This paper provides the first observations of EIA asymmetry by receiving beacon transmissions onboard low earth orbiting satellites from a single station ground-based receiver. The EIA strength and asymmetry are derived from the latitudinal profiles of TEC obtained from a radio beacon receiver at Trivandrum (8.5°N, 77°E, diplat ∼0.5°N). These two parameters, obtained well ahead of the onset time of ESF, are shown to have a definite role on the subsequent ESF activity. In the present paper, both these factors are combined to define a new ‘threshold parameter’ for the generation of ESF. It has been shown that this parameter can define the state of the ‘background ionosphere’ conducive for the generation of ESF irregularities much prior to its onset.
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 The Equatorial Spread F (ESF) is a nighttime equatorial F- region ionospheric phenomenon, characterized by the presence of a wide spectrum of field-aligned irregularities extending over nearly seven orders of magnitude generated due to a hierarchy of plasma instability processes. Since, the scintillations caused by these irregularities result in outages of the communication and navigation systems, understanding the variability of this phenomenon and forecasting/nowcasting the same are of considerable practical importance. The ESF is known to vary with season, local time, geographical location and solar activity, and over the years all these aspects have been studied extensively [Chandra and Rastogi, 1970; Woodman and LaHoz, 1976; Fejer and Kelley, 1980; Basu and Coppi, 1999; Hysell and Burcham, 2002]. The theoretical foundation based on the Rayleigh-Taylor (R-T) instability mechanism [Haerendel, 1973] has been widely accepted and used successfully in simulations [Ossakow, 1981]. As is known, the instability is excited on the bottom side F layer, where a steep density gradient develops after sunset due to the combined effects of the pre-reversal enhancement (PRE) in the vertical plasma drift over the magnetic equator and the fast chemical recombination of the dominant molecular ions. The primary requisite for the triggering of ESF is the lifting of F- layer to the region of low collision frequency, and a seed perturbation at the altitude of the steep density gradient. If seed perturbations are considered omnipresent, which is usually the case, the variability encountered by the phenomenon depends on the delicate balance of the background conditions and the operative forcing terms in the evening time equatorial F-region. The instability thus generated, is controlled by both ionospheric and neutral atmospheric parameters like plasma scale length (L), ion-neutral collision frequency (νin) and neutral winds in addition to the main driving force, viz. gravity. The generalized expression for the growth rate of the R-T instability in terms of the various forcing parameters is [Sekar and Raghavarao, 1987; Kelley, 1989]
where Ex is the F-region zonal electric field, B is the geomagnetic field, Ωi is the ion gyro frequency, Wx and Wz are the zonal and vertical winds respectively. It has been shown that for estimating the growth rate, the flux-tube integrated ionospheric conductivity should be taken into account [Kelley, 1989; Sultan, 1996]. Once triggered, the instability grows nonlinearly and the prevailing ionospheric conditions become important for its further development and sustenance.
 The importance of ambient ionospheric and thermospheric conditions such as vertical plasma drifts, plasma density gradients, zonal, meridional and vertical neutral winds, initial seed perturbations etc., in the initiation and non-linear development of ESF and its dynamics has been well recognized [Hanson et al., 1986; Sekar and Raghavarao, 1987; Sekar and Kelley, 1998] However, the enigmatic day-to-day variability presents a challenge to the complete understanding of this phenomenon. For instance, in a given season and solar epoch, under seemingly identical ionospheric conditions, ESF might occur on a day and might be absent on another.
 The first indication of the significant role of intensification of the Equatorial Ionization Anomaly (EIA) in the initiation of ESF was given by Raghavarao et al. . They found that large crest to trough ratio of the EIA in the 270 to 300 km altitude range between 1700 and 1900 LT favored post sunset ESF occurrence. Sridharan et al.  followed it up and demonstrated that, there existed a precursor in the OI 630 nm dayglow, that represented the EIA strength, which enables the prediction of ESF at least 3hours prior to its actual occurrence. Results in similar lines based on TEC and also E × B drifts over the dip equator have also been reported in the literature. Based on 13 days of observations, Mendillo et al.  have pointed out that the best available precursor for pre-midnight ESF is the EIA strength at sunset and 85% successful "forecasts' had been achieved. Anderson et al.  showed that the scintillation activity is related to the maximum E × B drift velocity between 1830 and 1900 LT. Valladares et al.  showed using latitudinal distribution of TEC measurements at 2000 LT, that there is a high crest to trough ratio prevalent on ESF days. It is also shown by Valladares et al. , using differential TEC profiles, i.e., using the profiles of TEC (at 1800 hrs) – TEC (at 2000 hrs) that a fully developed pre-reversal enhancement (PRE) of the vertical drift (during nights of ESF activity) is able to reenergize the fountain effect. Recently, using a chain of GPS receivers on the west side of America, it was observed that the seasonal variation in the ESF occurrence is associated with that in the PRE of E × B drift and EIA asymmetry. All these results conclusively show that EIA and ESF are related. However, when it comes to forecasting, it could only be stated that stronger the EIA, more probable is the occurrence of ESF. In addition, no quantitative assessment of the ESF variability had been attempted in relation to the EIA parameters [Lee et al., 2005]. Recently, Devasia et al.  provided the missing link between EIA and ESF through meridional winds. They showed that during the equinoctial months when the base height of the F layer (h′F) at 1900 IST was in the range of 270–300 km, the meridional wind should be equator ward for ESF to occur over the equator. However, when h′F is >300km the ESF occurred irrespective of the polarity of the meridional wind. It was suggested that the observed equatorward winds could be an outcome of the pressure bulges associated with Equatorial Temperature and Wind Anomaly (ETWA) [Raghavarao et al., 1993], which in turn is linked to EIA [Devasia et al., 2002]. Further, it was shown that the threshold h'F for the triggering of ESF irrespective of the polarity of the meridional wind has a linear relationship with solar activity [Jyoti et al., 2004]. All these studies demonstrate that the daytime electrodynamics play a decisive role in the initiation of post sunset ESF.
 In this context, the present paper attempts to address the particular aspect of the day-to-day variability of the occurrence of ESF and also its deterministic prediction. We are presenting the variabilities of EIA strength and asymmetry as observed in the ground based, single station Total Electron Content (TEC) measurements using radio beacon transmissions from Low Earth Orbiting Satellites (LEOS), which are a part of the NIMS (Navy Ionospheric Monitoring System - USA). These two factors observed at the time interval 1600–1845 IST have been uniquely combined to relate it with the generation of post sunset ESF. The important aspect, which makes this study unique is that only single station TEC data is used to derive the ‘forecast parameter’ logically taking into account the background ionospheric conditions and attempting to predict the post sunset ESF.
 The CRABEX receiver located at Trivandrum (8.5°N, 77°E, diplat. ∼0.5°N), basically receives the two phase coherent signals 150 and 400 MHz transmissions from the NIMS satellites, and measures the differential Doppler between them. This is a part of the Coherent Radio Beacon Experiment (CRABEX), in which a chain of 6 receivers in the 77–78°E meridian is being operated, for ionospheric tomography. The details of converting the measured relative phase to the latitudinal profiles of relative vertical TEC is given elsewhere [Thampi et al., 2005]. For the present study the data for the period August 16, 2005–January 13, 2006 are used. Since the present attempt is to define the ‘forecast parameter’ for magnetically quiet time ESF activity, the days with Ap > 12 have been omitted from the analysis. It is well accepted that the role of geomagnetic activity in modulating the occurrence pattern of ESF should be treated separately. The ESF, in our study refers to the occurrence of bottom side spread F as seen by the co-located ground-based ionosonde at Trivandrum.
 The TEC data in the time interval 1600–1845 IST has been chosen for the analysis. Owing to the fact that only 3–4 satellites are being tracked, there are days when we do not have data within this specific time span. Apart from this, since the polar orbiting satellites are being tracked, it is not possible to get the data exactly at the same time on two consecutive days. These are the inherent limitations of our data set itself.
 Since the receiving station is situated at the trough of the EIA, right over the dip equator, the latitudinal gradients on both northern (N) and southern (S) sides are a measure of the EIA strength. Using these, the strength of the anomaly (SF) and the asymmetry in the anomaly (AF) are defined as
 It should be noted that all these definitions make use of only the latitudinal gradients and not the absolute TEC values. In the present analysis only those satellite passes with maximum off axis elevation >50° are selected and for estimating the latitudinal gradients we have used only the part of the data for which the direct elevation angles are >35°.
3. Results and Discussion
Figure 1a shows the day-to-day variability of EIA strength as inferred from the latitudinal gradients of TEC in the time interval 1600–1845 IST. The thick filled bars represent the ESF days and the hollow bars represent the days without ESF. In general, it is seen that the stronger EIA favors the generation of ESF in accordance with the earlier results cited in the previous section. However, it is clear that, taking the EIA strength alone, all ESF events cannot be ‘predicted’. For example, on day numbers 229, 233, 241, 242 and 244, the EIA is nearly equally strong but ESF is present only on day number 242. This implies that forecasting ESF purely based on the strength of EIA alone is probabilistic in nature. Since the triggering of the R-T instability basically depends on the base height of the F region (h′F), an attempt was made to see whether a relation exists between EIA strength and h′F. Figure 1b shows the EIA strength determined in the time interval 1600–1845 IST vs. h'F at 1900 IST, for each day obtained from the ionograms from Trivandrum. Similarly, Figures 2a and 2b show the day-to-day variability of EIA asymmetry in the time interval 1600–1845 IST, and the asymmetry factor vs. h′F at 1900 IST. It should be noted that this is the first systematic observation of EIA asymmetry using ground based radio beacon receiver from the Indian longitudes. In general, it is seen that strong meridional winds are capable of creating an asymmetry in the EIA strengths in the northern and southern hemispheres. This would also facilitate significant changes in the E region integrated conductivities that could control the F- region dynamics, which in turn would get reflected in the post sunset F region height and eventually in ESF. This mechanism suggested by Maruyama and Matuura  would operate irrespective of the winds being northward or southward. From Figure 2a, one could notice that stronger asymmetry seems to inhibit the ESF in conformity with the above mentioned relationship. However, there are days even when the EIA is significantly asymmetric, still ESF had occurred (for example day number 297 and 302). This implies that EIA asymmetry alone does not suffice to make a deterministic forecast for the generation of ESF on a given day.
 As mentioned earlier, the ESF occurrence or non-occurrence on a given day could be determined by the combined effect of the prevailing electrodymamical as well as the neutral dynamical conditions. The effects of these two independent aspects are manifested in the strength (SF) and asymmetry (AF) of the EIA, giving us an idea on the background conditions of the equatorial ionosphere in the evening hours. In other words, since the layer height at the post sunset hours bears a relation to the evening time SF and AF, we can combine these factors together in the form of a single parameter. In short, SF α h′F and AF α (1/h′F) and hence a combined parameter (C) could be defined as
Figure 3 shows the variation of C with the day number. It can be seen that C is systematically higher for ESF days, having a seasonal variability. Since the ionospheric and thermospheric parameters show a distinct seasonal variability, it is logical to expect a seasonal behavior in the background conditions that are conducive for ESF to occur. Even if we consider a single threshold C = 2, it clearly separates the ESF days except for a few days. This could be attributed to the seasonal variability as we can see that the threshold could be slightly higher for equinox than for solstice. A larger database would be able to bring out this aspect more clearly. The importance of the combined parameter, ‘C’ lies in the aspect that it systematically separates out the ESF days, unlike EIA strength or asymmetry taken independently as it inherently incorporates the EIA/ETWA effects into the ESF prediction.
 In the presence of seed perturbations, the background ionospheric conditions dictate whether the R-T instability could destabilize the equatorial F-region. It is conjectured that the electrodynamics and the neutral dynamics during the course of the day set the stage for the post sunset ESF. The relation between the daytime EIA and the post sunset ESF [Raghavarao et al., 1988; Sridharan et al., 1994] and their possible linkage through meridional winds [Raghavarao et al., 1993; Devasia et al., 2002] only reiterate the complexities of the equatorial thermosphere ionosphere system. Jyoti et al.  highlighted the solar activity effects on the threshold height (h'Fc) for the equatorward wind to play a deterministic role in triggering ESF. All the above studies though broadly refer the background conditions, do not attempt to bring out one single parameter, which could be used for forecasting the ESF well in advance. The present study is the first of its kind making use of the earlier knowledge and attempting toward accomplishing the same. However, when we say ‘deterministic’ we do not anticipate 100% predictability to ESF, at least at this stage since the occurrence depends also on many other factors like the seeding perturbation amplitudes, magnetic activity and so on. The seasonal, solar activity and the longitudinal variability of this parameter, if any, are also to be studied in detail with a more expanded database.
 The first systematic observations of EIA asymmetry as well as EIA strength are presented using latitudinal profiles of TEC obtained from a single ground based radio beacon receiver at Trivandrum. A combination of these values is used to arrive at a forecast parameter that represents the background ionosphere so as to predict the occurrence of post sunset ESF in the time interval 1600–1845 IST. A critical value for the forecast parameter has been identified such that when the estimated value for ‘C’ exceeds it, ESF is seen to occur. It is believed that this new parameter defined in this preliminary study, while providing confirmation to the link between EIA and ESF is through the ETWA related circulations, would turnout to be extremely important in understanding the enigma of the day- to- day variability of the occurrence/non-occurrence of ESF.
 This work was supported by Department of Space, Government of India. One of the authors, Smitha V Thampi, gratefully acknowledges the financial assistance provided by the Indian Space Research Organization through the research fellowship. The useful suggestions from the referees are duly acknowledged.