Aspect Angle of Plasma Irregularities in the Ionosphere Measured by Using the Radar Imaging of VHF Arrayed Radar

Multireceiver and multifrequency radar imaging techniques, implemented in the 46.5 MHz MU radar in Japan (34.85°N and 136.10°E), were employed to investigate the aspect sensitivity of field-aligned plasma irregularities (FAIs) in the mid-latitude ionospheric E region. Aspect sensitivity of refractive irregularities in the atmosphere describes the radar echo intensity varying with the radar beam direction. For the FAIs in the ionosphere, the radar echoes are generally strongest at the beam direction perpendicular to the geomagnetic field line, and decay rapidly with off-perpendicular angle along the geomagnetic field line. This feature relates partly with the values of electron-neutral collision frequency (ν_e), electron gyrofrequency (Ω_e), ion-neutral collision frequency (ν_i), ion gyrofrequency (Ω_i), the ratio ν_e/Ω_e, among others, and moreover, is also subjected to nonlinear coupling process of unstable waves in the plasma irregularities. In practical observation, the radar beam of the MU radar was directed to geographic north and at 51° zenith angle, which was normal to the geomagnetic field line around 100-110 km height (range: ~160-175 km along the beam direction). Five carrier frequencies and nineteen receivers were operated for radar imaging to retrieve the power distribution in the radar volume, and then the angular power distribution was used to estimate the aspect angle along the geomagnetic field line. Retrieval algorithms such as Fourier, Capon, and norm-constrained Capon (NC-Capon) were utilized, in which the NC-Capon was applied to FAIs for the first time and found to be more suitable for the present study. The aspect angles estimated by the NC-Capon algorithm ranged between 0.1° and 0.4° mostly, which are close to the previous measurements with the radar interferometry (RI) made for the lower mid-latitude sporadic E region and the equatorial electrojet irregularities.