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

Abstract

A relatively new method based on measurements by multipoint 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 ~200 m s −1 during nighttime hours. The mean observed horizontal velocity was 140 m s −1 over Tucumán and 107 m s −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.

Spread-F characteristics over Tucumán near the southern anomaly crest in South America during the descending phase of solar cycle 24

“…Alfonsi et al (2013) reported a maximum spread-F occurrence in local summer in Tucumán from October 2010 to September 2011. Using continuous Doppler sounding, Chum et al (2016) reported that most of the spread F events over Tucumán were observed during local summer. Chen et al (2006) described the equatorial F region irregularities in two Peruvian stations from April 1999 to March 2000.…”

“…In the Brazilian region, earlier works showed a high plasma bubble occurrence rate during the December solstice and a low occurrence during the June solstice (Abdu et al 1998;Sahai et al 1994). Chum et al (2016) examined the spread-F behaviour over Tucumán and Taiwan in 2014 based on measurements by continuous Doppler sounding. They observed the highest occurrence rate from September to March in Tucumán, and around equinoxes in Taiwan.…”

Near Real‐Time Global Plasma Irregularity Monitoring by FORMOSAT‐7/COSMIC‐2

“…By using a constellation of satellites equipped with the GNSS receivers, radio occultation (RO) scintillation is utilized to detect the location and height of the plasma irregularities (e.g. Chang et al., 2018; Chum et al., 2016; Huang et al., 2014; Liu et al., 2016; Wu et al., 2005). Those RO scintillation intensities are measured in a very similar magnitude of strength in comparison with the ground‐based scintillation observations (Chen et al., 2017), and show a similar distribution of EPBs to those using satellite in situ plasma measurements (e.g.…”

Ground-based measurements of ionospheric dynamics

“…Both the ground based measurements and satellite radio occultation measurements of S4 were used. Chum et al (2016b) studied occurrence rates of spread F, longitudinal distribution of spread F occurrence rates and zonal drift velocities of spread F and concluded that all the instruments provided consistent results.…”

“…spread F. Spread F, especially at low latitudes, can significantly deteriorate the quality of GPS (GNSS) signal and decrease the precision of position determination. Chum et al (2016b) compared the observation of equatorial (low-latitude) spread F by ionosondes, CDSs, and GPS (GNSS) receivers. The spread F was detected by GPS receivers as high values of the scintillation index S4.…”