On the estimation of solar wind velocity under varying solar activity conditions using <i>Akatsuki</i> measurements

ABSTRACT We present an analysis of solar wind dynamics based on Doppler spectral width measurements of X-band radio signals from the Japanese Akatsuki spacecraft. The data set includes two solar conjunction occultation experiments conducted in 2016 and 2022, capturing the transition from the descending phase of Solar Cycle 24, a period of low solar activity, to the ascending phase of Solar Cycle 25, which exhibited moderate to intense activity. Our study demonstrates the utility of this technique for estimating both slow and fast solar wind velocities across different phases of solar activity. A key focus is the 2022 experiment, which probed the solar corona near coronal holes at heliocentric distances ranging from 1.4 to 10 $\mathrm{ R}_\odot$. We also investigate the impact of electron density estimates on the accuracy of solar wind speed determinations, underscoring the need for improved electron density modelling to enhance the robustness of such measurements.