Coronal Electron Density Fluctuations Inferred from Akatsuki Spacecraft Radio Observations

Abstract

Trans-coronal radio observations were taken during the 2011 observing campaign of the Akatsuki spacecraft through superior conjunction. The observed X-band (8.4 GHz) signals exhibit frequency fluctuations (FF) that are produced by temporal variations in electron density along the radio ray path. A two-component model for interpretation of the FF is proposed: FF scales largely with acoustic wave amplitude through the inner coronal regions where the sound speed dwarfs the solar wind outflow speed, while FF in the region of solar wind acceleration is dominated by the increased density oscillation frequency on the sensing path that results from bulk advection of the plasma inhomogeneities. An estimate of fractional electron density fluctuation is obtained from the mid-corona. A radial profile of slow solar wind speed is determined in the extended corona using mass-flux continuity principles. The coronal sonic point for slow solar wind is estimated to range from 4 to 5 solar radii from the heliocenter.

Detecting the Oscillation and Propagation of the Nascent Dynamic Solar Wind Structure at 2.6 Solar Radii Using Very Long Baseline Interferometry Radio Telescopes

“…We are able to measure the radial velocity of the solar wind because Hh related baselines cover a broad range in the projected radial distances, finding the streamer wave because Zc-Mc, Mc-Ys, and Zc-Ys cover a broad range in latitudinal distances, but a comparatively short range in radial distance, and finding the field-aligned fast stream because it happened to be highly anisotropic along Hh-Ys. The IPS or FF power spectra analysis of spacecraft signals could also be used to infer the solar wind velocity in the corona (Imamura et al 2014;Wexler et al 2020). These methods require only one telescope.…”

“…However, the IPS focuses on the short-period waves around the Fresnel frequency. We can detect the field-aligned density irregularities caused by the propagation of Alfvén waves with a longer period of 100-500 s. Wexler et al (2019Wexler et al ( , 2020 adopted the electron density model in their FF analysis, whereas, it is difficult for the model to depict the instantaneous variations of the electron density in the case of the CME. We should combine the advantages of different methods to study the solar wind velocity in the future.…”

“…Wexler et al (2019) measured the speed of solar wind by examining the power spectral density of FFs. Wexler et al (2020) presented a two-component model for interpretation of the FF from the Akatsuki spacecraft and determined the radial profile of slow wind speed in the extended corona using mass-flux continuity. Ma et al (2021a) measured the radial solar wind velocity within 10 Rs with very long baseline interferometry (VLBI) radio telescopes.…”

Multipoint radio probe of the solar corona: The trans-coronal radio array fleet

“…Because T-CRAF uses multiple downlink frequencies, we can use radio frequency fluctuations (FF) to infer V SW . This method, as detailed in Wexler et al (2019a), Wexler et al (2020), requires calculating the FF variance, σ 2 FF , generated by density oscillations of length L RAD advected across the LOS. The FF variance is then related to the solar wind speed according to Eq.…”

“…SO < 6 R ⊙ require higher frequencies like C-and X-bands (e.g. Kooi et al, 2014;Jensen et al, 2018;Wexler et al, 2020) because the side lobes of the receiving antenna's beam response are smaller and, therefore, less susceptible to solar interference (e.g. solar flares, radio bursts, active regions, the Sun itself, etc.)…”