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II: f layer Postsunset height rise due to electric field prereversal enhancement: 2. traveling planetary wave ionospheric disturbances and their role on the generation of equatorial spread f


This paper investigates the role played by traveling planetary wave ionospheric disturbances (TPWIDs) on the modulation of the electric field prereversal enhancement (PRE) amplitude, near-sunset hours, in the equatorial ionosphere, and their influence on the day-to-day equatorial spread F (ESF) variability. We show that ''fresh'' (ESF) day-to-day variability is closely related with the postsunset height rise due to the electric field PRE, and there is some indication that the altitude of 300 km may be a threshold height for the generation of fresh ESF. Observations of F layer virtual heights during evening hours (1800 LT, 1900 LT, and 2000 LT) present a strong day-to-day variability, even during geomagnetically quiet periods. In fact, this well-organized variability is induced by TPWIDs-type oscillations that push the F layer up or down according to the TPWID phase. The preliminary analysis presented here show that, when the planetary TPWID oscillations or geomagnetic activity raises the F layer around sunset hours, the occurrence of fresh ESF is increased, but when the TPWID-type oscillations or geomagnetic activity pushes the F layer downward, the fresh ESF occurrence is reduced. Also, the TPWID phase modulated the onset time of the fresh ESF during high fresh ESF season. The modulation acts in such way that, when the TPWID phase maximizes, the fresh ESF onset time occurs earlier than when the TPWID phase minimizes and the fresh ESF onset time occurs later. (AU)

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