Some characteristics of spread F at the magnetic equatorial station Fortaleza

Abstract

Spread F in ionograms of both the range and frequency spread types is studied for the magnetic equatorial station Fortaleza (geographic coordinates 38°W, 4°S; dip latitude −1.7°) for a 1‐year period during 1978–1979). Satellite traces suggesting ionization structures with scale sizes of 100–200 km are found to be necessary precursors to the formation of the irregularities that give rise to range type spread F, whereas frequency type spread F was observed under different circumstances. Striking differences are noted in the spread F characteristics and related parameters over Fortaleza as compared with Huancayo (geographic coordinates 75.3°W, 12.05°S; dip latitude 0.6°) and Jicamarca (geographic coordinates 76.8°W, 11.95°S; dip latitude 0.9°), situated within only ∼38° in longitude west of Fortaleza. The postsunset maximum in h′F, in winter, is delayed by about 1½ hr with respect to other seasons over Fortaleza, as compared with Huancayo, where it occurs earlier in winter than in other seasons. Similar differences also exist in the prereversal peak near sunset in the vertical drift velocity (Vz) between Fortaleza and Jicamarca. The differences are attributed possibly to the large difference in the magnetic field declination and to other factors such as the difference in the magnetic field intensity and the relative separation between geographic and magnetic equators that exist between Fortaleza and the other stations.

Satellite traces, range spread-F occurrence, and gravity wave propagation at the southern anomaly crest

“…Satellite traces in equatorial ionograms appear as one or more quasi replicas of the Flayer trace at displaced virtual heights prior to the ESF onset, and are probably produced by oblique reflections from upwelling walls, that is, corrugations in the isodensity surfaces. This issue was very well illustrated by Tsunoda (2008) who provided further evidence, besides those advanced by Wright (1959), Lyon (1961), Abdu et al (1981), andMcNamara et al (2008), that STs are a characteristic precursor of ESF over the equatorial belt. Fagundes et al (2009a, b), as well, showed that PSSR can be strongly modulated by traveling planetary wave ionospheric disturbance oscillations.…”

Equatorial scintillations in relation to the development of ionization anomaly

“…3.3 Post-sunset F-region height rise over the magnetic equator and scintillations Generation of equatorial irregularities over the magnetic equator in the post sunset hours is intimately related to the variation of height of the F-layer around sunset (Farley et al, 1970; Abdu et al, 1981). It was established that a sharp rise of F-region altitude due to the pre-reversal enhancement of the eastward electric field is often followed by the generation of intense irregularities through Rayleigh-Taylor instability mechanism.…”

A comparative study of GPS ionospheric scintillations and ionogram spread F over Sanya

“…The irregularities exist at altitudes from 200 to 1000 km or even higher, with scale sizes ranging from tens of centimeters to hundreds of kilometers. Numerous means of observations such as satellite in situ and ground-based receivers, radar, rocket, optical imager and ionosondes have been applied to study the spatial and temporal distributions and characteristics of the irregularities during the last few decades at equatorial, low, middle and high latitudes (Abdu et al, 1981;Hysell and Burcham, 1998;Sahai et al, 2000;Su et al, 2008;Lynn et al, 2011).…”