The F1 ledge: density, height and slope

Diurnal variations of the F1 region ionization at 170 km altitude and the slope ?170 = dN/dh at this height are analyzed for mid-latitude and equatorial stations: Millstone Hill, Ramey, Puerto Madryn and Jicamarca. Both the density N(t) and the slope ?(t)at 170 km show well defined diurnal variations with day-to-day variabilities of less than 10%. The heights of the F1 ledge, hmF1, are spread over ± 20 km and are therefore of limited value for modeling purposes.