Modeling of the electron density depletion in the storm-time trough on April 20, 1985

Multi-instrument experimental data are analyzed to determine the main processes forming a deep trough in the electron density at F-peak altitudes during a strong magnetic storm (K_p?5). Previous attempts to explain the observations were not successful. The model we use to interpret the data includes production of vibrationally excited N₂ in the region poleward of the trough and its transport into the trough region by a southward wind. The main source of the vibrationally excited N₂ is secondary electrons created by precipitating electrons. Joule heating and dissipation of precipitating electron energy create a pressure gradient and induce the southward wind. According to the model calculations, such a system of processes can cause the very strong electron density depletion observed by the Millstone Hill incoherent scatter radar on April 20, 1985. An important additional condition for such a deep trough is a decrease in the O]/N₂] ratio. The total energy flux and average energy of precipitating electrons just poleward of the trough is also a factor.