The Possible Effect of Turbopause on Formation of Mid-Latitude Sporadic E layers

Sporadic E (Es) layers are thin enhanced ionization patches. The electron density of Es layer is very high, up to 100 times that of the regular E layer. Therefore, Es layer has a significant effect on radio wave propagation. The formation mechanisms of Es layers is very complicated, so it needs multi-observations and simulations research. Combined observations using an ionosonde, very high frequency (VHF) coherent backscatter radar, VHF broadband backscatter radar, and meteor radar were performed. The height and irregular structure of mid-latitude Es layers observed by ionosonde, VHF coherent backscatter radar and VHF broadband backscatter radar were compared with the zonal and meridional winds observed by meteor radar, the role of wind shear on the formation of Es layer was confirmed, and examples were also found where wind shear theory could not explain and apply to the formation of Es layer. We simulated the formation of Es layers based on wind shear theory. The results of simulations and observations revealed other potential role for Es layer formation besides wind shear. By comparing the height of turbopause with that of Es layer, it was found that the variation of the altitudes of turbopause and Es layer was consistent over different latitude stations, and the effect of the turbopause on the formation of Es layer was proposed. The combined effects of eddy diffusion below and molecular ion collision above turbopause can prevent metal ions from drifting downward and promote their convergence to form thin plasma layers at the turbopause height. The turbopause accumulation effect is considered as a potentially important mechanism for the formation of Es layers besides wind shear theory. The study in the paper is helpful for analysis of physical origin and modeling of Es layer.