Statistical Study on the Occurrences of Postsunset Ionospheric E, Valley, and F Region Irregularities and Their Correlations Over Low‐Latitude Sanya

Abstract

The coupling of equatorial and low-latitude ionosphere has been an active area of research for many years. At low latitude, the occurrences of E and valley region irregularities during postsunset hours are obviously different from those around the magnetic equator. In this work, we statistically investigated the occurrences of postsunset ionospheric E, valley, and F region irregularities, the postsunset rise of F layer, and their correlations by using the Sanya (18.4°N, 109.6°E, dip latitude 12.8°N) very high frequency radar and ionosonde observations during equinoctial months of 2012-2016. A statistically significant correlation between the presence of valley region irregularities and that of equatorial spread F (ESF) was found over Sanya. For the days without ESF, valley region irregularity was rarely observed. The Doppler velocities of valley region irregularities were mainly upward which resemble those of simultaneous ESF irregularities. On the other hand, an occurrence peak of E region irregularities was detected around sunset. After sunset, E region irregularity echo was frequently disrupted. The disruption can appear on both ESF and non-ESF days, when the amplitudes of F layer postsunset rise were significantly different. The effects of polarization electric field associated with ESF and of equatorial ionospheric background condition on the E and valley region irregularities over Sanya are discussed.

A Statistical Study of F‐Region 3.2‐m‐Scale Field‐Aligned Irregularities Occurrence and Vertical Plasma Drift Over Hainan: Solar Activity, Season, and Magnetic Activity Dependences

“…The altitude of the FAIs usually kept declining as it went into the midnight and postmidnight sector, meaning that the FAIs occurred during this period were usually in the decay phase and were mostly continuation of postsunset FAIs which drifted from outside the FoV. This altitude‐universal time distribution of FAIs occurrence rate is similar to the Sanya VHF radar observation during equinoctial months of 2012–2016 (Xie et al., 2018 and see Figure 2 in their paper). The Doppler velocities of postmidnight echoes were mostly negative (see Figure 3 in their paper), which reinforces our assertion that postmidnight FAIs in equinox over Hainan were mostly remnant structure of the premidnight echoes.…”

“…VHF radars have been used for detecting backscatter echoes caused by the Bragg scatter from FAIs with a scale size of half of the radar wavelength. For instance, Xie et al (2018) investigated the occurrences of ionospheric E-region, valley and F-region FAIs in the postsunset period by using the observations from the low-latitude fivebeam Sanya, Hainan Island of China (18.4°N, 109.6°E) VHF radar and Sanya Digisonde during equinoctial months of 2012. Hu et al (2020 studied the statistical characteristics and correlation of equinoctial bottom-type and plume irregularities over Sanya during equinox of 2011-2018 and found that bottom-type and plume irregularities showed opposite correlation with F-layer virtual height.…”

Statistical Characteristics and Correlation of Low‐Latitude F Region Bottom‐Type Irregularity Layers and Plasma Plumes Over Sanya

“…This could be related to the altitude extension of plasma plumes over the magnetic equator. Over low‐latitude station Sanya, only the plume structure rising higher than ~500 km apex altitude over the magnetic equator can extend along magnetic field lines to the Sanya latitude and be detected (e.g., Xie et al, 2018). Under low solar flux condition, the plume‐type structure generated around the magnetic equator may be confined within a small altitude range at later night and could not be observed by the radars far away from the magnetic equator.…”

IONISE: An Ionospheric Observational Network for Irregularity and Scintillation in East and Southeast Asia

“…Now, the subradar works with an operating frequency of 47.5 MHz and a peak power of 24 kW. The radar is sensitive to irregularities of 3‐m scale size and is a useful tool to study the ionospheric E , valley, and F ‐region irregularities over Hainan (e.g., Li et al, 2017; Ning et al, 2012; Xie et al, 2018). The antenna array is composed of six identical modules (each module consists of 2 × 2 five element Yagi antennas) aligned in the EW direction that can receive backscatter echoes separately and independently.…”