Off‐great‐circle paths in transequatorial propagation: 2. Nonmagnetic‐field‐aligned reflections

Abstract

There is considerable evidence that plasma structure in nighttime equatorial F layer develops from large‐scale wave structure (LSWS) in bottomside F layer. However, crucial details of how this process proceeds, from LSWS to equatorial plasma bubbles (EPBs), remain to be sorted out. A major obstacle to success is the paucity of measurements that provide a space‐time description of the bottomside F layer over a broad geographical region. The transequatorial propagation (TEP) experiment is one of few methods that can do so. New findings using a TEP experiment, between Shepparton (SHP), Australia, and Oarai (ORI), Japan, are presented in two companion papers. In Paper 1 (P1), (1) off‐great‐circle (OGC) paths are described in terms of discrete and diffuse types, (2) descriptions of OGC paths are generalized from a single‐reflection to a multiple‐reflection process, and (3) discrete type is shown to be associated with an unstructured but distorted upwelling, whereas the diffuse type is shown to be associated with EPBs. In Paper 2 (P2), attention is placed on differences in east‐west (EW) asymmetry, found between OGC paths from the SHP‐ORI experiment and those from another near‐identical TEP experiment. Differences are reconciled by allowing three distinct sources for the EW asymmetries: (1) reflection properties within an upwelling (see P1), (2) OGC paths that depend on magnetic declination of geomagnetic field (B), and (3) OGC paths supported by non‐B‐aligned reflectors at latitudes where inclination of B is finite.

Post-sunset rise of equatorial F layer—or upwelling growth?

“…6. There is mounting evidence that EPBs develop where there are pre-existing upwellings (LSWS) [e.g., Tsunoda and White (1981); Kelley et al (1981); Singh et al (1997); Eccles (2004); Fukao et al (2006); Thampi et al (2009); Tsunoda et al (2011Tsunoda et al ( , 2016aTsunoda et al ( , 2016b; Tulasi Ram et al 2012; "Equatorial Atmosphere Radar (EAR) and ionosondes (CPN, KTB)" and "Case study: 27 March 2006, " and additional references in Tsunoda (2015)].…”

Arrival Angle and Travel Time Measurements of HF Transequatorial Propagation for Plasma Bubble Monitoring

“…On 14–17 April 2011, eastward intrusion of DOAs associated with the sunset effect (Maruyama & Kawamura, ; Tsunoda et al, ) was seen around 10 UT. According to Tsunoda et al (), it is considered to be due to a strong postsunset rise of the ionosphere around the day‐night boundary near the magnetic equator slightly to the east of the great‐circle propagation path. In the dayside (before 10 UT), the Radio Australia signals were received at the azimuth angle around the great circle direction.…”

“…Recently, Tsunoda et al (, ) analyzed DOAs of OGCP of HF radio waves together with ionosonde measurements between the transmitter and receiver and showed that they could be classified into two groups, discrete and diffuse types. They concluded that the discrete‐type OGCP was due to multiple reflections of HF radio waves by unstructured upwellings associated with plasma bubbles, while the diffuse‐type OGCP was due to reflection from plasma structure in plasma bubbles.…”