Ionogram signatures of large‐scale wave structure and their relation to equatorial spread F

Abstract

[1] Observations of signatures of the large-scale wave structure (LSWS) and subsequent development of equatorial spread F (ESF) by using data from a newly installed digital ionosonde at Trivandrum (8.5 N; 77 E; 0.5 N magnetic dip latitude) are presented. The LSWS signatures are observed using the ionograms with echo directions. The 'satellite' traces are shown as echoes from oblique directions, when an upwelling (associated with LSWS) is not directly over the ionosonde location. When an upwelling is centered overhead, and the radius of curvature of the isodensity contours matches the height of reflection, signals from various directions can add in phase to enhance the ionospheric reflection coefficient, and produce another ionogram signature, referred to as multireflected echoes (MREs). It is also shown that the generation of ESF is related to the presence of LSWS. The skymaps also show the presence of small-scale wave structures in the bottomside F region. The investigation also indicates that the range spread F (RSF) occurs because of the reflections from the embedded smaller-scale structures within the large-scale upwelling.

Upwelling: a unit of disturbance in equatorial spread F

“…Additional evidence regarding upwelling presence comes from two ionogram signatures, namely multi-reflected echoes (MREs) (Tsunoda 2009;Thampi et al 2012;Tsunoda et al 2013) and satellite traces (STs) (Abdu et al 1981;Tsunoda 2008;Thampi et al 2012). The finding that MREs appear most often around the time of SS E (Tsunoda et al 2015) is consistent with the finding that upwellings are amplified during the PSSR; in fact, the maximum in PSSR rate is centered more or less about SS E (Tsunoda 2009;Tsunoda et al 2015).…”

“…Enhancement is possible, when reflection occurs from concave isodensity contours, such as those associated with an upwelling (Tsunoda 2009;Thampi et al 2012). Enhancement should be strongest, when the radius of curvature of the reflecting surface is approximately equal to the height of reflection.…”

“…To illustrate, we present a sketch in Figure 10 that illustrates what likely happens to paths of radio signals, when an upwelling with plasma structure is situated to the west of an ionosonde, and when the same structure is situated to the east of the station. To the west, rays can find surfaces of reflection along the west wall of the upwelling (where the bottomside shape of secondary EPBs are encountered) that will allow the rays to return to the ionosonde (see Thampi et al (2012) for further discussion). We refer to these rays as monostatic rays; they are drawn with red lines.…”

Off‐great‐circle paths in transequatorial propagation: 1. Discrete and diffuse types

“…The ESF appeared to the east at 1730 UT (Figure f); it appeared to recede in the ionogram taken at 1745 UT and was gone by 1800 UT (Figure h). Strong MREs [ Tsunoda , 2009; Thampi et al ., ] were observed at 1845 UT, which could be interpreted in terms of upwelling presence in the vicinity. Eastward drifting ESF structures have been reported before, for example, by Argo and Kelley [, Figure 6] and by Tsunoda [, Figure 3].…”

Study of Large-Scale Wave Structure and Development of Equatorial Plasma Bubbles Using the C/NOFS Satellite

“…Other than scanning radars (e.g., ALTAIR), the only instrument capable of providing information about spatial ionospheric structure (without horizontal transport) is an ionosonde [Tsunoda, 2005]. Toward this end, we have shown that multi-reflected echoes (MREs) are ionogram signatures for LSWS [Tsunoda, 2009;Thampi et al, 2012]. Figure 4 presents an example of MREs in an ionogram taken at Kwajalein.…”