Geostationary L-band signal scintillation observations near the crest of equatorial anomaly in the Indian zone

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Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 500–514 www.elsevier.com/locate/jastp

Geostationary L-band signal scintillation observations near the crest of equatorial anomaly in the Indian zone S. Raya, A. DasGuptaa,b, a

S. K. Mitra Center for Research in Space Environment Institute of Radio Physics and Electronics, University of Calcutta, Kolkata, India

b

Received 29 March 2006; received in revised form 23 August 2006; accepted 20 September 2006 Available online 22 November 2006

Abstract Microwave L-band signal at 1.5 GHz from geostationary satellite INMARSAT (641E) has been recorded on a continuous basis since 1990 at Calcutta (22.581N, 88.381E geographic; 321N magnetic dip). The location of Calcutta, virtually under the northern crest of the equatorial anomaly in the Indian longitude sector, provides an excellent platform for studying equatorial scintillations. This paper presents a long-term study of scintillations at 1.5 GHz observed from Calcutta extending from minimum to high sunspot number (1996–2000). The variations of scintillations with local time, season, solar and magnetic activity are presented. L-band scintillations have been found to be essentially a pre-midnight phenomenon during the equinoctial months of high sunspot number years. Both the occurrence and intensity increase dramatically with solar activity level. During the solar maximum phase of 1998–2000, scintillations in excess of 20 dB occurred for 20% of total time in the pre-midnight hours of March 2000. Considering individual hours, a maximum percentage occurrence of 37 was recorded in the interval 19–20 LT. Although the percentage occurrence appears to be low, scintillations with Scintillation Index (SI)420 dB were recorded on 23 out of 31 nights during the same month, indicating that scintillations do not occur continuously over a long interval of time, but in small patches with clear gaps in between. The distribution of patch duration during years 1998–2000 has a maximum in the time interval of 1–3 min. From the cumulative distributions of patch duration with SIX10 and 20 dB, the 99th percentile values of patch duration have been found to be 49.3 and 39.6 min, respectively. This information may be used to obtain an estimate of longitudinal separation of geostationary satellites to be used in Satellite Based Augmentation System (SBAS). During the equinoctial months of August–October and February–April, occurrence of scintillations is less during magnetically active (Kp ¼ 3+–9) conditions than during quiet (Kp ¼ 03). In the local summer months of May–July, the probability of occurrence of scintillations is more under magnetically active condition than in quiet condition. Equatorial scintillation is essentially controlled by the pre-reversal enhancement of the eastward electric field near sunset, which in turn has season and solar activity dependences. During magnetic activity, the generation of equatorial irregularities resulting in scintillations may be ascribed to the prompt penetration of magnetospheric electric field and disturbance dynamo effects. r 2006 Elsevier Ltd. All rights reserved. Keywords: L-band scintillations; Equatorial plasma bubbles; Equatorial anomaly crest; Scintillation patches

Corresponding author. Institute of Radio Physics and Electronics, University of Calcutta, Kolkata, India. Tel.: +91 33 23509115; Fax: +91 33 23515828. E-mail address: [email protected] (A. DasGupta).

1364-6826/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2006.09.007

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1. Introduction The phenomenon of scintillations, i.e. amplitude and phase fluctuations of satellite signals propagating through the ionosphere particularly in the lower microwave band, has received considerable attention in recent years because of its detrimental effects on communication and navigational systems like Global Positioning System. Amplitude scintillations induce signal fading and when the depth of fading exceeds the fade margin of a receiving system, message errors in satellite communication systems are introduced. In GPS, amplitude scintillation may cause degradation of position fixing by standalone GPS receivers (Bandyopadhyay et al., 1997; DasGupta et al., 2004), data loss and cycle slips (Kelly et al., 1996). Severe phase scintillations may stress phase-lock loops in GPS receivers and give rise to loss of phase lock. During scintillations, irregular phase fluctuations are imposed on a transionospheric radio wave and amplitude fluctuations are developed by a diffraction process as the wave propagates in the free space beneath the ionosphere. The magnitude of the random phase perturbations depends on the inteR grated electron density deviation, DN dl, along the ray path. This parameter is controlled by the irregularity amplitude (DN/N) and the background electron density, N, and its distribution in the ionosphere. Satellite in situ measurements have shown that though the irregularity amplitude remains nearly constant, the background density in some regions undergoes a drastic variation with the sunspot cycle. This in turn affects the magnitude of amplitude and phase scintillations as a function of sunspot cycle at any point on the globe (Basu et al., 1988). Though the phenomenon of scintillations has been studied theoretically and experimentally for over four decades, it was mainly confined to VHF/ UHF for the first decade. Scintillation phenomenon in the GHz range was not investigated till the seventies. The discovery by Craft and Westerlund (1972) that ionospheric scintillations occur at 4 and 6 GHz in the equatorial region triggered many research activities to find an explanation of the same. Basu and Basu (1976) showed from in situ data of OGO-6 satellite that the large irregularity amplitude encountered in an environment of high ambient ionization density could cause GHz scintillations.

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The morphology of L-band scintillations during solar maximum and minimum years were reported by Basu et al. (1988). It has been found that L-band scintillations are most intense in the equatorial region, moderate at high latitudes and generally absent at mid-latitudes. For all latitude regions, scintillation occurrence peaks during solar maximum period when F region ionization density increases and the irregularities occur in a background of high ionization density. The equatorial F region possesses two unique features: (1) the equatorial or Appleton anomaly, which is the latitudinal variation of electron density with a trough at the magnetic dip equator and two crests at 15–201 north and south dip latitudes, and (2) intense irregularities in electron density distribution. The equatorial electric field plays a dominant role in shaping the development of both daytime equatorial anomaly and nighttime density irregularities. The field is eastward during the day and reverses to the west after sunset, at about 21LT. Before reversal, at the time of sunset, a dramatic increase in the electric field, known as pre-reversal enhancement, develops at F region heights (Fejer, 1991). The effect of prereversal enhancement on the eastward electric field is twofold and has both seasonal and solar activity dependences. The increased electric field causes a redistribution of ionization by a fountain-like effect, thereby increasing the ionization density near the crests at the expense of that at the trough over the magnetic equator. At the anomaly crests, fresh influx of ionization combined with the neutral wind counteracts the normal decay of ionization and produces a secondary peak or a ledge in the ionization distribution (Anderson and Klobuchar, 1983; DasGupta et al., 1985; Huang et al., 1989). Further, the pre-reversal enhancement of the eastward electric field raises the F layer at the magnetic equator to high altitudes, where recombination effects are negligible and conditions favourable for the generation of irregularities may be obtained (Haerendel, 1974; Woodman and La Hoz, 1976). Through the Rayleigh–Taylor mechanism, the irregularities then develop into plasma-depleted bubbles and are upwelled to the topside of the ionosphere. The polarization electric field within the bubbles is higher, and as a result, the bubbles rise to the topside at a velocity much greater than the ambient F region plasma drift (Anderson and Haerendel, 1979). Steep gradients on the edges of the depletions (Haerendel, 1974; Costa and Kelley, 1978) help to generate small-scale irregularities as

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the bubbles rise to great heights, sometimes exceeding 1000 km above the magnetic equator. These are widely recognized as plumes on radar backscatter maps (Woodman and La Hoz, 1976). The bubbles extended in altitude map down along the magnetic field line to anomaly locations of about 151N and 151S magnetic latitudes. The above effect is most pronounced during the equinoctial months of sunspot number maximum years. Persistence of high ambient ionization and injection of equatorial irregularities result in severe scintillation effects near the anomaly crest in the post-sunset period. The magnitude of scintillation varies with magnetic latitude since the background ionization density has a trough at the magnetic equator and maximum around the anomaly crests. Aarons et al. (1981) have compared the levels of scintillation activity of MARISAT I beacon at L-band (1541 MHz) recorded by Air Force Research Laboratory at three stations, namely, Natal (5.61S, 33.71W geographic; 5.51S dip latitude) in Brazil, Huancayo (111S, 68.51W geographic; 2.51N dip latitude) in Peru and Ascension Island (7.41S, 14.41W geographic; 171S dip latitude). Natal and Huancayo are located close to the magnetic equator while Ascension Island is near the anomaly crest. Peak to peak fluctuations greater than 27 dB were noted from Ascension Island while only 7–9 dB from Natal and Huancayo. This gave rise to the hypothesis that the dominant factor responsible for intense L-band scintillation is the traversal of the propagation path through the anomaly region. During years of high sunspot numbers, the high levels of DN constituting the F-region irregularity structure are due to very high ambient electron density in the anomaly region compared to that near the magnetic equator and the late appearance of these electron densities (20–22 LT) in the anomaly region. The patches or plumes of irregularities seen in the post sunset local time period have high DN. The intensity of scintillations depends on DN. The longitudinal variation of equatorial irregularities for different seasons has been studied based on in situ measurements by Basu et al. (1976). It has been found that the primary maxima occur near the equinoxes at most longitudes while the solstitial behaviour is a function of longitude. Maruyama and Matuura (1984) obtained the global distribution of equatorial spread F from ISS-b topside sounder data. The distribution shows enhancement at Atlantic longitudes during northern winter and at Pacific longitudes during northern summer and

moderate enhancement at the Indian longitudes during equinoctial periods. Tsunoda (1985) hypothesized that scintillation-producing irregularities are most likely to develop in the equatorial F-layer when the integrated E-region Pedersen conductivity experiences its maximum longitudinal gradient. Based on this postulate, Tsunoda predicted that scintillation activity would maximize at a given longitude at times of the year when the solar terminator is most nearly aligned with the geomagnetic flux tubes in that sector. The occurrence statistics of L-band scintillations reported so far are from the American longitude sector (Basu et al., 1980; Livingston, 1980; Aarons et al., 1981; Basu and Basu, 1981; Aarons et al., 1983; Basu and Basu, 1985; Basu et al., 1988; Groves et al., 1997; Basu et al., 2002); no such statistics have been reported from other longitudes. In fact, Calcutta is one of the very few stations outside the American zone having a continuous database of L-band scintillations for over a solar cycle (1990 to date) and it is one of the locations providing worst-case scenario of scintillations for communication and navigation links. This paper presents a study of L-band scintillations for half a solar cycle (1996–2000) from Calcutta, which is located under the northern anomaly crest in the Indian longitude sector. The year 1996 corresponds to sunspot number minimum (8.6) and 2000 to high sunspot number (119.6). The variations of occurrence with local time, season, solar and magnetic activity have been discussed. Satellite Based Augmentation System (SBAS) like the Wide Area Augmentation System (WAAS) in the United States, European Geostationary Navigation Overlay System (EGNOS) in Europe, Multifunctional Transport Satellite (MTSAT) Satellitebased Augmentation System (MSAS) in Japan and GPS and Geo Augmented Navigation (GAGAN) in India will provide precision guidance to aircrafts at a large number of airports distributed over extended areas. The area to be covered is divided into 51  51 grids. Precisely surveyed ground reference stations that will be linked to form a SBAS network will receive signals from GPS satellites and estimate errors due to propagation effects over individual grids. Each station will relay the data to Wide Area Master Stations (WMS) where correctional information for a specific geographical area will be computed and uplinked to a geostationary satellite (GEOS) via a Ground Uplink Station (GUS). This message will be broadcast on the same frequency as

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GPS L1 (1575.42 MHz) to the GPS receivers on board the aircrafts flying within the service area of SBAS. Through the use of GEOS carrying navigation payloads, SBAS will improve the basic GPS accuracy, system availability and provide integrity information about the entire GPS constellation. In SBAS, the GEOS link carrying the correction message at L1 will be subject to disruptions during periods of severe scintillations. Non-availability of the GEOS downlink may disable the entire SBAS over a large area in the equatorial zone. For the past four decades, extensive work has been carried out to find a correlation between geomagnetic activity and occurrence of small-scale irregularities causing scintillations. During geomagnetic storms, the ionospheric electric fields, plasma drifts, plasma density distribution and onset of plasma instabilities in the equatorial F region giving rise to scintillations differ from their quiet day pattern. Two major sources of storm time electric field perturbations have been identified. They are the magnetospheric and ionospheric disturbance dynamos. Aarons (1991) showed the presence or absence of irregularities versus longitude and storm time by considering the Universal Time (UT) of the storm’s peak effect (Dst). The use of a geomagnetic index to designate peak storm time can identify the longitude sector most susceptible to irregularity generation. Basu et al. (2001) related the occurrence of strong irregularities with rate of change of Dst (dDst/dt) greater that 50 nT/h. Huang et al. (2001) and Paul et al. (2002) also reported that equatorial irregularities occur in the specific longitude sector for which post-sunset period corresponds to the time of rapid Dst variation. A robust climatological model of scintillations WBMOD, which is useful for planning purposes, is now available (Secan et al., 1995, 1997). The model uses VHF scintillation data collected from Wideband Satellite at Ancon, Peru and Kwajalein Island and VHF and L-band data from MARISAT satellite recorded at Manila, Phillipines (VHF), Huancayo, Peru (VHF) and Ascension Island (Lband). Scintillation is a phenomenon exhibiting extreme variability in space and time. Thus climatological models are unsuitable for supporting space-based communication and navigation systems and weather models, which can provide real-time specification and forecasting, are required. Such a system, known as Scintillation Network Decision Aid (SCINDA) has been developed for the American, Indian Ocean, Far Eastern and the Pacific

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sectors (Groves et al., 1997; http://www.afrlhorizons.com/Briefs/Jun04/VS0306.html). The system combines scintillation magnitude and zonal drift measurements made at two sites, Ancon, Peru near the magnetic equator and Antofagasta, Chile at 111S magnetic latitude by using transmissions from two geostationary satellites, one to the east and the other to the west of the stations. The data drives the equatorial scintillation model, which uses upwelling and zonal motion to produce three-dimensional scintillation structures. The latitude variation of in situ electron density at 840 km by the polar orbiting DMSP satellites measured at 18 h local time at the equator gives an indication of the pre-reversal enhancement of the eastward electric field one and a half hour prior to sunset on a particular day. The DMSP data has been incorporated in the SCINDA system to provide actual forecast of scintillations to be observed one hour later at the longitude of DMSP transit on the same day (Basu et al., 2002). To specify and forecast scintillations, the Communication Navigation Outage Forecasting System (C/ NOFS) Satellite, a joint initiative of the US Air Force Research Laboratory, NASA and Naval Research Laboratory, is to be launched in 2008. The satellite will be placed in an elliptical (375 km  710 km) orbit with 131 inclination and will measure the ionospheric parameters such as ambient and fluctuating electron density as well as the drivers such as the electric and magnetic fields and neutral wind (de La Beaujardie´re et al., 2004) These data will drive an equatorial ionospheric model, which will be able to forecast the onset of plasma instability. The L-band scintillation statistics of Calcutta presented in this paper can provide useful inputs from the Indian sector to global climatological and weather models of scintillations to be developed in the future. The utility of the data in the design of SBAS for navigation is also indicated in the present paper. 2. Data Amplitude of the L-band carrier signal (1537.528 MHz) from INMARSAT (350 km-subionospheric point 21.081N, 86.591E geographic; 28.741N magnetic dip, 15.331N dip latitude) and VHF carrier signal (244.156 MHz) from FLEETSATCOM (FSC) (350 km-subionospheric point: 21.101N, 87.251E geographic; 28.651N magnetic dip, 15.281N dip latitude) has regularly been recorded at Calcutta since 1990 and 1981, respectively.

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ICOM Wideband Communication Receivers are used in series with preamplifier-converter. The detected outputs are simultaneously recorded on a PC-based Data Acquisition System and a Strip Chart Recorder. The receivers are calibrated once a week by a HP Signal Generator (model: HP8648C) following Basu and Basu (1989). The dynamic range of each receiver is 25 dB and the sampling frequency is 20 Hz. The Scintillation Index S4 has been computed at 15 minutes interval from the digitally sampled signal power data. The corresponding SI (dB) is obtained using the third peak method of Whitney et al. (1969). Following scattering theory, the signal fade depths in dB, which are more useful for communication engineers, have been computed for the SI (dB) levels. The 350-km subionospheric points of INMARSAT and FSC map up along the field line to a height of 700 km above the magnetic equator. Scintillations observed at Calcutta thus correspond to equatorial bubbles at or above this altitude over the magnetic equator. To study the behaviour of scintillations with geomagnetic conditions, Dst and AE indices have been used as measures for geomagnetic activity. The storms under study have been ordered both with respect to the maximum negative dDst/dt and AE maximum. The Dst and AE indices have been obtained from the World Data Center for Geomagnetism, Kyoto (URL address: http://swdcwww. kugi.kyoto-u.ac.jp/) and the IMF Bz and Solar Wind Speed from Advanced Composition Explorer (ACE) data (URL address: http://www.srl. caltech.edu/ACE/). Geomagnetic storms can be classified as intense, moderate or small (substorm) according to minimum Dst value, and duration and magnitude of southward component of Bz. A geomagnetic storm is said to be intense if minimum Dsto100 nT; Bz o10 nT for at least 3 h. It is moderate if minimum Dsto50 nT; Bzo5 nT for at least 2 h. The storm is small if minimum Dsto30nT;Bzo3 nT for at least 1 h (Gonzalez et al., 1994). During the period of study covering half a solar cycle from sunspot number minimum (1996) through maximum (2000) years, 40 moderate storms with sudden commencement were observed, out of which scintillations, at least in VHF link were present in 9 storms and absent in 31. Out of the 40 moderate storms, 21 occurred in the irregularity season (February–April and August–October). Restricting our study to intense magnetic storms with sudden commencement, it is observed that 17 such storms occurred

during 1996–2000 of which 4 showed scintillation activity. 3. Results 3.1. Local time, season and solar activity dependence Fig. 1 shows the hourly percentage occurrences of L-band (1.5 GHz) scintillations with SI410 and 20 dB during 1996–2000. Saturated VHF (244 MHz) scintillation occurrence is also shown in the same figure for comparison. It is observed that scintillations at L-band essentially occur between local sunset and midnight with a maximum in the local time interval 19–20. On the contrary, at VHF, it extends well beyond midnight; the maximum occurring around 20 LT. Fig. 1 also shows two clear seasonal maxima corresponding to the two equinoxes namely, February–April and August– October except in low sunspot number years. A maximum hourly occurrence of 37% was observed for scintillations in excess of 20 dB in March 2000 (monthly smoothed sunspot number: 119.9) during 19–20 LT. In the month of September 2000 (monthly smoothed sunspot number: 116.3), for the same scintillation intensity level, the hourly occurrence was observed to be 15% in the same local time interval. A remarkable solar activity dependence is also evident. Scintillations at L-band are practically absent during 1996–1997 for SI410dB which are years of low sunspot number. The maximum hourly percentage occurrences are 39% for same SI (dB) level in March 2000, a period of high sunspot number. For saturated VHF scintillations, the solar activity dependence is similar; with little or no scintillations during 1996–1997 and the maximum hourly percentage occurrence is 45% in March 2000. It should however be noted that during low sunspot number years, there are individual occasions of scintillations at both VHF and L-band frequencies at the latitude of Calcutta caused by high altitude equatorial bubbles. Their occurrence frequency is no doubt much less but they are occasionally observed. A distribution of monthly percentage occurrence during 20–24 LT over the rising solar epoch 19962000 with smoothed monthly mean sunspot number has been found for fade depth levels 6, 10 and 20 dB, corresponding to SI (dB) levels of 8, 13 and 23, is shown in Fig. 2. The increase in both percentage occurrence and intensity of scintillations with sunspot number is very much evident with a maximum

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Fig. 1. Hourly percentage occurrence of 1.5 GHz scintillations with SI410 and 20 dB and 244 MHz scintillations with SI420 dB observed during 1996–2000 at Calcutta.

Fig. 2. Monthly percentage occurrence of 1.5 GHz scintillations during pre-midnight hours 20–24 LT for different fade depth levels46, 10 and 20 dB observed during 1996–2000 at Calcutta. The black circles indicate the monthly smoothed sunspot number.

occurrence 30% for fade depth46 dB in March 2000. Although the maximum hourly percentage occurrence of scintillations at L-band is in the range 35–40 (Fig. 1) during the sunspot maximum years, it is more or less a daily feature in the equinoctial

months of high sunspot number years. For example, in March 2000, out of 31 nights, scintillations were observed on 23 nights (Fig. 3). The apparently small percentage occurrence in time is due to the fact that scintillations do not occur continuously but occur in well-defined patches with clear gaps in between.

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Fig. 3. No. of days of 1.5 GHz scintillation occurrence with SI410 and 20 dB observed during 1996–2000 at Calcutta.

Fig. 4. Frequency distributions of patch duration for scintillations with SIX10, 15 and 20 dB observed during 1998–2000 at Calcutta.

In other words, the irregularities causing scintillations drift across the line of sight of the satellite in the form of clouds. Fig. 4 shows the distribution of patch duration for the solar maximum years 1998–2000, for SI greater than 10, 15 and 20 dB. As a visual examination of the records shows that scintillations frequently occur in patches of duration less than the routine scaling interval of 15 min, the data have been scaled at 3 min interval to obtain the distribution of patches. The maximum of the distribution of patches during 1998–2000 occurs in the time interval

1–3 min with a median value of 9 min for SIX10 dB and 6 min for SIX15 and 20 dB. Fig. 5 shows the distributions of patch duration individually for the years 1998, 1999 and 2000 for scintillations in excess of 10 dB. The number of observed patches increased from 68 in 1998 to 158 in 1999 and 295 in the year 2000. The median value of the distributions varied from 6 min in 1998 to 9 min in 1999 and 2000. The patchy nature of equatorial scintillations near the anomaly crest has been utilized in increasing the reliability of a SBAS (Ray et al., 2003). The correction messages applicable to each

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Fig. 5. Frequency distributions of patch duration for scintillations with SIX10 dB observed for the years 1998, 1999 and 2000 at Calcutta.

Fig. 6. Cumulative distributions of patch duration for SIX10 and 20 dB observed during 1998–2000 at Calcutta. The arrows indicate the 99th percentile values for the two distributions.

51  51 grid of service area of SBAS will be broadcast to the GPS users in that particular grid via GEOS at GPS L1 frequency. Severe scintillations at L-band are likely to render the GEOS links carrying correction messages unusable and disable the SBAS over a large area in the equatorial zone. Fortunately, the irregularities producing scintillations occur in patches of varying horizontal extent and drift across the ray path of the satellite. In order to have a fail-safe system, it is suggested that at least two geostationary satellites, well separated in space, be used for downlinking so that when one link exhibits scintillations, another may remain free of the phenomenon. According to scattering theory, scintillation indices of 10 and 20 dB correspond to signal fades of about 7.5 and 17 dB, respectively.

For SBAS receivers having fade margins less than 7.5 dB, the GEOS link will be disrupted for scintillations with SI exceeding 10 dB. Similarly for receivers with fade margins less than 17 dB, scintillations with intensity greater than 20 dB will disrupt the GEOS link. Fig. 6 shows the cumulative percentage distributions of patch duration for SIX10 dB and SIX20 dB only during high solar activity years 1998–2000. The 99th percentile values (shown with arrows) are 49.29 min for SIX10 dB and 39.56 min for SIX20 dB, which may be used to estimate the minimum longitudinal separation between two GEOS for uninterrupted SBAS operation. Following the procedure adopted in Ray et al. (2003), the separation comes out to be 70.111 for SIX10 dB and 59.101 for SIX20 dB (vide Appendix).

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3.2. Magnetic activity dependence Figs. 7(a) and (b) compare the monthly occurrence of scintillations at different fade depth levels for magnetically quiet (Kp ¼ 03) and active (Kp ¼ 3+–9) days for local time 20–24 h during 1996–2000. It is observed that the occurrence of scintillations at all fade levels is higher under quiet magnetic conditions excepting during October 1998, September 1999 and April 2000. Fig. 8 clearly shows the above features at 10 dB fade depth. At a lower level of 6dB, however, occurrence of scintillations is more under magnetically active conditions in the local summer months of May 1998, May 1999 and July 2000. The L-band scintillation occurrence at Calcutta is compared with that in the same local time interval

and magnetic activity conditions at Ascension Island (7.41S, 14.41W geographic; 171S dip latitude) (Basu et al., 2002). The data sets are however for different solar epochs, 1996–2000 at Calcutta and 1986-1989 at Ascension Island. The location of Ascension Island is near the southern anomaly crest in the American-Atlantic longitude sector. Both at Ascension Island and Calcutta, occurrence of scintillations is in general less during magnetically active periods than under quiet conditions. To examine the nature of scintillation occurrence during magnetic storms, the 17 intense geomagnetic storms with sudden commencement that occurred during the period of observation 1996–2000 have been selected. Fig. 9 shows the frequency distribution of time of (a) AE maximum, (b) maximum negative dDst/dt, (c) minimum Dst and (d) storm

Fig. 7. Monthly percentage occurrence of 1.5GHz scintillations during pre-midnight hours 20–24 LT for different fade depth levels46, 10 and 20 dB for (a) Kp ¼ 3+–9 and (b) Kp ¼ 0–3 observed during 1996–2000 at Calcutta. The black circles indicate the monthly smoothed sunspot number.

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Fig. 8. Monthly percentage occurrence of 1.5 GHz scintillations during pre-midnight hours 20–24 LT for fade depth level410 dB for Kp ¼ 3+–9 and Kp ¼ 0–3 observed during 1996–2000 at Calcutta. The black circles indicate the monthly smoothed sunspot number.

sudden commencement for these 17 storms. The results presented are in UT and the local time of the station is UT+06 h. It is observed that the AE maximum of most (12) of the (17) storms occurs in the local daytime sector (00–12 UT). For the time of maximum negative dDst/dt, the distribution shows a minimum (1 out of 17 storms) during the local post-sunset sector (12–18 UT) and maximum (13 out of 17) in the local daytime sector. Out of a total of 17 storms, 11 storms have minimum Dst in the local daytime sector. Scintillations were recorded in 4 out of 17 storms (23.53%) and absent in 13 (76.47%). It is observed that during the equinoctial months of February– April and August–October, out of a total of 10 storms, 9 (90%) did not show any scintillation activity. In the local summer months of May–July, 50% of the storms showed scintillation activity. For the local winter months of November-January, scintillations were present in 66.67% storms. In the present study, the delay of scintillation onset from the time of occurrence of maximum AE, maximum negative dDst/dt and minimum Dst respectively, have been examined. Based on the above sample size, the response is taken to be ‘‘prompt’’ when the offset in time from AE maximum and dDst/dt maximum is 3 h or less and it is ‘‘delayed’’ when this time difference is more than 3 h. The delayed response may be divided into two classes. In the first case, the difference in time is more than 3 h but less than 12 h while in the other class; it is more than 12 h but less than 30 h. In the May–July months, out of the 2 storms showing scintillation activity, one could be categorized as ‘Prompt’ and the other ‘Delayed’ of the first kind.

Fig. 9. Distribution of time of (a) maximum AE (b) maximum negative dDst/dt, (c) minimum Dst and (d) storm sudden commencement (SSC) during major geomagnetic storms of 1996–2000.

For the equinoctial months, February–April and August–October, the only storm with scintillations present falls in the second ‘Delayed’ category. In the

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November–January season also, there is only one storm with scintillations in this latter category. 4. Summary and discussions This paper presents a study of equatorial scintillations particularly at L-band observed at Calcutta. Scintillations at this location near the crest of the equatorial anomaly are essentially controlled by solar flux under quiet geomagnetic conditions. The confinement of the occurrence of L-band scintillations to pre-midnight hours and extension of VHF scintillations to post-midnight hours has been explained by DasGupta et al. (1982). L-band scintillations are effectively caused by irregularities of scale size of a few hundreds of metres and VHF scintillations by irregularities in the scale size range of 1 km. During the initial phase of development in the post sunset hours, irregularities of different scale sizes coexist and scintillations in excess of 20 dB are present in both L-band and VHF. In the later phase, during post-midnight hours, the overall strength of the irregularities is eroded, the smaller-scale irregularities decaying earlier (Basu et al., 1978, 1980) and hence scintillations at 1.5 GHz are practically absent after local midnight. The continuation of VHF scintillations beyond midnight is due to the longer lifetimes of the km-scale irregularities. The seasonal and solar activity dependences of equatorial scintillations can be explained in terms of the effect of variation of pre-reversal enhancement of the eastward electric field with season and solar activity. Fejer et al. (1979) have shown from incoherent scatter radar measurements of F region vertical drift at Jicamarca Observatory, Peru that during solar maximum years, the evening prereversal enhancement is observed throughout the year but its amplitude is smallest from May to August. For solar minimum years, the amplitude of pre-reversal enhancement is much less compared to the solar maximum years. Also during low solar activity years, the height of the F region decreases resulting in sparse scintillation occurrence. The equinoctial asymmetry in occurrence, i.e. higher occurrence in vernal equinox than in the autumnal equinox may be attributed to difference in ambient ionization between the two equinoxes resulting from composition changes, particularly that of atomic oxygen density (DasGupta et al., 1983). In the present paper, the response of scintillations observed at Calcutta during seventeen intense (Gonzalez et al., 1994) storms with sudden

commencement during 1996–2000 has also been studied. It is observed that the probability of scintillation occurrence is more on disturbed days in the May–July months. Scintillations were observed on two occasions out of four magnetic disturbance cases studied in this season of generally scarce scintillation activity. Mullen’s (1973) observations may also be seen in conformity with the present inferences. During the equinoctial months of August–October and February–April, when scintillations are normally more frequent, the generation of equatorial irregularities is in general absent during magnetic disturbances. Of the 10 storms analyzed, on nine occasions no scintillations were recorded. During November–January, when occurrence of scintillations is not as high as in the equinoctial months, subsequent to magnetic storms, scintillations were observed on one occasion. The magnetospheric dynamo converts the solar wind energy into electromagnetic energy in the magnetosphere through reconnection process between the solar wind and the Earth’s magnetic field. This reconnection is favoured when the direction of the Interplanetary Magnetic Field (IMF) is southward and is opposed when the direction of the IMF is northward. The magnetosphere intercepts the solar wind voltage and this magnetospheric potential is impressed upon the ionosphere as high latitude polar cap potential (Fpc) that generates field aligned Region I currents (higher latitude current systems in the auroral region). Prompt penetration of electric field is caused by rapid changes in Fpc that produce sudden Region I current variations. Region II currents (the lower latitude current system in the auroral zone) cannot change at the same rate and there is an ‘‘undershielding’’ condition whereby the high latitude electric field can penetrate to much lower latitudes (Kikuchi et al., 2000). Magnetospheric dynamo process generates short-term (‘‘prompt’’) electric field perturbations with time scales 2 h or less (Jaggi and Wolf, 1973; Senior and Blanc, 1984). The penetration electric fields are eastward during the day and westward at night (Nopper and Carovillano, 1978) producing equatorial perturbation plasma drifts, which are upward during the day and downward at night. The drift pattern is in good agreement with the Rice Convection Model (RCM) (Spiro et al., 1988; Fejer et al., 1990) and other global convection models (Senior and Blanc, 1984). A few hours after the onset of geomagnetic activity, when Fpc suddenly decreases, Region II

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currents remain large and electric field of opposite polarity can propagate to lower latitudes. This ‘‘overshielding’’ condition drives in low latitude westward electric field during the day and eastward fields at night, which result in upward drifts at night and downward during the day. When minimum Dst occurs during the local daytime sector, normally the effect would be inhibition of the pre-reversal peak due to the disturbance dynamo electric field and consequently the inhibition of scintillations the same day. Theory suggests that the disturbance drifts result from the dynamo action of storm time winds driven by enhanced energy deposition into the high-latitude ionosphere during geomagnetically active periods. The effect is thus delayed and longer lasting (Blanc and Richmond, 1980). Scherliess and Fejer (1997) showed that there is also a long-term disturbance dynamo component with time delays of about 20–30 h, which generates relatively large upward perturbations at night and small downward drifts during the day. These longterm disturbance dynamo drifts maximize during geomagnetically quiet times preceded by strongly disturbed conditions, and are probably due to disturbance electric fields resulting from storminduced composition changes, which alter the magnetic field line integrated conductivities. In view of the dramatic effects of scintillations on the GPS networks in the American sector during the superstorms of October 30, 2003 and November 20, 2003, intensive studies using global GPS network and in-situ measurements have been undertaken (Basu et al., 2005a, b). It has been suggested that the storm induced irregularity generation by the prompt penetration electric field is confined to the postsunset hours of a narrow longitude sector. Scintillations occurring with a time delay of more than 3 h after dDst/dt maximum may be attributed to the disturbance dynamo effects under overshielding condition, which prevails over all longitude sectors and local times. Beyond 12 h, the scintillation occurrence may be attributed to the long-term ionospheric disturbance electric fields resulting from storm induced composition changes (Fuller-Rowell et al., 1994; Pro¨lss, 1995). The prompt penetration of high latitude electric field to the equatorial locations in an ‘‘undershielding’’ environment by Region II current occurs within three hours of dDst/dt maximum or AE maximum and much enhanced eastward electric field develops in the equatorial region during the post-sunset hours. Such electric fields form equator-

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ial irregularities by plasma instability processes that cause scintillations of satellite radio signals. Since the present study is based on observations at a particular location, the UT corresponding to the prompt phase of the storm may not correspond to the post-sunset time but it may be detected at other favourable locations. From Fig. 9 it is observed that the majority of the storms have AE maximum and dDst/dt maximum in the local daytime sector for Calcutta. A long-term analysis of this feature at other longitudes may establish if there is any longitudinal asymmetry in this response to magnetic disturbance from the global perspective. The essential difference between equatorial scintillations and scintillations at higher latitudes is that the latter is mainly controlled by transient phenomena of the Sun like the solar wind, IMF components and geomagnetic storm while the former is primarily controlled by the Sun’s 11-year cycle. Intense equatorial scintillations may disable satellite-based communication and navigation systems like GPS and SBAS for a considerable period of time resulting in position errors and loss of data. The study of the dependence of such worst-case scintillation events on local time, season, solar and magnetic activity is required in order to provide a complete characterization of equatorial scintillations observed from the crest of the equatorial anomaly. Acknowledgements The authors thank Dr. Santimay Basu and Dr. Sunanda Basu for stimulating discussions. Private communication with Dr. M. Bakry El-Arini regarding the SBAS geometry is acknowledged. This research has been sponsored in part by the Indian Space Research Organization (ISRO) through a project at the S.K. Mitra Center for Research in Space Environment. The ACE MAG instrument team and the ACE Science Center are thankfully acknowledged for providing the ACE data. The authors also thank the Geomagnetic Data Service at the World Data Center for Geomagnetism, Kyoto for providing the Dst and AE indices. Appendix Geometry of GEOS Separation in SBAS Total number of patches with SIX10 dB during 1998–2000 ¼ 521, The 99th percentile value corresponds to patch duration of 49.29 min for SIX10 dB in Fig. 6.

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Taking ionospheric drift to be 200 m/s corresponding to 20–21 LT (Bhattacharya et al., 1989) and assuming that the two geostationary links skirt the two edges of the patch at 350 km, the separation between the two subionospheric points at 350 km height ¼ (49.29  60  200)/1000 km ¼ 591.48 km. This is equivalent to the earth central angle 2y ¼ 591.48/(6378+350) ¼ 5.041 (Fig. 10). [Since the earth’s radius ¼ 6378 km]. Therefore, y ¼ 2.521 ¼ +OCI. From DOCI, CO ¼ 6378 km, CI ¼ 6378+350 ¼ 6728 km. Therefore, OI ¼ O(CO2+CI22COCI  cosy) ¼ 453.21 km and a ¼ 1801—+COI ¼ 1801 sin1{(CI/OI)  sin y} ¼ 139.281. From DOCG, b ¼ sin1 {(CO/CG)  sin+COG} ¼ 5.661. [Since +COG ¼ +COI ¼ 1801a; sin+COG ¼ sin (1801a) ¼ sin a]. From DOCG, g ¼ 1801(a+b) ¼ 35.0551. Longitudinal Separation between the two GEOS ¼ 2g ¼ 70.111. Similarly for SIX20 dB, the longitudinal separation between the two GEOS is 59.101 with a 99th percentile value of 39.56 min.

Fig. 10. Geometry of the GEOS longitudinal separation in SBAS (not to scale). The inner circle represents the globe with center C, the outer circle the geostationary orbit and the intermediate circle the ionosphere. O is the observing station, I the ionospheric pierce point at one edge of the irregularity cloud as shown by bold line, G1 and G2 the two geostationary satellite locations for the rays skirting the irregularity cloud. The angles are as indicated.

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