4:00 PM - 4:15 PM
[PEM16-03] MHD waves and a CME in the solar corona observed by using radio occultation technique with Akatsuki spacecraft
The solar wind is a supersonic plasma flow streamed from the solar corona. The solar wind acceleration mainly occurs in the outer corona at heliocentric distances of about 2-10 RS (= solar radii), where the coronal heating by magnetohydrodynamic waves and the wave-induced magnetic pressure are thought to play significant roles in the acceleration. The mechanisms have not been fully confirmed because the acceleration region is where no spacecraft has ever reached to date. Recently, however, an inner heliosphere observation network is getting ready, such as NASA's Parker Solar Probe and ESA's Solar Orbiter and BepiColombo.
Radio occultation observations cover the acceleration region fully and can obtain information complementary to in-situ observations. The radio occultation observations are conducted during the passage of a spacecraft on the opposite side of the Sun as seen from the Earth. Inhomogeneity of coronal plasma density structure traversing the ray path disturbs radio waves' frequency so that we can interpret the received frequency fluctuations as density fluctuations in the coronal plasma. Previous observations detected quasi-periodic components thought to represent magnetoacoustic waves (e.g., Efimov et al., 2012; Miyamoto et al., 2014). The details of the detected waves still have not been investigated.
The magnetic field fluctuations in the solar wind are extracted by applying the FR analysis to the dual-polarization data. Following Wexler et al. (2017), the FR associated with the magnetized plasma is derived from the cross-spectra of the dual-circular polarization signals transmitted from the Akatsuki spacecraft. The amplitude of the position angle increases as the tangential point gets closer to the Sun, and the fluctuations of amplitudes become gradual beyond 3 RS. The oscillations considered to be Alfvén waves were detected from the FR fluctuations obtained at closer to 3 RS. Furthermore, the FR fluctuations with a large amplitude were observed at 2.5 RS, and these fluctuations are identified as the fluctuations of a CME event based on the coronagraph images obtained from the LASCO C2 onboard the SOHO spacecraft.
Radio occultation observations cover the acceleration region fully and can obtain information complementary to in-situ observations. The radio occultation observations are conducted during the passage of a spacecraft on the opposite side of the Sun as seen from the Earth. Inhomogeneity of coronal plasma density structure traversing the ray path disturbs radio waves' frequency so that we can interpret the received frequency fluctuations as density fluctuations in the coronal plasma. Previous observations detected quasi-periodic components thought to represent magnetoacoustic waves (e.g., Efimov et al., 2012; Miyamoto et al., 2014). The details of the detected waves still have not been investigated.
The magnetic field fluctuations in the solar wind are extracted by applying the FR analysis to the dual-polarization data. Following Wexler et al. (2017), the FR associated with the magnetized plasma is derived from the cross-spectra of the dual-circular polarization signals transmitted from the Akatsuki spacecraft. The amplitude of the position angle increases as the tangential point gets closer to the Sun, and the fluctuations of amplitudes become gradual beyond 3 RS. The oscillations considered to be Alfvén waves were detected from the FR fluctuations obtained at closer to 3 RS. Furthermore, the FR fluctuations with a large amplitude were observed at 2.5 RS, and these fluctuations are identified as the fluctuations of a CME event based on the coronagraph images obtained from the LASCO C2 onboard the SOHO spacecraft.