Abstract The sporadic E (Es) layer is a prominent ionospheric irregularity mainly driven by vertical wind shear at mid‐latitudes. Hereby we statistically investigate for the first time Es responses to variations of the northern polar vortex (represented by NAM index) using long‐term ionosonde observations over Japan (44 years) and Australia (34 years). The analysis reveals clear polar vortex modulation of Es, with increasing/decreasing foEs on low/high NAM days over Japan and decreasing foEs over Australia on low NAM days with a time lag of 5–7 days. This hemispheric asymmetry is largely attributed to nearly anti‐phase wind shear responses in two hemispheres. Our results demonstrate the modulation of deep connection between stratosphere dynamics and ionospheric irregularities, emphasizing the importance atmosphere‐ionosphere coupling. It suggests that the NAM index could be used to increase the accuracy of Es layer prediction and serve as indicator for assessing the risk of Es layer occurrence in advance. This has practical implications for fields such as radiocommunications and over‐the‐horizon radar.
Maeda et al. (Fri,) studied this question.