Observation of the Solar Corona Using Radio Scintillation with the Akatsuki Spacecraft: Difference Between Fast and Slow Wind

• Published in: Solar physics 2022-03, Vol.297 (3), p.34, Article 34
• Main Authors: Chiba, Shota, Imamura, Takeshi, Tokumaru, Munetoshi, Shiota, Daikou, Matsumoto, Takuma, Ando, Hiroki, Takeuchi, Hiroshi, Murata, Yasuhiro, Yamazaki, Atsushi, Häusler, Bernd, Pätzold, Martin
• Format: Article
• Language: English
• Subjects: Acoustic waves; Acoustics; Alfven waves; Amplitudes; Astrophysics and Astroparticles; Atmospheric Sciences; Corona; Density; Electron density; Energy dissipation; Energy flux; Fluid flow; Japanese spacecraft; Kinetic energy; Magnetohydrodynamic waves; Periodic variations; Photosphere; Physical properties; Physics; Physics and Astronomy; Power spectra; Scintillation; Solar corona; Solar energy; Solar physics; Solar wind; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Spacecraft; Time series; Venus probes; Wave energy; Wave power; Wavelet analysis; Wind speed; Wind velocities
• ISSN: 0038-0938; 1573-093X
• DOI: 10.1007/s11207-022-01968-9

The properties of the coronal plasma at heliocentric distances of 1.5 – 8.9  R ⊙ (solar radii) were studied with radio-occultation observations using JAXA’s Akatsuki spacecraft in 2016. Physical parameters that characterize the solar wind were retrieved from the intensity-scintillation time series by fitting a theoretical spectrum to the observed power spectra. The derived solar-wind velocity clearly shows a difference between the fast wind and the slow wind, which was identified based on IPS observations. The inner scale, at which fluid motions dissipate and kinetic energy is converted to heat, increases with the heliocentric distance, and the fast wind has larger inner scales than the slow wind. By applying wavelet analysis to the frequency time series, we detected quasi-periodic fluctuations in the electron density. The density oscillations are considered to be manifestations of acoustic waves, which were generated from Alfvén waves originating from the photosphere, and the energy fluxes of those acoustic waves were estimated. The relative density-amplitude peaks around 4 – 6  R ⊙ and the wave-energy flux decreases beyond ≈ 6  R ⊙ , implying that the acoustic waves dissipate to heat the corona. The phase-scintillation spectrum that we obtained cannot be expressed by a single power law. A break is seen around the frequency of 0.5 – 2 Hz beyond ≈ 6  R ⊙ , suggesting an excess power other than turbulence at lower frequencies. The enhancement of the relative density amplitude around 6  R ⊙ found by the wavelet analysis might explain this excess power. The acoustic wave-energy flux in the fast solar wind tends to exceed that in the slow wind, suggesting that the fast wind is powered by a larger injection of Alfvén-wave energy than the slow wind.