Sporadic E layer characteristics at equatorial latitudes as observed by GNSS radio occultation measurements

Abstract

Scintillations in the signal-to-noise (SNR) profiles of GNSS radio occultation (RO) measurements at lower ionospheric altitudes are caused by compact layers of high ionization also known as sporadic E (Es) layers. It is widely accepted that Es layers are formed by the wind shear mechanism at northern and southern midlatitudes. However, approaching the equatorial regions, electric fields also control the formation and dispersal of these layers. In this paper we concentrate, in particular, on the occurrence rate and on the altitudes of Es layers appearing in a narrow band along the Earth’s magnetic equator. We analyzed several million of RO profiles concerning sporadic E occurrence as well as altitude and observed a high daily, seasonal and longitudinal variability which is controlled partly by zonal winds and electric fields. Especially Es layers at higher altitudes show a clear anticorrelation with the zonal electric field intensity measured by the Swarm satellites. Further, we solve the existing contradiction of Es layer signatures being present in equatorial ionosonde measurements while they are only rarely seen in RO recordings.