Vertical fine structure and time evolution of plasma irregularities in the Es layer observed by a high-resolution Ca+ lidar

The vertical fine structures and the time evolution of plasma irregularities in the sporadic E (E-s) layer were observed via calcium ion (Ca+) density measurements using a resonance scattering lidar with a high time-height resolution (5s and 15m) at Tachikawa (35.7 degrees N, 139.4 degrees E) on December 24, 2014. The observation successfully provided clearer fine structures of plasma irregularities, such as quasi-sinusoidal height variation, localized clumps, cats-eye structures, and twist structures, in the sporadic Ca+ (Ca+s) layers at around 100km altitude. These fine structures suggested that the Kelvin-Helmholtz instabilities occurred in the neutral atmosphere whose density changed temporarily or spatially. The maximum Ca+ density in the Ca+s layer was two orders of magnitude smaller than the maximum electron density estimated from the critical frequency (f(o)E(s)) simultaneously observed by the ionosonde at Kokubunji (35.7 degrees N, 139.5 degrees E). A strong positive correlation with a coefficient of 0.91 suggests that Ca+ contributes forming the E-s layer as well as major metallic ions Fe+ and Mg+ in the lower thermosphere. Moreover, the formation of a new Ca+s layer at 110km and the upward motions of the Ca+s layers at 100km and 110km were observed before the local sunrise and just after the sunrise time at the conjugation point. Although the presence or absence of a causal relationship with the sunrise time was not clear, a possible explanation for the formation and the upward motions of the Ca+s layers was the occurrence of strong horizontal wind, rather than the enhancement of the eastward electric field.