal (BHO) at three different times on 9 November 2004 (storm day) and 4 November 2004 (quiet day). At 09:30 IST on the quiet day as well as on 9 November 2004, the VTEC variations show the development of the EIA, with the trough close to the magnetic equatorial location of TRV (8.51N) and the crest over HYD (17.31N). But the VTEC variations at 12:30 IST shows the inhibition of the EIA on the disturbed day while on the quiet day at the same time it is seen that the anomaly is well developed, i.e., on the quiet day at 12:30 IST, the VTEC variations show the trough over TRV and crest over HYD, whereas on the disturbed day, the anomaly seems to be inhibited. This situation of an inhibited anomaly persists at 15:30 IST also, on the disturbed day, while the variations on the control day show a well-developed anomaly. During magnetic disturbances, the disturbance electric fields could enhance the development or inhibition of the equatorial ionization anomaly, whereas the disturbance winds seem to always inhibit EIA (Abdu, 1997). It is reported that EIA can undergo enhancement (expansion) due to the magnetospheric disturbance electric fields, which penetrate to low latitudes during the growth phase of a storm/substorm,
whereas EIA inhibition occurs more often under a disturbance dynamo (DD) electric field. During very severe magnetic storms the EIA has been shown to undergo dramatic expansion even up to midlatitudes with a very wide low latitude trough (Mannucci et al., 2005). The relative roles of different mechanisms, responsible for the suppression of the equatorial anomaly were studied numerically by Pavlov et al. (2004). In the presence of any such effect during the storms, inhibition of the EIA can occur and this inhibition will be seen in the ionosonde measurements also. In this context, it is interesting to examine the latitudinal variations of peak electron density (Nmax) as depicted by ground based ionograms. Nmax (m3) is given as 1/81[f2], where f is the critical frequency of the F layer in MHz. The latitudinal variations of the foF2 values at 12:30 IST as deduced from the TRV and Shriharikota, SHAR (13.71N, 80.21E; dip 101N) ionograms on the storm day (Fig. 4) reveal, that the EIA has indeed developed at 12:30 IST, while the TEC values show the contrary. This implies that the increased electron content shown at TRV by the GPS data is probably produced by an increase in the topside content extending up to the plasmasphere. In order to cross check the above statement, PEC values were deduced from GPS and CRABEX data. Fig. 5 depicts the variations of GPS-TEC (top left panel), CRABEX-TEC (bottom left panel) and PEC (right panel) on the quiet day (4 November 2004) and the disturbed day (9 November 2004). The development of EIA at 12:30 IST is brought out by the CRABEX data on both the quiet day and the disturbed day, while as mentioned previously, the GPS-TEC shows the inhibition of the anomaly on the disturbed day. Using the
Fig. 3. The latitudinal variations of VTEC from GPS data at three different times on 9 November 2004 (disturbed day) and 4 November 2004 (control quiet day).
Fig. 4. The latitudinal variations of the foF2 values at 12:30 IST as deduced from TRV and SHAR ionograms on 9 November 2004 (disturbed day).
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Fig. 5. The variations of GPS-TEC (top left panel), CRABEX-TEC (bottom left panel) and PEC (right panel) on the control day (4 November 2004) and the disturbed day (9 November 2004).
simultaneous GPS and CRABEX TEC values the PEC is estimated for both the quiet and disturbed days. While the PEC deduced for the quiet day is in agreement with the PEC estimated for the previously presented quiet days both in magnitude and percentage, for the disturbed day at TRV it is seen that the PEC shot up to 23 TECU which is significantly higher than the typical values of 4–6 TECU during quiet times. For BLR, the PEC is seen to be depleted to 4.5 TECU, much lower than the quiet time values of 7–9 TECU. Thus, the percentage contribution of PEC to GPS-TEC at TRV turned out to be very large during this particular phase of the storm. The present analysis shows large TEC enhancements at TRV and depletions at BLR, which is only 51 to the north of Trivandrum indicating the presence of large spatial fluctuations in TEC. This shows that during storm time, there is significant redistribution of plasmaspheric ionization, and this aspect has to be investigated in detail using a larger database. Large spatial fluctuations in topside electron content during the storm of 20 November 2003 as measured by the CHAMP space craft (at 400 km) have been reported by Yizensaw et al. (2006). They have observed very large spatial topside electron content fluctuations on the storm day in relation to
the measurements made on the control day of 19 November 2003. The observations made during the present study are also showing a similar pattern. It therefore seems that it is not possible to completely summarize the scenario during storms merely by discussing about the effect of the different disturbance electric fields and winds on the ionosphere. A full understanding of the behavior of the storm time upper atmosphere requires more quantitative information on the plasmasphere and factors that modulate the PEC. In the context of the increased use of GPS system for positioning and navigation purposes, measurement accuracies of better than 3 TECU are ideally required. With single frequency GPS users resorting to ionospheric models without including PEC variations for removing ionospheric error, the derived position information will be highly erroneous at low latitude regions more so during magnetically disturbed periods as shown in the present analysis. The PEC values even during quiet times are 10–20% of TEC and during storm time they are substantially more at the equator (30%), thus greatly degrading the quality of single frequency GPS based positioning. These observations thus highlight the need to develop the climatology of PEC variations.
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3.3. Nighttime plasmaspheric content on 1 February 2006 Fig. 6 shows the temporal variation of nighttime electron content on 1 February 2006, which is a magnetically quiet day (Ap ¼ 4) obtained from the Indian Geostationary satellite GSAT. Here, TEC corresponds to the TEC in the entire ray path up to 36,000 km, while IEC refers to the Faraday rotation based electron content, which represents the ionospheric part (IEC) of the total content. The IEC is found to be higher (27 TECU) in the evening hours at 18:30 h while it decreased subsequently and stabilized at a level of 23 TECU. The reduction in IEC in the later part of the night is essentially due to the chemical recombination effects. The almost constant level of IEC in the latter part of the night is attributed to increased
Fig. 6. The temporal variation of nighttime total electron content and IEC on 1 February 2006.
lifetime of the ions. The TEC is found to show temporal fluctuations, which are believed to be due to the fluctuations in the plasmaspheric part of the electron content. Fig. 7 illustrates the nighttime variations of PEC (left panel) and percentage PEC (right panel) on 1 February 2006. The PEC value increased in the initial part of the night and attained a maximum value of 15 TECU at 20:00 h. Thereafter, it decreases to 12 TECU at 21:30 h. The PEC variation is over a range of 9–15 TECU during the entire duration for which data were available. The percentage variation of PEC ranged from 25% to 40%. Earlier measurements of nighttime PEC, of the order of 56% of the total content have been reported in the mid-latitude American sector, during low solar activity by Poletti-Liuzzi et al. (1976) and Soicher (1976). In the mid-latitude Asian sector, during high solar activity, Balan et al. (2002) have reported, nighttime PEC variations of the order of 60% of the TEC. The present estimates made for the first time at low latitude regions around the dip equator in the Indian sector, seem to be of the same order as has been obtained by previous workers. These estimates highlight the significant modulation of TEC by the PEC especially at night. It underlines the need to quantify the nighttime PEC variabilities for reliable position estimation using GPS. Balan and Bailey (1995) have shown using the SUPIM model that strong nighttime increase occurs in TEC due to reverse fountain effects. During daytime, when the ExB drift is upward, the fountain rises to about 800 km at the equator and covers a wide range of latitudes; at regions above the fountain there is a plasma flow towards the equator
Fig. 7. The time variations of PEC (left panel) and percentage PEC (right panel) on 1 February 2006.
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from both sides. In the evening as soon as the drift turns downwards, the fountain becomes a reverse fountain with supply of ionization in both hemispheres from regions above the fountain. But the ionization in the region below 1000 km decreases because of rapid recombination while at higher altitudes in the plasmasphere there is no such effect resulting in an increase in PEC during nighttime. 4. Conclusions The first estimates of PEC in the low latitude Indian sector have been presented here for both daytime as well as nighttime. The results bring out the significant contribution of PEC to the TEC (along the ray path from the satellite to the receiver), at these locations at all local times. It brings into focus the inadequacy of present day models being used in single frequency GPS receivers to accurately predict the error induced in GPS positioning by the ionized medium. The present study therefore emphasizes the urgent need to quantify the PEC variations during different geophysical conditions so that the models can be fine tuned accordingly. References Abdu, M.A., 1997. Major phenomena of the equatorial ionosphere–thermosphere system under disturbed conditions. Journal of Atmospheric and Terrestrial Physics 59, 1505–1519. Balan, N., Bailey, G.J., 1995. Equatorial plasma fountain and its effects: possibility of an additional layer. Journal of Geophysical Research 100, 21421–21432. Balan, N., Bailey, G.J., Jayachandran, B., 1993. Ionospheric evidence for a non-linear relationship between the solar EUV and 10.7 cm fluxes during an intense solar cycle. Planetary and Space Science 41, 141–145. Balan, N., Otsuka, Y., Tsugawa, T., Miyazaki, S., Ogawa, T., Shiokawa, K., 2002. Plasmaspheric electron content in the GPS ray paths over Japan under magnetically quiet conditions at high solar activity. Earth Planets and Space 54, 71–79. Ciraola, L., Spalla, P., 1997. Computation of ionospheric total electron content using NNSS and the GPS. Radio Science 32 (3), 1071–1080. Davies, K., 1980. Recent progress in satellite radio beacon studies with particular emphasis on the ATS-6 radio beacon experiment. Space Science Reviews 25, 357. Doherty, P.H., Klobuchar, J.A., Bailey, G.J., Balan, N., Fox, M.W., 1992. Determinations of protonospheric electron content from TEC measurements from GPS and Faraday rotation and comparisons against the Sheffield plasmasphere
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model. In: Lee, M.C. (Ed.), Proceedings of the International Beacon Satellite Symposium. Massachusetts Insttiute of Technology, Cambridge, pp. 118–121. Kersley, L., Klobuchar, J.A., 1978. Comparison of protonospheric electron content measurements from the American and European sectors. Geophysical Research Letters 5, 123–126. Klobuchar, J.A., 1987. Ionospheric time-delay algorithm for single frequency GPS users. IEEE Transactions on Aerospace and Electronic System AES-23, 325–331. Leitinger, R., Schmidt, G., Tauriainen, A., 1975. An evaluation method combining the differential Doppler measurements from two stations that enables the calculation of electron content of the ionosphere. Journal of Geophysics 41, 201–213. Lunt, N., Kersley, L., Bailey, G.J., 1999. The influence of the protonosphere on GPS observations: model simulations. Radio Science 34, 725–732. Mannucci, A.J., Tsurutani, B.T., Iijima, B.A., Komjathy, A., Saito, A., Gonzalez, W.D., Guarnieri, F.L., Kozyra, J.U., Skoug, R., 2005. Dayside global ionospheric response to the major interplanetary events of October 29–30, 2003 ‘‘Halloween storms’’. Geophysical Research Letters 32, L12S02, doi:10.1029/2004GL021467. Ogawa, T., Sinno, K., Fujita, M., Awaka, J., 1980. Severe disturbances of VHF and GHz waves from geostationary satellites during a magnetic storm. Journal of Atmospheric and Terrestrial Physics 42, 637–644. Otsuka, Y., Ogawa, T., Saito, A., Tsugawa, T., Fukao, S., Miyazaki, S., 2002. A new technique for mapping of total electron content using GPS network in Japan. Earth Planets and Space 54, 63–70. Pavlov, A.V., Fukao, S., Kawamura, S., 2004. F-region ionospheric perturbations in the low-latitude ionosphere during the geomagnetic storm of 25–27 August 1987. Annals of Geophysics 22, 3479–3501. Poletti-Liuzzi, K., Yeh, C., Liu, C.H., 1976. Simulation and measurement of plasmaspheric electron content. In: COSPAR Satellite Beacon Symposium, Boston University, June. Ramarao, P.V.S., Gopi Krishna, S., Niranjan, K., Prasad, D.S.V.V.D., 2006. Temporal and spatial variations in TEC using simultaneous measurements from Indian GPS network of receivers during the low solar activity period of 2005–2006. Annals of Geophysics 24, 3279–3292. Soicher, H., 1976. Diurnal, day-to-day and seasonal variability of NF, NT and NP at Fort Monmouth, New Jersey. In: COSPAR Satellite Beacon Symposium, Boston University. Thampi, S.V., Ravindran, S., Devasia, C.V., Pant, T.K., Sreeelatha, P., Sridharan, R., 2005. First observation of topside ionization ledges using radio beacon measurements from low earth orbiting satellites. Geophysical Research Letters 32, L11104. Titheridge, J.E., 1973. The electron content of the southern midlatitude ionosphere 1965–1971. Journal of Atmospheric and Terrestrial Physics 35, 981. Yizensaw, E., Moldwin, M.B., Komjathy, A., Mannucci, A.J., 2006. Unusual topside ionospheric density response to the November 2003, superstorm. Journal of Geophysical Research 111, A2308.