| Summary: | Abstract 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 (5 s and 15 m) at Tachikawa (35.7°N, 139.4°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+ ($${{\text{Ca}}^{+}}_{\text{s}}$$ Ca+s ) layers at around 100 km 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 $${{\text{Ca}}^{+}}_{\text{s}}$$ 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°N, 139.5°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 $${{\text{Ca}}^{+}}_{\text{s}}$$ Ca+s layer at 110 km and the upward motions of the $${{\text{Ca}}^{+}}_{\text{s}}$$ Ca+s layers at 100 km and 110 km 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 $${{\text{Ca}}^{+}}_{\text{s}}$$ Ca+s layers was the occurrence of strong horizontal wind, rather than the enhancement of the eastward electric field.
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