Distortion calibrations and specifications of ionospheric plasma bubbles using fisheye lens (OI 630-nm airglow) images over Taiwan

This paper presents all-sky airglow image analyses from OI 630-nm fisheye lens (FEL) photographs on the equatorial plasma bubble (EPB) observations over Taiwan and the nearby area. A FEL image calibration procedure of using single image with concentric circle and radial line control features has been described and applied to transform the raw FEL images into the expected perspective projection images. Two approximations on FEL image transformation have been proposed and evaluated to model the varying spatial resolution using polynomial and cubic spline interpolation functions. The validity and error analyses of the FEL image transformations are also demonstrated by testing a synthetic spiral coil image. For further applications, a low-cost miniature all-sky imaging system using Sony IMAX462-mode FEL camera and open-source indi-allsky operation software was set up and operated at the Lulin Mountain (23.47°N, 120.87°E; 2842m altitude), Taiwan. The obtained field of view is from around 19.2°N to 27.8°N latitude (geomagnetic latitude 10.12°N∼18.74°N) and 116.6°E to 125.2°E at an assumed airglow emission height of 250 km. We demonstrate the experimental all-sky airglow observations and obtain bifurcated and parallel plasma depletions separately during two events of post-sunset and midnight EPBs occurred before and after the spring equinox of 2025, respectively. The FEL image distortion calibration technique yields a “consistent” determination of nighttime plasma drifts from the transformed all-sky images on EPB observations and motion analyses. The derived eastward plasma drifts were higher at lower latitudes than those at higher latitudes and showed negative latitudinal gradients of the zonal plasma velocities during both events.