The 27-day modulation of the low-latitude ionosphere during a solar maximum

Abstract

The 27-day solar modulation of the low-latitude ionosphere is investigated for the solar maximum period using in situ satellite measurement data and the total electron contents (TEC) estimated from the satellite signals of the global positioning system (GPS). Whereas the density and temperature of the topside ionosphere observed at an altitude of 685 km manifest delayed responses to the 27-day variations in the daily F10.7 values, similar to those previously reported for an altitude of 840 km, the nighttime-scale height, obtained by comparing the densities observed at altitudes of 685 and 840 km at similar local times, was shown to vary in accordance with the changes in F10.7 with the same time delay. The oxygen ion fraction measured at an altitude of 840 km shows a similar response regardless of the local time. Moreover, the GPS TEC values, most of which come from the F peak region, also exhibit similar delayed modulations in accordance with solar rotation. The TEC value correlates well with the thermospheric neutral density, and both are observed to be modulated with the solar rotation with time delay, especially when a long-term variation is filtered out. The present result confirms that the whole thermospheric and ionospheric system is modulated with solar rotation.