Topside Ionospheric Temperatures As a Function of Solar Cycle Measured By Cindi from the C/NOFS Spacecraft

Abstract:

Since the April 26, 2008 launch of C/NOFS, the Ion Velocity Meter (IVM), a part of the CINDI instrument package, has made in-situ measurements of plasma temperature, composition, density, and velocity in the 400-850 km altitude range of the equatorial ionosphere. The low inclination orbit caused the perigee to advance through all local times in about 67 days allowing seasonal sampling of ionospheric temperature (T_i), density, and composition. Here we examine the variations of these quantities as a function of altitude, local time, and solar cycle. The time period immediately following launch had unusually low solar activity with F10.7 cm radio fluxes consistently around 70 sfu. This condition resulted in a cold (T_i ~600 K in the pre-dawn region) low-density ionosphere near the spacecraft’s 400 km perigee altitude and a low O⁺/H⁺ transition height. In contrast, by 2012 the average F10.7 was ~130 sfu and only in the pre-dawn region (T_i ~ 800K) did the transition height drop below the spacecraft’s 800 km apogee. Using the IRI-2012 model we observe that T_i varies between the neutral temperature at low altitudes at night and the electron temperature (T_e) in the morning above 500 km. At all levels of solar activity there is a midday trough in T_i. In the 400-650 km altitude range it is observed that T_i is actually higher in the daytime during the low solar activity period of 2008-2009 than in the moderate solar activity period of 2011-2012.