The Height-Dependent Delayed Ionospheric Response to Solar EUV

Based on the analysis of electron density Ne profiles (Grahamstown ionosonde), a case study of the height-dependent ionospheric response to two 27-day solar rotation periods in 2019 is performed. A well-defined sinusoidal response is observed for the period from 27 April 2019 to 24 May 2019 and reproduced with a Thermosphere-Ionosphere-Electrodynamics General Circulation Model simulation. The occurring differences between model and observations as well as the driving physical and chemical processes are discussed based on the height-dependent variations of Ne and major species. Further simulations with an artificial noise free sinusoidal solar flux input show that the Ne delay is defined by contributions due to accumulation of O+ at the Ne peak (positive delay) and continuous loss of O2+ in the lower ionosphere (negative delay). The neutral parts' 27-day signatures show stronger phase shifts. The time-dependent and height-dependent impact of the processes responsible for the delayed ionospheric response can therefore be described by a joint analysis of the neutral and ionized parts. The return to the initial ionospheric state (and thus the loss of the accumulated O+) is driven by an increase of downward transport in the second half of the 27-day solar rotation period. For this reason, the neutral vertical winds (upwards and downwards) and their different height-dependent 27-day signatures are discussed. Finally, the importance of a wavelength-dependent analysis, statistical methods (superposed epoch analysis), and coupling with the middle atmosphere is discussed to outline steps for future analysis