Previous results
Estimation of the layered conductance from optical data
Scientific objective of this paper is development of methodology to estimate the ionospheric conductance at auroral latitudes using data from a multi-wavelengths photometer (427.8, 557.7, and 630.0 nm). An advantage of this study is derivation of the layered conductance separating the ionosphere according to individual emission heights. The best-fitted function to provide the layered conductance from the optical data was determined by using height-resolved conductivity from the European Incoherent Scatter (EISCAT) Tromsø UHF radar. This study presented that the developed method was able to provide the conductance from the optical data with some confidence (at least in a same level as the previous method) even after separating the ionosphere into three layers, 95-110 km, 110-170 km, and 170-300 km.
Estimation of the ion–neutral collision frequency
Collision between ions and neutral particles is one of the important fundamental processes in the upper atmosphere, but many unresolved problems remain. The ion-neutral collision frequency, νin, a physical parameter representing how often such collisions occur per unit time, is a function of various ionospheric parameters necessary for understanding magnetosphere–ionosphere–thermosphere coupling processes at high latitudes. Although several methods for deriving νin have been proposed, some of these are not suitable for estimating height-dependent temporal variations. We applied a new method to the EISCAT radar data and obtained νin in the lower thermosphere at a height resolution of 3 km. This application makes it possible to reproduce temporal variations in νin associated with geomagnetic activity.
Variance of the vertical ionospheric motion
The vertical component of the ion velocity measured with the EISCAT-UHF radar in the lower ionosphere (from 95 to 130 km) was characterized by notably large variances at oscillation periods of 2－8 minutes. Height profiles of variance of the vertical ion speed during geomagnetically disturbed periods indicated a peak at 120 km. This peak was well reproduced by a theoretical calculation assuming oscillations in the meridional component of the electric field. The statistical and theoretical calculations provided important insight into the magnetosphere－ionosphere coupling study.
Ionospheric/Thermospheric fine structures in the Auroral Arc Vicinity
Simultaneous observations were made with the Reimei satellite and the Sondrestrom IS radar to study the ionospheric/thermospheric fine structures in the vicinity of the auroral arc. The horizontal patterns of electron density, electric field, and auroral emission intensity were measured in association with evolution of the ionospheric closure current system formed by the upward/downward field-aligned currents (FAC) and the Pedersen current. Detailed analysis focused on the horizontal fine structures and found that the ion temperature showed a clear horizontal shear on the scale of a few tens of kilometers, though the ion speed did not exhibit obvious shears corresponding to the ion temperature pattern. This result suggested that horizontal fine structures were also present in the thermosphere.