Open AccessOnset conditions of equatorial (range) spread F at Fortaleza, Brazil, during the June solstice
Author(s) -
Sastri J. H.,
Abdu M. A.,
Batista I. S.,
Sobral J. H. A.
Publication year - 1997
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/97ja02166
Subject(s) - solstice , atmospheric sciences , earth's magnetic field , morning , latitude , climatology , range (aeronautics) , f region , sunset , thermosphere , ionosphere , geology , geodesy , physics , geophysics , astronomy , materials science , quantum mechanics , magnetic field , composite material
Case studies are made of the ionospheric, thermospheric, and geomagnetic conditions associated with the occasional onset of range spread F at Fortaleza (4°S, 38°W, dip latitude −1.8°, and magnetic declination 21°W), Brazil, in the June solstice, a season in which frequency spread F is typically seen in the postsunset hours (after 2000 LT). It is found that the F layer experiences quite consistently a large vertical drift early in the evening hours (1815–1915 LT) on days of range spread F in comparison to days of only frequency spread F . The anomalously large dusk time vertical plasma drift occurs under both geomagnetically disturbed and quiet conditions. There is no significant change in the pattern of meridional neutral winds (at and prior to the time of onset of range spread F ) estimated from the data of F layer peak height ( h max ) at the low‐latitude station, Cachoeira Paulista (23°S, 45°W, and dip 26.5°S), using a modified form of the servo model. The results strongly suggest that though the meridional (poleward) wind is very effective in inhibiting the growth of Rayleigh‐Taylor (R‐T) instability and hence range spread F at Fortaleza during the June solstice as shown by Maruyama [1996], its variability does not play an important role in creating favorable conditions for R‐T instability on a day‐to‐day basis. The prerequisite for the occasional occurrence of range spread F at Fortaleza in the June solstice seems to be the presence of an impulsive and large F layer vertical plasma drift, a condition favorable for destabilizing the bottomside F region through R‐T instability mechanism because of the high altitude of the layer.