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

Abstract

The postsunset equatorial ionosphere is studied with rocket‐borne plasma diagnostic instruments within the Dynamics of the Equatorial Ionosphere Over Shriharikota Range (DEOS) campaign. Data from an impedance probe and a Langmuir probe allow for a critical comparison between the instruments, with ionosonde data, and with the International Reference Ionosphere model. During flight F05, which was launched before the onset of equatorial spread F (ESF), a generally smooth density profile was found. Density depletions (bubbles) are only found when the rocket crosses the lower edge of the F layer during the downleg of the trajectory. In flight F06, launched shortly after onset of ESF, a large‐scale depletion is found on the upleg of the trajectory below 360‐km altitude, where a transition to an unperturbed density profile 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 previous investigations. Electron temperature measurements with Langmuir probes show that the temperature in the depleted region is lower than that in the ambient plasma.