Abstract
AbstractThe wind shear theory is widely accepted as an explanation for the formation of a sporadic E (Es) layer, but the direct comparison of Es with the local wind shear has been limited due to the lack of neutral wind measurements. This study examines the role of the vertical wind shear for Es, using signal‐to‐noise ratio profiles from COSMIC‐2 radio occultation measurements and concurrent measurements of neutral wind profiles from the Ionospheric Connection Explorer. It is observed that the Es occurrence rate and average S4 index are correlated with the negative vertical shear of the eastward wind, providing observational support for the wind shear theory. Es can be observed even when the vertical wind shear is positive, which is interpreted as metallic ion layers generated at an earlier time.
Highlights
Metallic ion layers are commonly observed in the ionosphere
We have investigated the relationship between Es and vertical wind shear, using vertical profiles of COSMIC-2 radio occultation signal-to-noise ratio (SNR) and Ionospheric Connection Explorer (ICON)/Michelson Interferometer for Global High-Resolution Thermospheric Imaging (MIGHTI) neutral winds
Global seasonal climatologies of the Es occurrence rate and S4 scintillation index derived from COSMIC-2 SNR profiles are in good agreement with those previously reported using other data, which validates the COSMIC-2 Es data
Summary
Metallic ion layers are commonly observed in the ionosphere. Those which occur at E-region heights show high plasma concentrations, and they can cause anomalous high frequency (HF) radio propagation, known as Es. The characterization of the phenomenon and the formation mechanism for Es layers have been a subject of study for many decades (see, e.g., reviews by Haldoupis, 2011; Mathews, 1998; Whitehead, 1989). The formation of an Es layer requires the convergence of ion flux. The wind shear theory (Axford, 1963; Whitehead, 1961) suggests that the required ion flux convergence can be realized by the vertical shear of horizontal neutral winds. The following simple expression can be obtained for the vertical component of the ion velocity, under the assumption of balance between the Lorentz force due to ion motions across the ambient geomagnetic field and the frictional force due to ion-neutral collisions (e.g., Haldoupis, 2011):
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