R.G. Ezquer

Propagation of gravity waves and spread F in the low-latitude ionosphere over Tucumán, Argentina, by continuous Doppler sounding: First results

Results of systematic analysis of propagation directions and horizontal velocities of gravity waves (GWs) and spread F structures in low-latitude ionosphere (magnetic inclination ~27°) in Tucuman region, Argentina, are presented. Measurements were carried out by multipoint continuous Doppler system during 1 year from December 2012 to November 2013. It was found that meridian propagation of GWs dominated and that southward propagation prevailed in the local summer. Oblique spread structures observed in Doppler shift spectrograms and associated with spread F propagated roughly eastward at velocities from ~70 to ~180u2009m/s and were observed at night fromu2009~u2009September tou2009~u2009March. The velocities were computed for 182 events and the azimuths for 64 events. Continuous Doppler sounding makes it possible to analyze more events compared to optical observations often used for propagation studies since the measurements do not depend on weather.

Scintillations observed at Tucuman as observed from GPS signals. First results

The occurrence of ionospheric irregularities was studied using Global Positioning System (GPS) measurements at Tucuman (26.9° S, 294.6° E; geomag. lat.: −15.5), a station placed near the souther peak of the equatorial anomaly. Radio scintillations at the GPS L1 frequency (1.575 GHz) were monitored using the GPS receiver installed at the mentioned station. The considered measurements were obtained during September 1998 – April 1999 period. The results show that amplitude scintillations frequently occur between 21 and 02 LT. Scintillations are more frequent when the satellites are in the northwest — northeast sector of Tucumans sky. Strong scintillations were observed in about 40% of the nights.

Spread F occurrence and drift under the crest of the equatorial ionization anomaly from continuous Doppler sounding and FORMOSAT-3/COSMIC scintillation data

AbstractA relatively new method based on measurements by nmultipoint continuous Doppler sounding is applied to study the occurrence rate, propagation velocities, and directions of spread F structures over Tucumán, Northern Argentina, and Taiwan, both of which were under the crest of the equatorial ionization anomaly in 2014. In addition, spread F is studied globally over the same time period from the S4 scintillation index measured onboard FORMOSAT-3/COSMIC (F3/C) satellite. It is shown that the continuous Doppler sounding gives results that are consistent with S4 data and with previous optical, global positioning system (GPS), and satellite measurements. Most of the spread F events were observed from September to March, i.e., during the local summer half of the year in Tucumán, whereas in Taiwan, the highest occurrence rate was observed around equinoxes. The occurrence rate in Tucumán was about four times higher than that in Taiwan. The propagation velocities and directions were estimated from the Doppler shift spectrograms. The spread structures related to spread F propagated roughly eastward at velocities from ~70 to ~200xa0mxa0s−1 during nighttime hours. The mean observed horizontal velocity was 140xa0mxa0s−1 over Tucumán and 107xa0mxa0s−1 over Taiwan. The local times at which the highest velocities were observed roughly correspond to local times with highest values of scintillation index S4, at ~20 to 23 LT. In addition, a comparison of measured drift velocities with neutral wind velocities predicted by models is provided. The observed velocities usually exceeded the horizontal neutral wind velocities predicted by the HWM14 model for the locations and times of observations.