Using a dual-frequency high-resolution software-based GPS receiver, TEC and phase have been monitored from Calcutta, India situated near the northern crest of the Equatorial Ionization Anomaly for studying some Space Weather events during 2008–2010. Data from a dual-frequency Ionospheric TEC and Scintillation Monitor operational at this station under the international SCINDA program of the U.S. Air Force have also been used. This paper presents two cases of intense Space Weather events occurring in the equatorial latitudes under magnetically quiet conditions during the abnormally prolonged minimum of solar cycle 24. High values of S4 with maximum ∼0.8 were noted on GPS links located almost due south of Calcutta (22.58°N, 88.38°E geographic; magnetic dip: 32°N) when the look angles of the satellites are more-or-less aligned with the axis of the anisotropic field-aligned irregularities over the magnetic equator. Associated bite-outs in TEC of amplitude 40 units were recorded in the local post-sunset hours. Well-defined patches of phase scintillations and associated cycle slips were identified. On these days, higher values of ambient ionization were noted and the diurnal maximum of the electrojet strength was found to be delayed followed by a significant rise of the F region with a high upward drift velocity over the magnetic equator around sunset indicated by ionosonde. Measurements of in situ ion density using LEO DMSP corroborate the F region height rise. Presence of irregularities in ionization density distributions around 450km was found from C/NOFS measurements.
 Space Weather, a relatively new terminology, loosely defines the hierarchy of all phenomena within the Earth-Sun environment that may affect systems operating within that environment. Space Weather refers to adverse conditions on the Sun, the solar wind, and in the Earth's magnetosphere, the ionosphere and the thermosphere. The effects of severe Space Weather events constitute one of the most intense propagation phenomena on satellite-earth station links, often causing complete outages of signals for prolonged periods of time thereby jeopardizing human lives in the modern space-based society. Global Positioning System (GPS) satellite signals traveling from the satellite to the Earth are subject to a variety of error sources, the most significant among them being multipath and the effects of the ionosphere. Of the ionospheric contributions, the background ionosphere introduces both delay and frequency dispersion whereas small-scale time-varying irregularities introduce phase and amplitude scintillations of the received signal. Presently, techniques have been devised to correct the ionospheric delay using dual-frequency GPS, Differential GPS (DGPS), and Satellite Based Augmentation System (SBAS). However, the phenomenon of scintillation remains an enigma and cannot be corrected for, being essentially a quasi-random process. Thus either mitigation and/or enhanced robustness of GPS receivers to scintillations are the only sensible strategies.
 The most severe scintillation occurrences in the world are recorded at the low latitudes (±30° about the geomagnetic equator). In the equatorial region, scintillation occurrences maximize during the post-sunset to midnight local time interval of equinoctial months around the sunspot number maximum years. While phase scintillation occurrences are often not associated with corresponding amplitude scintillations in the polar regions, it is suggested there is almost always a correspondence between the two in the equatorial region. Fremouw et al.  noted that assuming scintillations to arise out of relatively small-scale (of the order of a kilometer or less) irregularities imbedded in an otherwise smooth and stratified ionosphere, one would expect a phase record to be easily separable into a smoothly varying trend (caused mainly by the changing path length through a slab-like ionospheric layer) and relatively fast fluctuations i.e., phase scintillations. In practice, however, it appears that, when structured, the ionosphere contains irregularities on a vast spectrum of scales extending at least from the order of hundreds of kilometers to a few centimeters. A radio wave passing through such a structured plasma develops phase perturbations across the same extended spatial range. On the other hand, the spectrum of intensity scintillations is bound at the low-frequency end by the phenomenon of Fresnel filtering [Rufenach, 1972].
 Phase scintillation measurements are sparse all over the world and totally absent from the geophysically sensitive Indian longitude sector. Phase scintillations based on observations of coherent signals from sun-synchronous satellites at equatorial and auroral latitudes were studied about three decades back [Fremouw et al., 1978; Livingston et al., 1981]. The occurrence pattern for phase and amplitude scintillations is generally correlated although the characteristics may differ. Measurements of satellite beacon differential phase scintillations were made at high latitudes by Kersley et al.  using the phase differences between 150 and 400 MHz transmissions from the Navy Navigational Satellite System (NNSS). Aarons et al. [1996, 1997] used GPS phase fluctuations as an indicator to study occurrence of irregularities. Mendillo et al.  further developed an hourly index to quantify GPS phase fluctuations and used the index to study the irregularities in the South American sector. Many comparisons between GPS phase fluctuations and traditional techniques such as VHF radar, scintillations, optical imager and ionosonde have been made and show good consistency [Basu et al., 1999; Chu et al., 2005; Chen et al., 2006]. More recently the occurrence probability of spread-F, GPS phase fluctuations and plasma bubbles have been studied near the crest of equatorial anomaly [Lee et al., 2009]. Long-term climatology of nocturnal equatorial F region irregularities have been investigated using GPS phase fluctuations during the 23rd and 24th solar cycle covering the period 1996 through 2006 at the West Pacific longitudes [Chu et al., 2009].
 Geostationary VHF satellite beacon from FLEETSATCOM (FSC – 250MHz; 73°E) has been routinely recorded at the Institute of Radio Physics and Electronics (IRPE), University of Calcutta, Calcutta, India (22.58°N, 88.38°E geographic; magnetic dip: 32°N) since 1980. A dual-frequency software-based high resolution GPS receiver capable of providing the raw phase of the GPS L1 (1575.42 MHz) and L2 (1227.6 MHz) signals is operational at IRPE. IRPE is also part of the international SCINDA (SCIntillation Network Decision Aid) program of the U.S. Air Force where a dual-frequency Ionospheric TEC and Scintillation Monitor operates round-the-clock since November 2006. This station, being situated virtually under the northern crest of the Equatorial Ionization Anomaly (EIA) in the Indian longitude sector, has a unique advantage of studying the equatorial ionospheric irregularities.
 Simultaneous observation of ionospheric irregularities using geostationary VHF satellite beacon and GPS has the advantage that scintillations affecting the former may act as an indicator of scintillations on GPS links [DasGupta et al., 2004]. This may be understood from the fact that at equatorial latitudes in the early evening hours, intense to saturated scintillations with a very fast fading rate are noted on VHF satellite links caused by irregularities of scale sizes 800–1000m. At this time smaller scale irregularities of scale sizes 300–400m are also present and may cause scintillations on geostationary L-band as well as GPS links mainly concentrated around the northern crest of the EIA. With the progress of the night, the smaller scale irregularities gradually decay but the larger scale irregularities still persist. As a result, geostationary L-band scintillations die down but VHF links are still affected. However, around this time, GPS links looking south from the northern crest of the EIA ‘end-on’ through an irregularity which exists in the form of a ‘peeled-banana’ section experience scintillations.
 A combined GPS-GLONASS C/A code receiver was operational at IRPE during the high solar activity period 1999–2002 for studying errors in position-fixing by GPS in an environment of strong equatorial scintillations. The SNRs of many GPS and GLONASS links, particularly in the southern sky and near overhead, have been found to scintillate frequently in between the local sunset and midnight hours. Scintillations of satellite signals near overhead are caused by irregularities in electron density distribution in an environment of high ambient ionization occurring near the crest of the equatorial anomaly. For the links at lower elevation angles in the southern sky, scintillations occur when satellites are viewed “end-on” through the field aligned plasma bubbles. During periods of intense scintillations, in the high sunspot number years 1999–2002, it has frequently been observed that seven or eight GPS/GLONASS satellite links may simultaneously show scintillations in excess of 10 dB [DasGupta et al., 2004; Ray, 2007].
 Outages in transionospheric communication and navigation links from a location like Calcutta present the worst case figures. Parameterization of scintillations, both amplitude and phase, using GPS data from Calcutta may provide a benchmark for the international Space Weather program. The major advantage of using GPS for monitoring phase fluctuations of a transionospheric signal stems from the fact that the data could be recorded continuously at different look angles. However, this demands a very careful analysis because the received phase of a radio signal includes significant contribution arising from the path differences and the Doppler shift. The ionospheric contribution to the phase fluctuations is very small in comparison to that occurring due to the path length differences at consecutive sampling instants. The GPS receivers have simultaneously monitored GPS TEC and phase to study some Space Weather events from Calcutta particularly during 2008–2010.
 The daytime equatorial electrojet (EEJ) controls the development of the EIA [Rastogi and Klobuchar, 1990] and it was suggested that a developed EIA in the daytime plays a crucial role in the subsequent development of F region irregularities in the post-sunset hours [Raghavarao et al., 1988]. However, no idea about the pre-reversal enhancement of the vertical upward drift in the post-sunset hours can be obtained from magnetogram records since the electrojet current, which is conducting in nature, disappears in that local time interval. Among the many highly variable drivers that control the occurrence of equatorial spread-F on a given evening, it is generally agreed that few are as crucial as the vertical rise velocity of the ionospheric F region following sunset, which determines how high the F layer would rise into a region where collisional frequencies are low and the growth rate for the generalized Rayleigh-Taylor instability is large [Kelley, 1989]. Generation of equatorial ionization density irregularities over the magnetic equator in the post sunset hours is intimately related to the variation of height of the F layer around sunset [Farley et al., 1970; Abdu et al., 1981; Fejer et al., 1999; Sales, 1999; Tulasi Ram et al., 2006]. It has been established that a sharp rise of F region altitude due to the pre-reversal enhancement of the eastward electric field is often followed by the generation of intense irregularities through Rayleigh-Taylor instability mechanism. The enhanced electric field is attributed to the polarization effect due to the F region dynamo [Rishbeth, 1971]. Anderson et al.  reported a threshold value of 20m/s for the F region upward drift when UHF S4 index greater than 0.5 was expected in the Peruvian/Chilean longitude sector during 1998–1999. A threshold value of 25m/s was reported by Ray et al.  from observations near the northern crest of the EIA during the sunspot number maximum years 2000–2001. The coupling between the E and F layers around the solar terminator has been suggested as a spillover of the dayside EEJ into the F layer [Haerendel and Eccles, 1992; Eccles, 1998], which causes the low-latitude plasma to rise.
 Recent investigations leave open the scientific question of whether an enhancement in upward drift of the ionospheric F layer in the post sunset hours is necessary and sufficient or simply necessary for creating the ambient conditions conducive to scintillation occurrence. It is suggested that there exists a ‘threshold’ in the upward drift that determines whether subsequent scintillation activity will occur or not. It is implied that the ‘seeding’ mechanism is always present and all that is required is for the upward drift velocity to be higher than a critical value before bottom-side spread F can percolate upward and form equatorial bubbles [Anderson et al., 2004; Ray et al., 2006].
 Enhanced electric field raises the electron density to the topside and reverses the process of normal decay of the EIA. Sometimes, the post sunset anomaly becomes more developed than the daytime phenomenon. The EIA is not confined to the maximum ionization height hmF2, but extends up to several hundred kilometers in the topside of the ionosphere. The locus of the ionization crests in the topside lies on a field-line. A larger post sunset enhancement of the eastward electric field may raise the apex of the equatorial ionization anomaly over the magnetic equator to heights above the nominal altitude of satellites like DMSP (840km). As the satellite moves across the equator, the ionization density would then either show a flat top or two crests at off-equatorial latitudes. It has been suggested that an idea about the occurrence of post-sunset scintillations may be obtained from the latitudinal distribution of ion density by DMSP in the afternoon hours [Basu et al., 2002; Paul and DasGupta, 2010].
 The present station at Calcutta is situated virtually beneath the northern crest of the equatorial ionization anomaly. The geostationary as well as GPS satellite signals recorded from Calcutta are frequently disrupted by intense amplitude scintillations during the post-sunset hours of equinoctial months. These scintillations are caused by ionospheric irregularities generated over the magnetic equator. On days when the pre-reversal enhancement of the eastward electric field forces the apex of the equatorial anomaly to rise to heights of 750km or higher over the magnetic equator, the irregularities map to off-equatorial latitudes like Calcutta (22.58°N geographic latitude) along the magnetic field lines in the post-sunset hours. Polar orbiting DMSP satellites measure the latitudinal distribution of ionization density at 840km altitude. The magnetic field line with the apex at 840km above the magnetic equator maps to 18°N magnetic latitude (23.8°N geographic latitude) at the mean ionospheric height 350km. Thus a strong association may be expected between the latitudinal variation of the in situ ion density measurements of DMSP around the magnetic equator and occurrence of scintillations near the northern crest of the equatorial anomaly at Calcutta [Basu et al., 2002; Ray et al., 2006; Paul and DasGupta, 2010]. Locations near the anomaly crests experience worst-case disruptions in satellite based communication and navigation links and provide ideal test bed to check the reliability of operation of such services.
 In the equatorial latitudes, isolated cases of occurrences of post-sunset ionospheric scintillations affecting GPS links have been recorded during the unusually prolonged minimum of solar cycle 24 in 2008–2010 even under magnetic quiet conditions. These events of scintillations, observed on both the amplitude and phase of the GPS signals, were found to cause tracking jitters on GPS receivers leading to cycle slips or phase lock loss. The associated bite-outs in the Total Electron Content (TEC) were significant and results in abnormally high range errors leading to a considerable compromise on the accuracy of position-fixing. The extremely sharp gradients of ionization existing on the walls of the TEC bite-outs may jeopardize the reliability of operation of satellite links operating through such regions. The present paper highlights some such events during February 2008 through March 2010 as representative cases of amplitude and phase scintillations, large amplitude TEC bite-outs and the sharp gradients of ionization present on the walls of the TEC bite-outs. These phenomena severely affect the positional accuracy of satellite-based communication and navigation systems. To the best of our knowledge, the results presented in this paper are perhaps the first records of GPS phase scintillations from the Indian longitude sector.
 Occurrences of scintillations at L-band have been extremely sparse during the abnormally low solar activity period of 2008 through March 2010 as observed from Calcutta (22.58°N, 88.38°E geographic; magnetic dip: 32°N), a station lying near the northern crest of the equatorial ionization anomaly. Only four cases have been recorded during 2008 through March 2010, namely, on February 2, 2008, September 25, 2008, October 8, 2009 and March 13, 2010, when amplitude and phase scintillations were observed on GPS L1 frequency (1575.42MHz) with associated deep bite-outs in TEC and fluctuations in carrier-to-noise (CNO) ratios at GPS L1 frequency. The present paper highlights two events, namely, the ones occurring on October 8, 2009 and March 13, 2010, for understanding the characteristics of these intense Space Weather events occurring in the equatorial region even during the bottom of the solar minima under magnetically quiet conditions.
 Amplitude of the VHF carrier signal (244.156MHz) from FLEETSATCOM (350km-subionospheric point: 21.10°N, 87.25°E geographic; magnetic dip: 28.65°N) has been regularly recorded at Calcutta since 1980 using a wideband communication receiver. The detected output is digitally recorded at a sampling frequency of 50Hz. The receiver is calibrated once a week using a HP Signal Generator (model: HP8648C) following Basu and Basu . The dynamic range of the receiver is ∼25dB. The scintillation data was scaled to obtain Scintillation Index [SI (dB)] and S4 following Whitney et al. . The logic of mutual corroboration of geostationary VHF and GPS satellite scintillations formed the starting point of the present investigation.
 From the dual-frequency software-based GPS receiver, carrier-to-noise ratio (CNO) and phase of the L1 signal, and TEC have been utilized at different elevation angles and local times on the days of the above mentioned Space Weather events. The TEC measured using this receiver has been sampled at 1-min interval while the CNO and phase data at 17 Hz. The plots corresponding to phase scintillations presented in this paper are not those of phase fluctuations associated with TEC perturbations. Rather they are plots of raw phase data recorded at GPS L1 frequency which have been detrended to remove the geometrical contributions and the effects of Doppler shift. The corresponding 1-min phase scintillation index σϕ expressed in radians has been calculated for the satellite links which exhibited fluctuations in S4 index. To avoid any multipath effects, GPS data above an elevation angle of 15° only have been used for the analysis.
 The signal-to-noise ratios (SNR) of the L1 (1.6 GHz) transmissions from the GPS and GLONASS satellites were recorded at Calcutta during 1999–2002 by a stand-alone C/A code receiver. The GPS/GLONASS receiver usually tracks 8–12 satellites, simultaneously sampling different sections of the ionosphere at different look angles from the station. It has been established using data from this receiver that the propagation geometry of the GPS satellite link plays an important role in intense Space Weather events under magnetically quiet conditions around the bottom of the sunspot number minimum [Bandyopadhyay et al., 1997; Ray, 2007].
 The data recorded by the dual-frequency Ionospheric TEC and Scintillation Monitor at Calcutta under the SCINDA program are sampled at 1sec and continuously uploaded to the website https://scinda.aer.com/users/india from where they are available in a post-processed form to authorized users. Plots of S4 and elevation angles from GPS satellite vehicles as a function of Zulu Time (local time = UT + 06:00), polar plots of GPS S4 indicating the intensities of amplitude scintillations along the tracks using different shades and diurnal variation of calibrated TEC from different satellite vehicles are available from the SCINDA website.
 The strength of the electrojet could be measured by the differences between the hourly inequalities (ΔH) of the horizontal component (H) of the geomagnetic field at a station near the magnetic equator, situated close to the axis of the EEJ, and another station located away from the magnetic equator, outside the electrojet region. This eliminates any non-ionospheric contribution to the magnetic field variation, e.g., magnetospheric contributions. In the Indian subcontinent, geomagnetic data from Tirunelveli (lat: 8.67°N, long: 77.82°E geographic; magnetic dip: 0.5°N) situated within the EEJ and Alibag (lat: 19.00°N, long: 72.83°E geographic; magnetic dip: 24.75°N) outside the EEJ have been used to obtain an estimate of the strength of the EEJ. Denoting the hourly inequality of the horizontal component of the geomagnetic field at Tirunelveli by ΔHT and that at Alibag by ΔHA, a measure of the electrojet is given by (ΔHT − ΔHA) [Kane, 1973; Rastogi and Klobuchar, 1990]. EEJ data for the period around March 13, 2010 was not available.
 Distribution of topside ionization density during the days of the intense Space Weather events as well as on days preceding and following them have been obtained using DMSP ion density data recorded with a resolution of 4secs, made available from the U.S. Air Force Research Laboratory.
 Ion density data from a satellite like Communication/Navigation Outage Forecasting System (C/NOFS) moving in a 375 × 710 km equatorial orbit with an orbital inclination of 13° may be utilized for detection of ionization density irregularities. Detailed descriptions of the objectives of the C/NOFS mission are available in literature [de La Beaujardiere et al., 2004]. The ionization density data from the C/NOFS satellite are available at the CINDI (Coupled Ion-Neutral Dynamics Investigation) data distribution website http://cindispace.utdallas.edu of the Center for Space Sciences at the University of Texas at Dallas. Ion density data over the Indian longitude sector for the post-sunset hours 14:50–16:56 UT of March 13, 2010 were available.
 Since the irregularities are generated over the magnetic equator and map to higher latitudes, ionization density (NmF2) and vertical height rise of the F region over Trivandrum (8.47°N, 76.91°E geographic; magnetic dip: 0.91°N) in the post-sunset period have been utilized for the days mentioned above. The hourly Dst indices have been obtained from the website of the World Data Centre for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp).
Figure 1 shows the locations of the stations, Calcutta situated near the northern crest of the EIA in the Indian longitude sector, Trivandrum and Tirunelveli near the magnetic equator and Alibag situated outside the EEJ belt. One of the cases of the effects of severe scintillations on satellite based systems occurred on October 8, 2009 which was magnetically quiet with a sunspot number of 0. Figure 2 shows the plot of S4 and elevation angle of different GPS satellites recorded by the SCINDA receiver from Calcutta on October 8, 2009. It may be noted from Figure 2 that the frame corresponding to SV12, enclosed by a box, shows fluctuations in S4 around 14:00 UTC with maximum value of nearly 0.8. A polar plot showing the track of SV12 is shown in Figure 3 with the portion marked in bold corresponding to the period of S4 > 0.15 from 13:30–14:15 UTC. During this interval S4 varied from a value of 0.26 at 13:33 UTC to a maximum of 0.78 at 13:56 UTC and finally down to 0.17 at 14:15 UTC. During the time interval 13:30–14:15 UTC, the satellite was located almost due south of Calcutta having azimuth lying in the range 209.68°–195.98°.
 Scintillations at GPS L1 frequency are caused by small-scale irregularities, which decay by midnight. In the case of slowly moving Medium Earth Orbiting (MEO) GPS, the geometry of propagation plays an important role in perturbing the signal characteristics. An examination of the geometry of propagation indicates that GPS satellite signals often exhibit scintillations when the raypath becomes highly field-aligned and the propagation angle attains a value of 169.21°. The propagation angle has been calculated using IGRF-10 (2005) magnetic field model. The calculated propagation angle for GPS satellites observed from Calcutta show that a near ‘end-on’ propagation is achieved with propagation angle in the range of 170.12°–169.80° at 18°–21° elevation around 180° azimuth from the station corresponding to 350km-subionospheric points 14.95°–15.94°N, 88.38°E geographic; 16.59°–18.39°N magnetic dip. This feature could be well understood from Figure 4a which shows contours of the calculated propagation angle for GPS satellites from Calcutta at different elevation and azimuth angles. The 350-km subionospheric track of GPS SV12 as observed from Calcutta on October 8, 2009 is drawn and the point of maximum S4 is indicated by a small triangle. The location of the geostationary satellite FLEETSATCOM is also marked with a star. Thus, even with the reduced irregularity strength, the GPS satellite CNO/SNR at 1.575GHz shows intense amplitude scintillations as the raypath traverses the bubble more or less ‘end-on’. This fact is well illustrated in Figure 4b which shows the GPS L1 amplitude Scintillation Index (SI) in dB mapped against time and subionospheric latitude for different nights of September 2000, an equinoctial month of high sunspot number 116.3 [Ray, 2007]. The dotted line indicates the magnetic equator and the arrow indicates the latitude of the station Calcutta. Two prominent regions of scintillations (SI > 15 dB) may be observed. One is at 18°–22°N around 15:00 UT and the other is around 12°N during late evening hours of 17:00 UT. The first one is attributed to irregularities in an environment of high ambient electron density near the northern crest of the EIA [Aarons et al., 1981] and the latter to the ‘end-on’ propagation geometry with respect to the magnetic field line [DasGupta et al., 2004].
 TEC measured by SV12 using the software-based GPS receiver and the calculated 90-min moving averaged TEC for the period October 7–9, 2009 are plotted in Figure 5a. Variation of TEC recorded during October 1–7, 9–11 and 13–15, 2009 along with the ±1σ error bars are indicated in this figure. The afternoon ionization on the day of occurrence of scintillations is perceptibly higher than the average value, much above the +1σ level. The elevation and CNO-L1 for October 8, 2009 are also shown in this figure. It is important to note that the ambient TEC was significantly higher on October 8, 2009 in comparison to that on October 7 and 9, 2009 as well as the average TEC over the period October 1–7, 9–11 and 13–15, 2009. On October 8, 2009, an intense depletion in TEC of 40 TEC units (1 TEC unit = 1 × 1016 electrons/m2) from the ambient level and CNO fluctuations of about 10 dB-Hz are noted during 13:30–14:15 UTC on the SV12 link. The satellite elevation during this time varied from 35°–20°. Variations of hourly Dst indices for October 2009 are plotted in Figure 5b. It is noted that the variations of the Dst index during October 6–10, 2009 ranged from −10nT to +8nT.
 The phase data recorded at GPS L1 frequency using the software-based receiver have been detrended to remove the path length and Doppler shift contributions. Figures 6a–6c show three frames of the GPS phase data for the time interval 13:30–15:00 UTC on October 8, 2009. Patches of phase scintillations may be identified starting around 13:50 UTC and continuing till about 14:35 UTC. The very large bite-out in TEC around 13:57 UTC of 40 TEC units evident in Figure 5a may have resulted in receiver loss of lock seen in Figure 6a as absence of phase data for the period 13:56–13:58 UTC indicated by the arrow. One cycle of carrier phase advance at GPS L1 frequency is equivalent to 1.173 TEC units. The sharp depletion in TEC of 40 TEC units would result in nearly 34 cycles of carrier phase advance over a time interval from 13:56–13:58 UTC. This assumes importance in view of the fact that the carrier phase tracking loops of GPS receivers typically have a bandwidth of 14–18 Hz. Figure 6d shows the 1-min phase scintillation index σϕ expressed in radians during 13:30–14:50 UTC. σϕ is also found to increase to a value of 0.3 radians around 13:57 UTC.
 The carrier amplitude of the VHF satellite beacon transmitted from FSC recorded at Calcutta is plotted in Figure 7a. Patches of amplitude scintillations are noted starting from around 14:00UT which continued till 17:00UT. The 1-min S4 index corresponding to the patches of amplitude scintillations are shown in Figure 7b.
 It has been suggested in recent years that there is a correspondence between the development of the EEJ during the daytime and the post-sunset enhancement of the eastward electric field, although the physical mechanism connecting the two is not fully clear. The development of the EIA is mainly controlled by the EEJ. Under strong electrojet conditions, the anomaly is developed in the afternoon hours with a steep gradient of the F region ionization in the region between the crest and the trough. Diurnal plots (LT at 75°E) of the strength of the EEJ Δ(HT − HA) for October 6–10, 2009 is shown in Figure 8. Data was not available for October 7, 2009. It may be noted that although the diurnal maximum value of EEJ was higher on October 6, 2009 (79nT) compared to that on October 8, 2009 (45nT), the time of occurrence of the diurnal maximum was delayed on October 8, 2009. On October 6 and 9, 2009 the diurnal maximum was attained at 11 LT (75°E) while on October 8, 2009 it was delayed to 12 LT (75°E).
 The equatorial electric field plays a dominant role in shaping the development of both daytime equatorial ionization anomaly and nighttime density irregularities. The field, which is eastward during the daytime reverses to the west around 2100LT. Before reversal, at the time of sunset, a dramatic increase in the electric field known as the pre-reversal enhancement develops at F region heights. The increased electric field causes a redistribution of ionization leading to a secondary peak or a ledge in the ionization distribution [Anderson and Klobuchar, 1983; DasGupta et al., 1985; Huang et al., 1989]. The pre-reversal enhancement of the eastward electric field is believed to be the key factor governing the development of ionospheric irregularities during the post-sunset hours over the magnetic equator. The diurnal variation of the maximum ionization (NmF2) and vertical rise of the evening F layer (h′F) from Trivandrum situated over the magnetic equator in the Indian longitude sector are plotted for 3 days from October 7–9, 2009 in Figures 9a and 9b, respectively. The variation of ionization starting from 10:00UT to 12:00UT was found to be much sharper on October 8, 2009 compared to that on October 7 and October 9, 2009. Figure 9a also shows that the ambient ionization was higher on October 8, 2009. An idea about the F region height changes may be obtained from the h′F values. The rate of change of h′F, d(h′F)/dt, from the time of the ionospheric E region sunset (at an altitude of 100km) till the time when the height rise stops (around 20:00LT), was used to estimate the upward drift velocity of the F region. The vertical rise velocity of the F region was 17.4m/s on the evening of October 8, 2009. However, the velocity was 5.6m/s on October 7, 2009. The value for October 9, 2009 could not be calculated because of absence of data during 12:00–14:30 UT. The higher vertical rise velocity of the F region on October 8, 2009 was in conformity with the suggestions linking pre-reversal enhancement of the eastward electric field for generation of ionospheric irregularities in the post-sunset hours [Raghavarao et al., 1988; Anderson et al., 2004; Ray et al., 2006].
 Topside sounder [King et al., 1967] and in situ observations have established that the EIA is not confined to the height of maximum ionization only; it also extends to the topside. The separation between the crests decreases with increasing altitude and the locus of the crest lies on a field line. A larger post-sunset enhanced eastward electric field may raise the apex to heights above the nominal 840km altitude of satellites like DMSP. DMSP satellites in polar orbit can measure the latitude variation of in situ ionization density. If the apex of ionization over the magnetic equator exceeds the DMSP orbital altitude, the DMSP in situ data would produce an ion density-magnetic latitude plot with a flat-top. On the other hand, if the apex lies below 840km, a round-top ionization density distribution will be indicated by DMSP. Third, if the topside ionization has two peaks at the DMSP orbit or higher altitude, an ionization density distribution with double inflection will be displayed by DMSP. Study of the topside ionization density distribution over the magnetic equator using DMSP around sunset has been suggested as a possible method for forecasting occurrence of post-sunset scintillations [Basu et al., 2002; Ray et al., 2006; Paul and DasGupta, 2010].
 The different panels of Figure 10 show ion densities measured by DMSP F15 on October 8, 2009 over a magnetic latitude range of ±70° in different longitude sectors around the globe. It is extremely interesting to note that only in the panels enclosed by bold lines with equator crossing longitudes of 75°E and 101°E at 11:05 UT and 12:47 UT respectively which correspond to local afternoon/early evening hours in the Indian longitude sector, flat-topped ionization density distributions have been recorded. This is indicative of the fact that the F region apex height exceeded the nominal 840 km altitude of DMSP satellites on October 8, 2009.
 The second intense Space Weather event to be presented in this paper occurred on March 13, 2010, a magnetically quiet day with a low sunspot number of 20. The SCINDA GPS S4 and elevation angle plots from Calcutta for March 13, 2010 are shown in Figure 11a. On this date six satellites, namely, SV3, 6, 11, 13, 20 and 23 exhibited fluctuations in S4 during 14:00–19:00 UTC with maximum S4 indices of nearly 0.8. Figure 11b shows the tracks of these 6 satellites on a polar plot with the station Calcutta at the center. The portion of the satellite tracks marked in bold corresponds to S4 fluctuations in excess of 0.15. It is important to note that all the affected satellites exhibited amplitude and phase scintillations with associated TEC bite-outs when located almost due south of Calcutta in the evening to midnight local time sector. SV23 which moved from northwest of the station to the southeast showed a large TEC depletion of nearly 30 TEC units around 17:30 UTC as evident from Figure 12a. The elevation, CNO at L1 frequency and 90-min moving average of TEC are also presented in this figure. Variations of hourly Dst indices for March 2010 are plotted in Figure 12b. The Dst values were found to vary between −25nT and +3nT during March 11–15, 2010. The corresponding phase data for SV23 recorded at L1 frequency for the time interval 17:00–18:00 UTC are shown in Figure 13. Patches of phase scintillations may be identified during 17:40–17:50 UTC. Plots of the carrier amplitude from the geostationary VHF link from FSC recorded at Calcutta are shown in Figure 14a. The corresponding 1-min S4 indices are plotted in Figure 14b.
 DMSP F18 plots of ionization density on March 13, 2010 as a function of magnetic latitude on a global scale are presented in Figure 15. A flat-topped ion density distribution over the magnetic equator at 95°E and 70°E around 13:35 UT and 15:17 UT respectively could be identified.
 Ion density measured by C/NOFS for March 13, 2010 during 14:50–16:56 UT is shown in Figure 16. Well-defined signatures of the presence of ionization density irregularities on that day are evident, enclosed by a box, during 15:53 UT to 16:14 UT at geographic latitude and longitude ranges of 12.6°N–1.8°S and 52.1°E–128.6°E respectively. The ion density was found to fluctuate from a value ∼106cm−3 to a value ∼104cm−3. The altitude of the satellite during the above time interval varied from 425.8 to 487.2km respectively.
 Occurrences of intense Space Weather events affecting transionospheric L-band communication and navigation systems under magnetically quiet conditions during the unusually benign minimum of solar cycle 24 may pose a serious threat to SBAS system designers in the equatorial region. The present paper reports some results of amplitude and phase scintillations, associated TEC depletions and possible loss of lock leading to cycle slips observed on GPS links from Calcutta during 2008 to March 2010. The present station is situated near the northern crest of the EIA in the geophysically sensitive Indian longitude sector and presents worst-case figures in relation to outages of satellite signals which may serve as benchmark for the international Space Weather program.
 Phase scintillation measurements are sparse globally and practically non-existent in the Indian longitude sector. The present paper reports, perhaps for the first time, GPS phase scintillation records from India. It is worthwhile to mention that these records do not correspond to the commonly referred phase variations associated with TEC fluctuations. These measurements when coupled with the amplitude data will provide an improved understanding of equatorial irregularity hierarchy. The event on October 8, 2009 resulted in nearly 40 TEC units of change from the ambient over the time interval 13:56–13:58 UTC. Over this time period, the GPS receiver possibly lost lock of the signal.
 This paper presents observations over the period 2008–2010 when scintillations were very sparse and it was intended to make a detailed study of the few cases observed during this period at the bottom of the solar cycle. Statistical studies of ionospheric scintillations from Calcutta and other stations show that equatorial scintillations is essentially an equinoctial phenomena with high occurrence frequency and intensity around the sunspot number maximum years. Scintillations are produced by ionospheric irregularities in the post-sunset period over the magnetic equator [Woodman and La Hoz, 1976]. An entire field tube takes part in the irregularity generation mechanism and the depleted field tube is upwelled to the topside of the ionosphere in the form of a ‘banana’ or ‘peeled-orange’ [Haerendel, 1974]. Within the large-scale bubble, there are smaller scale irregularities with scale sizes ranging from a few hundred kilometers to centimeters. Amplitude scintillations of transionospheric signals are effectively produced by irregularities having the dimensions of the order of the first Fresnel zone, sub-km scale sizes for VHF and hectometer scales for L-band signals. In the initial phase, the irregularities of all the scale sizes coexist. But with time, there is a gradual decrease of the strength of the irregularities with the erosion of the background ionization [Basu et al., 1978, 1980], the smaller scale irregularities decaying faster. Scintillations at VHF in the initial post-sunset hours are normally saturated and very fast. At microwave frequencies, it may be moderate to intense. But around midnight, the larger scale irregularities causing VHF scintillations may still have some power. But the smaller scales become too weak to produce scintillations at microwaves [DasGupta et al., 1982]. But under a special propagation condition when the ray makes a small angle with the field-line, even the weak field-aligned irregularities may produce large phase perturbations and enhanced amplitude scintillations. If the irregularities are anisotropic with a high axial ratio, a pronounced enhancement of scintillations may be observed around the region of field-alignment [Briggs and Parkin, 1963; Singleton, 1970]. On an earlier occasion [Guha, 2002], using the phase screen model [Rino, 1979], S4 contours were generated for Calcutta with the Parameterized Ionospheric Model (PIM 1.6) for background ionization on February 11, 1995 (sunspot number: 13) at 14:30UT. For the irregularity amplitude dn/n = 1%, the maximum value of S4 was found to be 0.22 for GPS SV25 with an irregularity axial ratio of 5 corresponding to an elevation of 10° along the meridian. When the axial ratio was assumed to be 10, the corresponding maximum S4 was 0.26. The condition of field-alignment cannot be observed with a geostationary satellite from a station situated in the equatorial region. With the GPS from an off-equatorial station like Calcutta, there would thus be two regions of enhanced scintillations: i) one around the crest of the Equatorial Ionization Anomaly due to a high ambient ionization [Aarons et al., 1981] and ii) the other in the direction looking toward the magnetic equator [DasGupta et al., 2004]. The second case is peculiar to stations near the crest of the anomaly around local midnight when VHF scintillations with a geostationary satellite is recorded with no scintillations at L-band. This is evident from Figure 4.
 The distribution of ambient ionization (TEC) and irregularities which prevail in the equatorial region around midnight hours during high sunspot number years may sometimes be obtained at the bottom of the solar cycle in the pre-midnight period. Near the bottom of the solar cycle, the ambient ionization is low and the overall strength of the irregularities, if any, would also be low to simultaneously cause VHF and microwave scintillations near the crest of the anomaly. However, under field-aligned conditions, microwave scintillations may be observed under conditions of anisotropy. This paper illustrates cases of scintillations with geostationary (VHF) and GPS (L-band) satellites in the pre-midnight hours in years of minimum sunspot number when scintillations, particularly at microwaves, are not normally expected.
 The cases of intense Space Weather events presented in this paper occurred under geomagnetic quiet conditions at the bottom of solar cycle 24. Present understanding of the background conditions suggested to be conducive for the generation of irregularities which produce ionospheric scintillations are, namely, i) relatively high ambient ionization, ii) delayed diurnal maximum of the electrojet in the afternoon hours, and iii) a pronounced pre-reversal enhancement of the eastward electric field followed by a rapid rise of the ionospheric F layer around sunset. As the magnitude of amplitude scintillations is proportional to the electron density along the raypath of the radio wave between the satellite and the receiver, a relatively high ambient ionization may be treated as an important factor for the occurrence of scintillations. It has been suggested in recent years that there is a correspondence between the development of the equatorial electrojet (EEJ) during the daytime and the post-sunset enhancement of the eastward electric field, although the physical mechanism connecting the two is not fully clear. Under strong electrojet conditions, the anomaly is developed in the afternoon hours with a steep gradient of the F region ionization in the region between the crest and the trough [Rastogi and Klobuchar, 1990]. A developed equatorial anomaly in the afternoon hours may be taken as a precursor to equatorial spread-F or scintillations on transionospheric links [Raghavarao et al., 1988; Ray et al., 2006]. However, no idea about the pre-reversal enhancement of the E × B drift in the post-sunset hours can be obtained from magnetogram records since the electrojet current, which is conducting in nature, disappears in that local time interval. The enhanced electric field is attributed to the polarization effect due to the F region dynamo [Rishbeth, 1971]. The coupling between the E and F layers around the solar terminator has been explained as a spillover of the dayside equatorial electrojet into the F layer [Haerendel and Eccles, 1992; Eccles, 1998], which causes the low-latitude plasma to rise. The electrojet region provides the best avenue to be channeled from the dayside to meet the vertical current demands of the F region neutral wind dynamo after sunset. The conductivity reduction in the E region due to the recombination of ionization and the plasma uplift enhances the horizontal (eastward) electric field and thereby increases the speed of the uplift. A delayed diurnal maximum of the EEJ in the afternoon hours may thus produce a better cascading between the dayside and the evening ionosphere thereby creating increased possibilities for the generation of the Rayleigh-Taylor instability mechanism.
 The rise of the apex of the F layer to heights in excess of 700–800km over the magnetic equator was corroborated by DMSP ion density measurements at 840 km by showing flat-topped ionization distribution over the Indian longitude sector. However, identification of the ‘necessary’ and the ‘sufficient’ condition for generation of ionospheric irregularities remains unresolved from the present exercise.
 The upward drift velocity of the F region due to pre-reversal enhancement of the eastward electric field may vary over the solar cycle [Fejer et al., 1979]. Incoherent scatter radar data from Jicamarca recorded on some days of September–October 1994 showed that the post-sunset enhancement of upward plasma drift is a necessary condition for the generation of ESF even though this enhancement amounted to only 10–20 m/s during the solar minimum period [Basu et al., 1996]. Irregularities causing scintillations may be generated following the rapid rise of the F region [Anderson et al., 2004; Ray et al., 2006]. The vertical drift may attain a high value of the order of 40m/s during solar maximum. At low sunspot number years, ionospheric scintillations are sparse but not altogether absent. On occasions, a smaller upward drift velocity may cause generation of irregularities to produce scintillations. In the present paper, a few such cases are reported at the bottom of the sunspot cycle.
 The performance of a single GPS receiver channel under scintillating conditions is dependent on the behavior of the code and carrier tracking loops. Phase scintillations have a significant effect on the receiver's carrier phase tracking loops. The impact of scintillations on system performance is related to the relative velocity between the satellite vehicle and the irregularities [Kintner et al., 2004; DasGupta et al., 2006]. Cases of loss of lock and cycle slips even during low sunspot number years and geomagnetically benign conditions near the northern crest of the EIA as reported in this paper present the international Space Weather community a challenging problem in its endeavor to specify and forecast their occurrences. The vertical rise velocity of the F region following the pre-reversal enhancement of the eastward electric field as reported in this paper occurred at the bottom of solar cycle 24. The upward velocity due to pre-reversal enhancement of the eastward electric field may have solar cycle dependence. The value of 17.4m/s may appear to be low compared to the suggested value of 20m/s [Anderson et al., 2004] which is based on observations at high sunspot numbers.
 This research has been sponsored in part by the Ministry of Human Resource and Development through the TEQIP Program at Calcutta University and the Asian Office of Aerospace Research and Development (AOARD), Japan through the SCINDA program of the U.S. Air Force. The authors are thankful to the Air Force Research Laboratory, Space Vehicles Directorate, Hanscom AFB, USA for providing the DMSP plots. Geomagnetic data have been provided by the Indian Institute of Geomagnetism, Navi Mumbai, India and the ionosonde data by the Space Physics Laboratory, Vikram Sarabhai Space Centre (VSSC), Trivandrum, India. CINDI data are provided through the auspices of the CINDI team at the University of Texas at Dallas supported by NASA grant NAS5–01068.
 Robert Lysak thanks the reviewers for their assistance in evaluating this paper.