Daiki Yamasaki

Solar filaments are dense and cool plasma clouds in the solar corona. They are supposed to be supported in a dip of coronal magnetic field. However, the models are still under argument between two types of the field configuration; one is the normal polarity model proposed by Kippenhahn & Schlueter (1957), and the other is the reverse polarity model proposed by Kuperus & Raadu (1974). To understand the mechanism that the filaments become unstable before the eruption, it is critical to know the magnetic structure of solar filaments. In this study, we performed the spectro-polarimetric observation in the He I (10830 angstrom) line to investigate the magnetic field configuration of dark filaments. The observation was carried out with the Domeless Solar Telescope at Hida Observatory with a polarization sensitivity of 3.0x10^-4. We obtained 8 samples of filaments in quiet region. As a result of the analysis of full Stokes profiles of filaments, we found that the field strengths were estimated as 8 - 35 Gauss. By comparing the direction of the magnetic field in filaments and the global distribution of the photospheric magnetic field, we determined the magnetic field configuration of the filaments, and we concluded that 1 out of 8 samples have normal polarity configuration, and 7 out of 8 have reverse polarity configuration.

Small flares frequently occur in the quiet Sun. Previous studies have noted that they share many common characteristics with typical solar flares in active regions. However, their similarities and differences are not fully understood, especially their thermal properties. In this study, we performed imaging spectroscopic observations in the H$\alpha$ line taken with the Solar Dynamics Doppler Imager on the Solar Magnetic Activity Research Telescope (SMART/SDDI) at the Hida Observatory and imaging observations with the Atmospheric Imaging Assembly onboard Solar Dynamics Observatory (SDO/AIA). We analysed 25 cases of small flares in the quiet Sun over the thermal energy range of $10^{24}-10^{27}\,\mathrm{erg}$, paying particular attention to their thermal properties. Our main results are as follows: (1) We observe a redshift together with line centre brightening in the H$\alpha$ line associated with more than half of the small flares. (2) We employ differential emission measure analysis using AIA multi-temperature (channel) observations to obtain the emission measure and temperature of the small flares. The results are consistent with the Shibata & Yokoyama (1999, 2002) scaling law. From the scaling law, we estimated the coronal magnetic field strength of small flares to be 5 --15 G. (3) The temporal evolution of the temperature and the density shows that the temperature peaks precede the density peaks in more than half of the events. These results suggest that chromospheric evaporations/condensations play an essential role in the thermal properties of some of the small flares in the quiet Sun, as does for large flares.

We often find spectral signatures of chromospheric cold plasma ejections accompanied by flares in a wide range of spatial scales in the solar and stellar atmospheres. However, the relationship between physical quantities (such as mass, kinetic energy, and velocity) of cold ejecta and flare energy has not been investigated in a unified manner for the entire range of flare energies to date. This study analyzed the spectra of cold plasma ejections associated with small-scale flares and solar flares (energy $10^{25}-10^{29}\,\mathrm{erg}$) to supply smaller energy samples. We performed H$\alpha$ imaging spectroscopy observation by the Solar Dynamics Doppler Imager on the Solar Magnetic Activity Research Telescope (SMART/SDDI). We determined the physical quantities of the ejecta by cloud model fitting to the H$\alpha$ spectrum. We determined flare energy by differential emission measure analysis using Atmospheric Imaging Assembly onboard Solar Dynamics Observatory (SDO/AIA) for small-scale flares and by estimating the bolometric energy for large-scale flares. As a result, we found that the ejection mass $M$ and the total flare energy $E_{\mathrm{tot } }$ follow a relation of $M\propto E_{\mathrm{tot } }^{2/3}$. We show that the scaling law derived from a simple physical model explains the solar and stellar observations with a coronal magnetic field strength as a free parameter. We also found that the kinetic energy and velocity of the ejecta correlate with the flare energy. These results suggest a common mechanism driven by magnetic fields to cause cold plasma ejections with flares on the Sun and stars.