Daytime Equatorial Spread F‐Like Irregularities Detected by HF Doppler Receiver and Digisonde

Abstract

The ionosphere is an indispensable medium for the propagation of communication and navigation signals. It has been extensively studied over the years to understand its properties and variability under different conditions. Spread F (SF) is a common feature of the ionosphere that is caused by irregularities in electron density distribution in the F-layer. Radio signals incident on these irregular plasma structures are scattered, thereby causing diffuse spreading in the F-layer trace of ionograms. Bottomside spread F was first reported by Booker and Wells (1938) and a number of mechanisms are understood to be responsible for its occurrence and development. Post-sunset SF is most commonly attributed to increased upward vertical plasma drift (also known as the pre-reversal enhancement [PRE]) in response to an enhanced eastward electric field. Due to the absence of a highly conducting E-layer at night, plasma instabilities can develop more easily at elevated F-layer heights and can subsequently grow via the Rayleigh-Taylor instability (RTI). This phenomenon is even more common at equatorial latitudes because the geometry of the geomagnetic field at these latitudes facilitates  E B plasma drifts (Sreehari et al., 2006). Apart from the PRE associated with post-sunset F-region electrodynamics, other mechanisms known to contribute to F-layer plasma instabilities include zonal winds, atmospheric gravity waves (AGWs) which manifest in the ionosphere as traveling ionospheric disturbances (TIDs), and trans-equatorial thermospheric winds (Abdu, 2012). The role of the RTI in the development of the equatorial spread F (ESF) was first suggested by Dungey (1956). The RTI develops when a heavy fluid rests on a lighter one. In such a state, gravity or other triggers can cause the heavier liquid to become unstable. Ossakow (1979) and references therein discussed the role of the F-layer peak and electron density scale length in the linear growth of the RTI. A higher F-layer peak and smaller electron density scale length are both favorable conditions for the linear growth of the RTI.In the mathematical formulation by Huang et al. (1993), it was shown that AGWs can directly seed the RTI. Huang et al. (1993) showed that AGWs with speed 5-20 ms −1 and wavelength in the order of 100 km Abstract Daytime equatorial spread F (ESF) is not as common as nighttime ESF due to the presence of a highly conducting E-layer during the daytime which counteracts the development of F-layer plasma irregularities. This study presents two rare daytime ESF-like events which occurred over an interval ∼2 h and were detected by the HF Doppler receiver located in Lagos (LAG: geographic: 3.27°E, 6.48°N; dip latitude −1.72°) and the Lowell Digisonde at Ilorin (ILR; 4.68°E, 8.50°N; dip latitude −1.25°), managed by Lowell GIRO Data Center (LGDC). Analysis of the first event revealed ∼30 min periodic oscillations in iso-heights of ionospheric electron density. Shorter period (∼15 min) oscillations appeared simultaneously in HF Doppler measurements and these oscillat...

Case Studies on the Day‐to‐Day Variability in the Occurrence of Post‐Sunset Equatorial Spread F

“…Olugbon et al. (2021) also pointed out that changes in ionospheric ED appeared about the same time that large amplitude oscillations appeared in F‐layer plasma. Changes in ionospheric conductivity have been previously reported to cause a change in the polarization of ultra‐low frequency waves at low and equatorial latitudes (Saka et al., 1980, 1982, 1988).…”

“…Interestingly, for the first event reported, harmonic frequencies were observed in the spectra of ΔsTEC from LGLA when compared with those from ABFC. Olugbon et al (2021) also pointed out that changes in ionospheric ED appeared about the same time that large amplitude oscillations appeared in F-layer plasma. Changes in ionospheric conductivity have been previously reported to cause a change in the polarization of ultra-low frequency waves at low and equatorial latitudes (Saka et al, 1980(Saka et al, , 1982(Saka et al, , 1988.…”

“…In Olugbon et al. (2021), two daytime ESF‐like events in Nigeria, which were associated with TIDs were investigated. Interestingly, for the first event reported, harmonic frequencies were observed in the spectra of ΔsTEC from LGLA when compared with those from ABFC.…”

Observations of Unusual Daytime Range Spread F at Middle Latitude During the Afternoon Hours

“…It is worthy to note that Olugbon et al. (2021) presented two daytime equatorial spread F events by the HF Doppler receiver and indicated that wave structures related to TIDs were responsible for the daytime ESF‐like irregularities. In the middle latitude regions, Bowman et al.…”

“…Luo et al (2020) reported the post-sunrise plasma irregularities at low latitude region during a moderate geomagnetic storm and considered that these plasma irregularities were not developed from the previous night, but were newly generated near sunrise hours due to the disturbance of the dynamo electric field. It is worthy to note that Olugbon et al (2021) presented two daytime equatorial spread F events by the HF Doppler receiver and indicated that wave structures related to TIDs were responsible for the daytime ESF-like irregularities. In the middle latitude regions, Bowman et al (1987) found that distorted traces of the main traces on daytime ionograms were caused by the passage of TIDs, and suggested these daytime distorted traces was a kind of the spread F-phenomenon.…”