A PHYSICAL MODEL OF IONOSPHERE FOR LOW LATITUDES AND ITS' APPLICATIONS IN DISCUSSIONS OF F₁ VALLEY FORMATION

F₁ valley has recently been one of the interesting issues in ionospheric F₂ region.In this paper,a physical model of ionosphere for mid-and low-latitudes is taking into account self-consistently both the neutral wind and motions of ions.The speeds of neutral particles and ions are obtained by simultaneously solving sets of dynamic equations satisfied by those two kinds of particles,with are coupled through collisions.Four species of ions are included to represent photo-chemical reactions.With this ionospheric model and focusing on the area of China's central part,i.e.,116° and 30℃,the effects of photo-chemical and dynamical processes on the F₂ region are discussed with particular emphasis put on the influence of ions' vertical motion on F₁ valley.Results show that under photo-chemical equalibrium,E region always forms and a F₁ ledge appears only in summer at low solar activity,but deep F₁ valley and formation of F2 region can not be explained solely by photo-chemical process.Ambipolar diffusion is falally important for F2 formation.Vertical motions of ions caused by neutral wind does not affect E region much,but can produce a considerably profound valley in summer at low solar activity with a depth of 0.05-0.1.Diurnal variations of F₁ valley is also examined and results show that this model can reflect some features of F₁ valley for China's mid-and low-latitude area.