On the Origin of Summer Sporadic E Periodicities at Midlatitudes

Summertime midlatitude sporadic E ( Es ) layers exhibit daily periodicities whose underlying physical causes remain to be fully identified. We investigate the origin of these periodicities by analyzing an 11‐year data set (2007–2017) comprising ionosonde measurements, FORMOSAT‐3/COSMIC radio occultation (RO) observations, and WACCM‐X neutral wind model simulations. The analysis focuses on four Northern Hemisphere midlatitude regions centered on four ionosonde sites. Wavelet spectral analysis of ionosonde and composite RO time series reveals dominant 12‐h and 24‐h periodic variations in both the intensity and altitude of sporadic E layers during the summer months. In addition to spectral analysis, height‐time‐intensity (HTI) plots are obtained to identify the presence of tidal‐like periodicities. It is found that the observed 12‐h periodicities are clearly manifested in the downward altitudinal movements of Es layers over time, which correspond to the vertical motion of metal‐ion plasma convergence nodes driven by semidiurnal tidal wind shears. This evidence confirms previously published results indicating that the 12‐h periodicity originates from tidal winds, a mechanism further supported here by WACCM‐X simulations of wind variations over altitude and time. Conversely, the regular 24‐h periodicity in Es height and intensity, which stands out prominently in the wavelet spectrograms, is shown to be of non‐tidal origin. Instead, it is likely associated with daytime Es enhancements driven by the photoionization of metal atoms, a mechanism proposed recently to account for the intensification of sporadic E during sunlit hours. This latter finding challenges published postulations that attribute the 24‐h sporadic E periodicity to diurnal tidal winds.