Spread-F occurrences and relationships with foF2 and h′F at low- and mid-latitudes in China

Abstract

Ionospheric irregularities are an important phenomenon in scientific studies and applications of radio-wave propagation. Spread-F echoes in ionograms are a type of high-frequency band irregularities that include frequency spread-F (FSF), range spread-F (RSF), and mixed spread-F (MSF) events. In this study, we obtained spread-F data from four ionosondes at low-and mid-latitudes near the 120°E chain in China during the 23rd solar cycle. We used these data to investigate spread-F occurrence percentages and variations with local time, season, latitude, and solar activity. The four ionosondes were located at Haikou (HK) (20°N, 110.34°E), Guangzhou (GZ) (23.14°N, 113.36°E), Beijing (BJ) (40.11°N, 116.28°E), and Changchun (CC) (43.84°N, 125.28°E). We also present possible correlations between spread-Fs and other ionospheric parameters, such as the critical frequency of the F2-layer (foF2) and the virtual height of the bottom-side F-layer (h′F). In particular, we investigated the possible threshold of the foF2 affecting the FSF and the relationship between the h′F and the RSF. The main conclusions are as follows: (a) the FSF occurrence percentages were anti-correlated with solar activity at all four sites; meanwhile, RSF occurrence rates increased with the increase in solar activity at HK, but not at the other three sites; (b) FSF occurrence rates were larger at the mid-latitudes than expected, while FSFs occurred more often after midnight; (c) the highest FSF occurrence rates mostly appeared during the summer months, while RSFs occurred mostly in the equinoctial months of 2000-2002 at HK and GZ; (d) a lower foF2 was suitable for FSF events; nevertheless, h′F and RSF occurrences satisfied the parabolic relationship; (e) the foF2 thresholds for FSFs were 15, 14, 7.6, and 7.8 MHz at HK, GZ, BJ, and CC, respectively. The h′Fs occurring between 240 and 290 km were more favorable for RSF occurrences. These results are important for understanding ionospheric irregularity variations in eastern Asia and for improving space weather modeling and forecasting capabilities .

Mid‐Latitude Spread‐F Structures Over the Geomagnetic Low‐Mid Latitude Transition Region: An Observational Evidence

“…To summarize, there are few earlier investigations that report the occurrence of mid‐latitude irregularity structures using multiple instruments such as all‐sky airglow imager, VHF radar and ionosonde above 25°N/S of geomagnetic latitudes. In these studies, all‐sky airglow imagers (e.g., Garcia et al, ; Otsuka et al, ; Sahai et al, ) were used to show the MSTID structures whereas, VHF radar (e.g., Fukao et al, ; Otsuka et al, ; Swartz et al, ) or the ionosonde (e.g., Wang et al, ; Yang et al, ) observations were primarily used to identify the mid‐latitude FAIs/Spread‐F signatures. The present investigation reports a nearly geomagnetic field aligned (slightly tilted) westward drifting airglow depletion over 25°N geomagnetic latitude which is different from the previous investigations.…”

Longitudinal differences in the statistical characteristics of ionospheric spread-F occurrences at midlatitude in Eastern Asia

“…The longitudinal and latitudinal effects of the spread-F occurrences at midlatitude have been discussed by many authors (Huang et al 2011;Kherani et al 2009;Wang et al 2018;Perkins 1973;Yakoyama et al 2008;Zhou et al 2005). They argued that AGW and Perkins instability may be the main mechanism for the occurrence and evolution of spread-F at midlatitude.…”

“…Spread-F has been widely studied since it was first defined on the ionogram in the early 1930s (Booker and Wells 1938). Several observations since then revealed the main morphological features of spread-F occurrence, including its dependences on solar and magnetic activities, season, longitude, latitude, local time and the background ionosphere (Aarons et al 1980;Abdu et al 1981Abdu et al , 1983Abdu et al , 1998Fukao et al 2004;Li et al 2013;Lynn et al 2011;Wang et al 2007Wang et al , 2018Xu et al 2010). Previous studies have focused on low latitudes especially for the equatorial region.…”

Statistical analysis of low-latitude spread F observed over Puer, China, during 2015–2016

“…Statistical characteristics of nighttime SF have been studied over the past several decades. Earlier observational evidences suggested that the occurrence of SF at equator and low latitudes has obvious diurnal, latitudinal, longitudinal, seasonal, solar cycle and geomagnetic activity variations (Abdu et al 1981(Abdu et al , 1985Li et al 2010;Upadhayaya and Gupta 2014;Rungraengwajiake et al 2013;Wang et al 2010;Dabas et al 2007;Zhu et al 2015;Wang et al 2018). Pezzopane et al (2013) performed a comparative analysis of equinoctial SF characteristics at different longitude sectors at low latitudes using observations of ionosonde.…”

“…Moreover, the negative correlations between Kp and RSF, MSF and SSF were also presented in their work, except FSF. On the other hand, Wang et al (2018), Dabas et al (2007) and Rungraengwajiake et al (2013) carried out comparative analysis of SF recorded by ionosondes at different latitudinal regions. Their results showed that RSF was dominant over the stations near equator which occurred mostly in equinoxes and at post-sunset.…”