BEHAVIOR OF THE IONOSPHERIC SLAB THICKNESS OVER CHINESE MID-LATITUDE REGION FOR A HALF SOLAR CYCLE

This paper deals with the variations in equivalent slab thickness of the ionosphere over Chinese mid-latitude region for a half solar cycle (1981-1985) using satellite ETS-2 radio beacon observations at Xinxiang (400km subionospheric point: 32.4°N,115.6°E) and Chongqing (400km subionospheric point;27.2°N,108.7°E).The contours and three-dimensional plots for diurnal and seasonal variations of slab thickness are obtained.The harmonic components of diurnal changes in slab thickness are given by Fourier analysis.By the method of linear regressions,a weakly linear dependence of slab thickness with 12 month running mean values of sun-spot number is also obtained.At the same time,the existence of a predawn peak in slab thickness around 0400 to 0700 LT is noted,and the mechanism causing the predawn peak in slab thickness is briefly discussed.