Midnight ionosphere collapse at Arecibo and its relationship to the neutral wind, electric field, and ambipolar diffusion

We report the analysis of midnight collapse, a large drop in the F-layer peak height (HmF2) around midnight, observed at Arecibo during Jan. 14-22, 2010. During the nine nights of observations, the first four nights (Jan. 14-17) exhibited modest drops in HmF2 while the last five nights (Jan. 18-22) showed more severe drops. We examine the roles played by the meridional wind, electric field, and ambipolar diffusion in driving the vertical ion motion. The collapse process can be classified into three stages: preconditioning, initial descent, and sustained descent. Severe collapses occur when HmF2 is preconditioned high prior to the collapse. Ambipolar diffusion is most important during the initial descent. Neutral wind and electric field are responsible for sustaining the collapse. During Jan. 18-22, HmF2 was pushed high by the neutral wind before the collapse started. Neutral wind and electric field were in phase during the sustained severe collapses. The diurnal tide of the meridional wind provided the general condition for the collapses. The terdiurnal tide was most important to cause the difference between the two periods in our observation. Previous studies largely emphasized meridional wind being the dominant mechanism causing midnight collapse. Our study suggests that electric field and ambipolar diffusion also play an important role and the former can be the most dominant factor in some cases.