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I: f layer postsunset height rise due to electric field prereversal enhancement: 1. traveling planetary wave ionospheric disturbance effects


An ionospheric sounding station is operational at Palmas (10.2o S, 48.2o W, dip latitude 5.5o S), Brazil, since 2002. Observations of F layer virtual height day-to-day variations during evening hours (1800 LT to 2000 LT) show a strong variability, even during geomagnetically quiet periods. From the ionospheric multifrequency virtual height variations (at 3, 4, 5, 6, 7, and 8 MHz), observed from July 2003 to May 2004, it is found that the virtual height day-to-day variability presents oscillations with periods of days during the evening hours. The thermospheric wind component perpendicular to the magnetic meridian (zonal wind) is one of the primary sources that generate the F region dynamo near sunset, which causes the zonal electric field prereversal enhancement (PRE) that induces the E _ B vertical F layer postsunset height rise. Therefore, the planetary wave (PW) component that flows superposed on the thermospheric wind induces a traveling planetary wave ionospheric disturbance (TPWID) on the vertical F layer displacement. This indicates that the postsunset ionospheric height rise can be strongly modulated by TPWID oscillations. Our study shows that TPWIDs with periods of several days control the strength of the electric field PRE and, therefore, slowly push the F layer heights up or down, according to the TPWID phase. Also, simultaneous virtual height variations at Sao Jose´ dos Campos (low latitude) and Palmas (equatorial region) for October and November show similar behavior. This suggests that TPWID oscillation is a manifestation of atmospheric equatorial Kelvin waves that modulate the thermospheric wind. (AU)