[P-EM16] Heliosphere and Interplanetary Space

[PEM16-P01] Source of the slow solar wind and acceleration mechanism of solar wind studied by Hinode satellite and interplanetary scintillation observations

Keita Morishima¹, *Kazumasa Iwai¹, Ken'ichi Fujiki¹ (1.Institute for Space–Earth Environmental Research (ISEE), Nagoya University)

Keywords:solar wind, active region, coronal magnetic field, Interplanetary scintillation

The source of the slow solar wind and the acceleration mechanism of the solar wind are both long-standing unsolved problems in solar wind physics. In this study, we focused on the plasma upflow (upflow) observed at the edge of active regions and aimed to clarify the relationship between the upflow and the solar wind. Previous studies have suggested that the upflow may be the source of the slow solar wind. However, the period in which it was verified is limited, and there have been few cases in which the solar wind speed has been verified using in-situ particle observations. In this study, as Objective 1, we examined whether the upflow could be the source of the slow solar wind by analyzing a large number of datasets spanning different levels of solar activity. Regarding the acceleration mechanism of the solar wind, the Alfvén wave model, which accelerates the solar wind by dissipating Alfvén waves, is consistent with the observations and simulations of the solar wind originating from coronal holes in the polar regions. However, the contribution of Alfvén waves to the solar wind originating from active regions is unknown. In this study, we verified the Alfvén wave model proposed by Suzuki (2006) for the solar wind, which is suggested to be originated from upflows, as Objective 2. This model suggests that the square of the solar wind speed is proportional to the magnitude of the photospheric magnetic field divided by the expansion factor of the magnetic flux tube at 1 AU from the photosphere. To verify this, we analyzed a total of 51 data sets constructed from the following observations and models over one solar activity cycle. The first data was a spectroscopic observation of coronal emission lines by the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode satellite, and the line-of-sight Doppler velocity of the Fe XIII (202 A) line was used to determine the upflow at the coronal base. The second data was the solar wind speed obtained from Interplanetary Scintillation (IPS) observations. The third data was the coronal magnetic field extrapolated by the Potential Field Source Surface (PFSS) model, and the solar wind speed obtained from the IPS observations was connected to the upflow determined from the Hinode satellite observations.
For Objective 1, we found that 29 samples had upflows connected to open magnetic field lines in the PFSS model, and 23 of those samples had open magnetic field lines connected to slow solar wind below 500 km/s. These results suggest that at least some upflows can be the source of the slow solar wind. For Objective 2, we found that in cases where the average expansion factor of the flux tubes was below 800, the relationship between solar wind speed and the magnetic field characteristics was consistent with the Alfvén wave model. However, for flux tubes with expansion factors exceeding 800, the observed relationship deviated significantly from the model. Futher, we compared the energy flux density required at the coronal base from the solar wind speed obtained by IPS observations (F_ips) with the energy flux density at the coronal base estimated from the non-thermal linewidth of the spectrum observed by EIS (F_eis). We found that F_ips exceeded F_eis in the group not reproduced by the Alfvén wave model. From these results, it is suggested that in the group reproduced by the Alfvén wave model, Alfvén waves likely play a significant role in accelerating the solar wind. Conversely, in the group not reproduced by the Alfvén wave model, other mechanisms may contribute to the acceleration of the solar wind originating from the upflow.