Low‐altitude image striations associated with bottomside equatorial spread F: Observations and theory

Abstract

Ionospheric plasma instabilities are usually discussed in terms of local parameters. However, because electric fields of scale size λ ≳ 1 km map along magnetic field lines, plasma populations far away from a locally unstable region may be affected by the instability process and vice versa. We present observations of electron density variations in the F1 region of the ionosphere at two locations near the magnetic equator. Oscillations in electron number density that were confined to a narrow wavelength regime were observed in a region of the ionosphere with a very weak vertical density gradient. Since magnetic flux tube interchange instabilities cannot create structure in such an environment we suggest that these are “images” of instabilities occurring elsewhere along the magnetic field line. A simple steady state theory of image formation is developed that is in good agreement with the observations. Moreover, this theory predicts a scale size dependent “effective diffusion” process in the F region that may dominate over classical cross‐field diffusion at kilometer scale sizes. Such a scale size dependent diffusion process is required to explain recent scintillation observations of decaying equatorial plumes.