Background/ Objective: University of Kerala, Thiruvananthapuram (8.5 • N ,77 • E, dip 0.5 • N) in India had a lively Space Physics research group since 1982, owing to proximity of the location to the geomagnetic dip equator. In 1982, a single frequency HF Doppler radar, operational at 5.5 MHz was installed in the University which provided many invaluable scientific results regarding the night time ionospheric F region. In 2003, it was upgraded to a new multifrequency HF Doppler Radar thereby enabling detailed examination of vertical plasma drift measurements at three different altitudes of F-region, in a nearsimultaneous manner. Methods: This study is basically a brief review of major results of F-region electrodynamics over Indian longitudinal sector obtained exclusively using HF Radar data. Findings: This review brings all the scientific results obtained using HF Radar about vertical drift studies over equatorial latitude and found that nearly about 40 research papers have been published in reputed national and international journals using the Radar data. Novelty: In a complementary point of view, this gives an opportunity to revisit and remember the important contributions of many of the earlier Indian scientists, who enriched ionospheric science using this unique facility.
Background/ Objective: The signatures of long period High Frequency Doppler (HFD) oscillations over a dip equatorial station, Trivandrum (8.33 • N, 77 • E, dip 0.4 • N) in the Indian longitudinal sector have been investigated. Methods: High cadence vertical plasma drift measurements obtained using a Multi frequency HF Doppler Radar over Trivandrum along with ground based magnetometer observations over several latitudes/longitudes have been used to address this very aspect. Findings: It has been observed that long period (~15 min.) oscillations of high latitude origin can instantaneously reflect over the equatorial latitudes in the form of coherent HFD oscillations. The study is very important as it provided a unique opportunity not only to understand various characteristics of HFD oscillations, but also expected to have significant implications in the understanding of equatorial ionospheric variability in context of solar wind-magnetosphere-ionosphere coupling. Novelty: In a complementary point of view, the study also highlights the potential of HF Doppler measurements to capture such transient events originating from interplanetary medium with an unprecedented temporal resolution.
Objective: A brief review on the effects of Sudden Stratospheric Warming (SSW) over the equatorial upper atmosphere (80-500 km) obtained using both ground and space based measurements are presented. The emphasis is given to understand the role of semi-diurnal tidal activity during the SSW. Methods: In order to address this aspect, zonal wind data obtained using meteor wind radar over a tropical station; Trivandrum in India (8.5 0 N, 77.0 0 E) has been used. Wavelet analysis has been carried out to investigate the role of semi-diurnal and terdiurnal tides during the SSW event of January 2008. For characterizing the event, polar stratospheric temperature (PST) at 10 hPa, as obtained using National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) reanalysis data has been used. In order to characterize the normal behavior, the aforementioned database during December 2007 has been also looked into. Findings: It has been understood that the tropical upper atmosphere exhibits substantial changes in connection with SSW with an increase in the amplitudes of semi diurnal tides of the order of 10-15 m/s in comparison with the normal period. Over the equatorial ionosphere, the changes were observed right from the E region, extending upto the topside F region. It was noticed that the imprints of SSW were not only restricted to the neutral atmospheric parameters such as temperature, density, wind, but also found to have distinct signatures in the electrodynamic processes such as Equatorial electrojet (EEJ), Equatorial Ionization Anomaly (EIA), Equatorial Spread-F (ESF) as well. Novelty: The study, probably for the first time, has conclusively shown that it is not the terdiurnal tides, but the semi-diurnal tides, which are responsible for the SSW induced effects due to its non-linear interaction with planetary waves.
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