Climatology analysis of the daytime topside ionospheric diffusive O+ flux based on incoherent scatter radar observations at Millstone Hill

This paper reports the characteristics of the topside ionospheric O+ diffusive flux (phi O+) during both geomagnetically quiet (0 <= Kp <= 2) and moderate (2 < Kp <= 4) times using incoherent scatter radar observations at Millstone Hill (42.6 degrees N, 288.5 degrees E) for solar minimum from 1970 to 2018. phi O+ partially characterizes plasma mass exchange between the upper and lower part of the topside ionosphere through diffusion and sometimes serves as upper boundary conditions for ionosphere-thermosphere models. The altitude where the flux sign changes (mainly during daytime) is termed the transition height and the time when the flux sign changes (mainly at dawn and dusk) is termed the transition time. At quiet times, the daytime transition height is similar to 100 km above the F2 peak height (h(m)F(2)) in summer, and it is about 50 km above h(m)F(2) in other seasons; the transition time is before 18 solar local time (SLT) in spring and winter, but after 18 SLT in summer and autumn. The daytime average upward phi O+ above the transition height shows a significant seasonal variation with a minimum of 2.2x108cm-2s-1 in summer and a maximum of 3.7x108cm-2s-1 in autumn. Under geomagnetically moderate conditions, the transition height increases by similar to 20 km in spring, winter, and autumn, but moves up by about 20-50 km in summer. The transition time occurs later by similar to 1 hr in summer but similar to 1 hr earlier in other seasons. The mean upward phi O+ peaks in summer and minimizes in spring.