Formation of Sporadic E (Es) Layer by Homogeneous Horizontal Wind

Abstract

Theoretically and by corresponding numerical simulations it is shown that the formation and localization of sporadic E (Es) layer at its mainly observable mid-latitude lower thermosphere heights can be determined by homogeneous horizontal wind velocity direction and value. In the suggested theory, differently from 'windshear' theory, the wind direction and value, in addition to geomagnetic field and vertically changing ion-neutral collision frequency, determine the minimal negative value of the divergence of heavy metallic ions drift velocity, which in turn causes ion convergence into Es type horizontal thin layer. Here, in the upper heights of the lower thermosphere, the Es layer peak density and thickness are also controlled by ion ambipolar diffusion.   

 In the lower thermosphere of the northern hemisphere, the Es layer caused by horizontal homogeneous wind can be located at height regions where (1) the ions vertical drift velocity is zero and its divergence is negative (east-northward wind), (2) the ions drift downward (northward and westward wind), which occurs more frequently,  or (3) the ions drift upward  (eastward wind)  and their negative divergences vanish   and  (4) in the case of dominance of southward wind the divergence of ion drift velocity is positive, consequently ion density divergence occurs and Es type layer formation is not expectable. The Es layer density increase and its vertical motion to its expectable location are faster for greater values of the horizontal wind velocity. The possibility of development of the suggested theory for vertically inhomogeneous wind is noted.