An analysis of the scale heights in the lower topside ionosphere based on the Arecibo incoherent scatter radar measurements

[1] We statistically analyze the ionospheric scale heights in the lower topside ionosphere based on the electron density (N-e) and temperature profiles observed from the incoherent scatter radar (ISR) at Arecibo (293.2 degrees E, 18.3 degrees N), Puerto Rico. In this study, a database containing the Arecibo ISR observations from 1966 to 2002 has been used in order to investigate the diurnal and seasonal variations and solar activity dependences of the vertical scale height (VSH), which is deduced from the electron concentration profiles defined as the value of -dh/d( ln(N-e)), and the effective scale height (H-m), which is defined as the scale height in the Chapman-alpha function to approximate the N-e profiles. As a measure of the slope of the height profiles of the topside electron density, the derived VSH and H-m show marked diurnal and seasonal variations and solar activity dependences. Their features are discussed in terms of thermal structures in the lower topside ionosphere. We also investigate the quantitative relationships between H-m, VSH, and plasma scale height (H-p) over Arecibo. The similarities and differences in these scale heights are discussed. Results suggest that both the contributions from topside temperature structure and diffusion processes can also greatly control VSH and Hm through changing the profile shape.