Understanding the Diurnal Cycle of Midlatitude Sporadic E. The Role of Metal Atoms

Midlatitude ionosonde observations show that there is a sporadic E (Es) diurnal cycle that starts in higher altitudes at sunrise. This property is labeled as the “Sporadic E sunrise effect.” Given that sporadic E layers are composed of metal ions, the sunrise effect implies that metal atom solar photoionization could play a role in the diurnal variability of sporadic E occurrence. This possibility is endorsed by Arecibo's incoherent scatter radar observations, showing that weak ion layers at lowest E and uppermost D region heights appear at sunrise to live during the daytime, apparently coming out of nighttime metal atom mesospheric layers. The solar photoionization of metal atoms increases the abundance of metal ions available for Es layer generation during the daytime, whereas this effect is absent at nighttime. This can explain why sporadic E layers start or intensify at sunrise all year round and why Es activity maximizes during sunlit hours, as has been reported in many ionosonde and satellite radio occultation studies. The significance of metal atom solar photoionization on the regular diurnal variation of Es went unnoticed, despite existing evidence for a long time. The present paper provides a base for a better physical understanding of the pronounced 24‐hr periodicity in Es layer intensity and places a step toward the improvement of simulation models for the predictions of sporadic E layer characteristics. Plain Language Summary “Sporadic E” (Es) is a term introduced in the ionosonde studies to describe layers of enhanced ionization that form in the midlatitude E region. The sporadic E formation mechanism involves long‐living metal ions of meteoric origin which converge vertically in a wind shear under the action of the geomagnetic field Lorentz force driven by neutral winds. In recent years, there has been evidence to suggest that sporadic E is not as “sporadic” as the name implies, but a regularly occurring phenomenon that is subject to a great deal of variability at various temporal and spatial scales. Among the Es variations, a prominent one is its diurnal variation, established from ionosonde and satellite radio occultation studies. The present research deals with the diurnal variation of sporadic E, aiming to provide an integrated physical understanding, which so far remained incomplete. Besides the tidal wind control of the diurnal variability and altitude descent of sporadic E layers, a new physical mechanism is introduced that involves me