The early phase of spread F development studied in situ with impedance andLangmuir probes

The postsunset equatorial ionosphere is studied with rocket-borne plasma di agnostic instruments within the Dynamics of the Equatorial Ionosphere Over Shriharikota Range (DEOS) campaign. Data from an impedance probe and a Lang muir probe allow for a critical comparison between the instruments, with io nosonde data, and with the International Reference Ionosphere model. During flight F05, which was launched before the onset of equatorial spread F (ES F), a generally smooth density profile was found. Density depletions (bubbl es) are only found when the rocket crosses the lower edge of the F layer du ring the downleg of the trajectory. In flight F06, launched shortly after o nset of ESF, a large-scale depletion is found on the upleg of the trajector y below 360-km altitude, where a transition to an unperturbed density profi le occurs. Impedance probe and Langmuir probe data show close agreement of the density depletions and their fine structure with a resolution of 100 m. The morphology of the plasma depletion is compared with models and previou s investigations. Electron temperature measurements with Langmuir probes sh ow that the temperature in the depleted region is lower than that in the am bient plasma.