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

Jaroslav Chum,Miguel Angel Cabrera,Jiří Fišer,Rodolfo Ezquer,Jann-Yenq Liu,Jiří Baše,Jan Laštovička,Shih-Ping Chen,Dalia Burešová,František Hruška

doi:10.1186/s40623-016-0433-1

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.

Highlights

Ionospheric irregularities known as spread F often prevent accurate scaling of ionograms and obtaining reliable values of F2 layer peak characteristics such as critical frequency foF2 and peak height hmF2 (McNamara et al 2008), which limits our knowledge of the ionosphere and predictability of conditions for radio wave propagation
The zonal velocities derived from observations of ionospheric irregularities at night are eastward with magnitudes decreasing from the geomagnetic equator to the equatorial ionization anomaly (EIA) crests (Kil et al 2002)
We present horizontal velocities of spread F structures observed by multipoint continuous Doppler sounding over Tucumán, Northern Argentina (27°S, 65°W, inclination of magnetic field I = 27°), and over Taiwan (24°N, 121°E, inclination of magnetic field I = 35°) and we compare the values with neutral wind velocities obtained from recent experimental model HWM14 (Drob et al 2015)

Summary

Introduction

Ionospheric irregularities known as spread F often prevent accurate scaling of ionograms and obtaining reliable values of F2 layer peak characteristics such as critical frequency foF2 and peak height hmF2 (McNamara et al 2008), which limits our knowledge of the ionosphere and predictability of conditions for radio wave propagation. It is assumed that plasma bubbles are generated above the geomagnetic equator and stretch along the magnetic field lines to low latitudes (Sultan 1996; Keskinen et al 1998; Bhattacharyya and Burke 2000). The zonal velocities derived from observations of ionospheric irregularities at night are eastward with magnitudes decreasing from the geomagnetic equator to the equatorial ionization anomaly (EIA) crests (Kil et al 2002). The mean zonal velocities of ionospheric irregularities at the Brazilian Saõ Luís equatorial station were larger during the December solstice and decayed during the equinoctial periods (Muella et al 2009). Low-latitude all-sky imager observations in northern Argentina and Peru revealed at stations closer to the magnetic equator weaker (stronger) eastward plasma drifts in the postsunset (postmidnight) period (Martinis et al 2003). Superrotation of the ionosphere was observed in zonal drift measurements, at lower latitudes

Objectives

Findings

Conclusion