Ionospheric disturbances in Japan detected by ALOS-2 subband azimuth offset

Abstract Global Navigation Satellite Systems (GNSS) and synthetic aperture radar (SAR) are valuable tools for observing ionospheric disturbances. While GNSS provides continuous temporal data with limited spatial resolution, SAR offers high-resolution spatial snapshots. This study utilizes the subband azimuth offset (SAzO) method, derived from single-day L-band SAR images, to comprehensively investigate ionospheric disturbances over Japan using Advanced Land Observing Satellite-2 (ALOS-2) Ultrafine mode data from 2016 to 2024. Unlike methods requiring image pairs (e.g., split spectrum method, multiple aperture interferometry, pixel offset), SAzO isolates signals from a single observation day and is particularly sensitive to short-wavelength total electron content (TEC) variations, as it measures the second derivative of TEC along the azimuth direction. Our comprehensive application of the SAzO method to a large ALOS-2 archive revealed distinct temporal and spatial characteristics of ionospheric disturbances. Nighttime disturbances (ascending orbits) are significantly more frequent, particularly in summer, consistent with the characteristics of medium-scale traveling ionospheric disturbances. These disturbances are extensive with a nearly uniform wavefront direction. In contrast, daytime disturbances (descending orbits) are less frequent and intense, typically localized with varying wavefront directions, presumably reflecting Sporadic-E (Es) layers. This extensive SAzO dataset provides novel insights into the detailed spatial structure and seasonal trends of ionospheric disturbances over Japan. The identification of numerous suspected disturbance signals provides crucial new data for a phenomenon whose detailed spatial structure has been poorly observed. The SAzO method's ability to process single images and its sensitivity to short-wavelength features make it a powerful tool. The SAzO could also be a useful metric for selecting less ionosphere-affected data and considering an appropriate ionospheric noise reduction strategy in deformation measurement using SAR interferometry. With the increased availability of L-band SAR data from ALOS-4 and NASA-ISRO SAR (NISAR), the SAzO method holds great potential for systematic global ionospheric disturbance monitoring, further advancing ionospheric research and benefiting interferometric SAR applications. Graphical abstract