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Several investigations have been carried out to identify the factors that are responsible for the day-to-day variability in the occurrence of equatorial spread-F (ESF). But the precise forecasting of ESF on a day-to-day basis is still far from reality. The nonlinear development and the sustenance of ESF/plasma bubbles is decided by the background ionospheric conditions, such as the base height of the F-layer (h'F), the electron density gradient (dN/dz), maximum ionization density (Nmax), geomagnetic activity and the neutral dynamics. There is increasing evidence in the literature during the recent past that shows a well developed Equatorial Ionization Anomaly (EIA) during the afternoon hours contributes significantly to the initiation of ESF during the post-sunset hours. Also, there exists a good correlation between the Equatorial Ionization Anomaly (EIA) and the Integrated Equatorial ElectroJet (IEEJ) strength, as the driving force for both is the same, namely, the zonal electric field at the equator. In this paper, we present a linear relationship that exists between the daytime integrated equatorial electrojet (IEEJ) strength and the maximum elevated height of the F-layer during post-sunset hours (denoted as peak h'F). An inverse relationship that exists between the 6-h average Kp-index prior to the local sunset and the peak h'F of the F-layer is also presented. A systematic study on the combined effects of the IEEJ and the average Kp-index on the post-sunset, peak height of the F-layer (peak h'F), which controls the development of ESF/plasma bubbles, is carried out using the ionosonde data from an equatorial station, Trivandrum (8.47° N, 76.91° E, dip.lat. 0.5° N), an off-equatorial station, SHAR (13.6° N, 79.8° E, dip.lat. 10.8° N) and VHF scintillations (244 MHz) observed over a nearby low-latitude station, Waltair (17.7° N, 83.3° E, dip.lat. 20° N). From this study, it has been found that the threshold base height of the F-layer at the equator for the development of plasma bubbles is reduced from 405 km to 317 km as the solar activity decreases from March 2001 (mean Rz=113.5) to March 2005 (mean Rz=24.5). This decrease in threshold height with the decreasing solar activity is explained on the basis of changes in the local linear growth rate of the collisional Rayleigh-Taylor instability, due to the variability of various terms such as inverse density gradient scale length (L−1), ion-neutral collision frequency (νin) and recombination rate (R) with the changes in the solar activity.

The GNU Radio Beacon Receiver (GRBR) Network has been recently established to provide coverage of Southeast Asia and Pacific low-latitude regions, with planned extensions into the Indian and African longitude sectors. With the availability of CERTO (Coherent Electromagnetic Radio Tomography) beacon transmissions from Communication/Navigation Outage Forecasting System (C/NOFS) satellite, which is in a unique low-inclination (13°) orbit, it is now possible to study zonal large scale wave structure (LSWS) in ionospheric total electron content (TEC) with fine spatial resolution over a wide longitudinal region. An automated procedure to determine absolute TEC from relative TEC measurements for low inclination CNOFS orbits has been implemented through a simple single station procedure for initial offset estimation, which is shown to be consistent with the better established two station method and with observations from a Digisonde. The LSWS is extracted by subtracting the background variation from longitudinal variation of TEC. The upwellings of LSWS manifest as depletions in the residual TEC variations. Further, these zonal structures have been found, in general, to be aligned with geomagnetic field lines, and the scintillation patches have been found to align with the west walls of TEC depletions. This spatial alignment recapitulates the premise that the observed zonal wave-like structures in TEC are the manifestations of bottom side LSWS. Hence, the methodology presented in this paper, would prove useful in future, to study the characteristics of LSWS on a regular basis.

Using ground observations of total electron content (TEC) and equatorial electrojet (EEJ) in the Asian sector, along with plasma and neutral densities obtained from the CHAMP satellite, we investigate the ionospheric electrodynamics and neutral background in this longitude sector during the major stratospheric sudden warming (SSW) in January 2009. Our analysis reveals the following prominent features. First, the TEC response in tropical regions is strongly latitude dependent, with monotonic depletion at the dip equator but a semidiurnal perturbation at low latitudes. Second, the TEC semidiurnal perturbation possesses a significant hemispheric asymmetry in terms of onset date and magnitude. It starts on the same day as the SSW peak in the Northern Hemisphere but 2 days later in the Southern Hemisphere. Its magnitude is twice as strong in the north than in the south. Third, strong counter electrojet occurs in the afternoon, following the strengthening of the eastward EEJ in the morning. Fourth, semidiurnal perturbation in both TEC and EEJ possesses a phase shift, at a rate of about 0.7 h/day. Comparisons with results reported in the Peruvian sector reveal clear longitude dependence in the amplitude and hemispheric asymmetry of the semidiurnal perturbation. Finally, thermospheric density undergoes ?25% decrease at low latitudes in the afternoon local time sector during the SSW, indicating significant cooling effects in the tropical upper thermosphere.

Using ground observations of total electron content (TEC) and equatorial electrojet (EEJ) in the Asian sector, along with plasma and neutral densities obtained from the CHAMP satellite, we investigate the ionospheric electrodynamics and neutral background in this longitude sector during the major stratospheric sudden warming (SSW) in January 2009. Our analysis reveals the following prominent features. First, the TEC response in tropical regions is strongly latitude dependent, with monotonic depletion at the dip equator but a semidiurnal perturbation at low latitudes. Second, the TEC semidiurnal perturbation possesses a significant hemispheric asymmetry in terms of onset date and magnitude. It starts on the same day as the SSW peak in the Northern Hemisphere but 2 days later in the Southern Hemisphere. Its magnitude is twice as strong in the north than in the south. Third, strong counter electrojet occurs in the afternoon, following the strengthening of the eastward EEJ in the morning. Fourth, semidiurnal perturbation in both TEC and EEJ possesses a phase shift, at a rate of about 0.7 h/day. Comparisons with results reported in the Peruvian sector reveal clear longitude dependence in the amplitude and hemispheric asymmetry of the semidiurnal perturbation. Finally, thermospheric density undergoes ∼25% decrease at low latitudes in the afternoon local time sector during the SSW, indicating significant cooling effects in the tropical upper thermosphere.

Periodicities in the area of coronal hole (CH) regions on the solar disk and solar wind (SW) high-speed streams (HSSs) have been studied, for the first time, during complete solar cycle 23 (SC 23) from 1996 to 2009 using solar EUV image data from the Solar and Heliospheric Observatory and STEREO satellites and ACE solar wind-magnetic data. Both the SW velocity and the area of CH regions consistently exhibit large values during the declining phase and the minimum of SC23 (2003–2009) due to equatorward extended holes and/or low-latitude isolated holes. Further, the SW velocity and CH area exhibit a greater tendency for subharmonic (13.5 and 9 day) periodicities during the declining phase and solar minimum. The response of Earths' geomagnetic and ionospheric properties to these periodicities associated with corotating interaction regions in SW HSSs is studied, with a focus on the extremely low-solar-activity period of 2008. Subharmonic oscillations in both day- and nightside ionospheric electron density are found to correlate well with oscillations in SW and Kp during 2008. The topside ionospheric response (above 350 km) appears to be dominated by changes in the plasma temperature and/or scale height and exhibits concurrent enhancements with the oscillations in geomagnetic activity during both day- and nighttime. However, the electron density response at altitudes between 200 and 350 km is dominated by changes in the neutral composition and exhibits significant latitudinal, local time, and seasonal variations. The results are discussed in light of equatorward wind perturbations during enhanced geomagnetic activity and summer-to-winter transequatorial neutral wind patterns.