[Edited on February 25, 2021]
Main Objective: A coordinated observation of HINODE, Akatsuki, and BepiColombo for investigating the corona where the
solar wind is accelerated.
Scientific Justification: We propose a coordinated observation of Hinode with BepiColombo and Akatsuki on March 2021 to investigate the source and acceleration regions of the solar wind. The radio scintillations are caused by the solar wind plasma, which provide the unique information to probe the region where the solar wind is accelerated in the corona. The occultation observations using radio beacon signals from the Akatsuki spacecraft have been carried out, providing new insights to the heating of the corona and the wind acceleration (Imamura et al. 2014, Miyamoto et al. 2014). On March 2021, BepiColombo and Akatsuki are going to be at the opposite side from the Earth at almost the same time for the first time.
BepiColombo will be closest to the Sun at 4.2 Rs on 17 March, whereas Akatsuki at 5.1 Rs on 26 March. Before their closest approaches (conjunctions), they will be located in a radial direction at the distance of 10 Rs and 11 Rs, respectively, at the southern hemisphere, on 13-14 March. This situation enables us to investigate the radial evolution of the solar wind around its acceleration region. For this unique measurements, some detailed observations by Hinode instruments at the solar surface and in the inner corona will help us to link magnetic and plasma properties in the source region to the solar wind which will be measured by both BepiColombo and Akatsuki. This unique coordinated observation will be the first opportunity to investigate the solar wind from its source region to the acceleration region.
The proposed coordinated observation has two phases:
Phase 1 (3, 6, 8, 10, and 12 March 2021)
Akatsuki and BepiColombo will be located at about 10 Rs from the Sun on 13-14 March, and, at this time, their radio occultation measurements are mostly sensitive to the coronal plasma that is closest to the Sun, i.e., about 90 degree position from the Earth, far above the south-west limb. To infer the magnetic field configuration in the corona above the south-west limb, an elaborate coronal field extrapolation is required with the photospheric magnetic field map covering the high latitude area of the south-western hemisphere including the region already rotated into the far side. A series of SOT/SP observation for the southern polar and high latitude region before the close approach is valuable to know the vector magnetic field that will be rotated to behind the south-west limb. About 8 days before the close approach, i.e. 6-12 March, are important for the SP observations when considering the differential rotation of the observing region. The observation period should start a few days earlier (3 March) to cover more western longitude area in case the coronal hole magnetic field has strong longitudinal expansion. The cadence in the series of the observations is about 2 days to create a synoptic map.
Phase 2 (13-14 March 2021)
On the 13-14 March, coronal observations by the EIS and XRT for the region around and above the south west limb. This region is expected to be magnetically connected to the region to which BepiColombo and Akatsuki’s measurements are the most sensitive. The steady structure of the solar corona is derived by the EIS and XRT; The EIS, especially, will provide the FIP bias and doppler and non-thermal speed maps, helping us to investigate the source region of the solar wind that is observed by BepiColombo and Akatsuki. When transient phenomena, such as jets, CMEs, and blobs, happen there, their counterparts will be identified in the BepiColomb and Akatsuki data more easily. If the common phenomena can be caught, we can trace their trajectory, i.e., the temporal and spatial evolution, from the inner corona to the inner heliosphere. The start time of the Hinode observations should be “7 h” earlier than the BepiColombo and Akatsuki operation times, assuming the average speed of 300 km/s for the solar wind and possible transient phenomena.
Imamura et al. 2014, ApJ, 788, 117
Miyamoto et al. 2014, 797, 51