Conjugate hemisphere asymmetric variation in sporadic E: Role of wind shear and meteor ions

The long-term (7 years) ionosonde observations of sporadic E (foEs) at two magnetically conjugate stations in the 95°E longitude, and the HWM vertical wind profiles are utilized to investigate the role of meteor ion deposition rates and vertical wind shear in the seasonal variation of Es. The chosen geometry of the two stations is such that southern station (Cocos islands,12.2°S, 96°N, geo-mag lat:21°S) is geographically at the low latitude, whereas the northern station (Dibrugarh 27.5°N, 95°E, geo-mag lat: 18°N) is at the low mid-latitude due to the north-shifted dip equator. The statistical observations show nearly invariant seasonal characteristics of Es at the southern low-latitude station (during low solar activity)in contrast to the summer maximum at the low mid-latitude station. Both low and high intensity Es over Cocos were found to be anti-correlated with solar activity. However, the high-intensity Es (>7 MHz) at the southern station is more frequent in the solstice. During the high solar activity period, the seasonal invariance is not maintained, and the minimum is recorded during the March equinox. The local time variation of foEs occurrence shows diurnal (semi-diurnal) oscillation at low (mid) latitude station. The HWM vertical profile of zonal winds show that wind shear depth at the low latitude station has low seasonal variation, whereas, at the low middle latitude station the maximum wind shear peaks during summer. Analysis of previously reported radar observations shows that the season of minimum (maximum) meteor deposition rate at equatorial/low-latitude (middle-latitude) regions are aligned with the observed season of minimum (maximum) of Es at low (middle) latitudes. Therefore, the difference of the seasonal variation of sporadic E (Es) at the two conjugate stations may be explained by a combined mechanism of wind shear variation and meteor deposition rates.