7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Understanding the universal heating mechanism of solar and stellar
atmospheres
Shin Toriumi1
1Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency
e-mail (speaker): toriumi.shin@jaxa.jp
One of the outstanding mysteries of the solar and stellar
plasmas is why these stars commonly host extremely hotouter atmospheres, including the corona with atemperature of over 1 million Kelvin and thechromosphere of about 10,000 Kelvin. It is evident thatthese plasmas are magnetically heated, which is explainedby the two representative scenarios: the wave heatingmodel, in which magnetohydrodynamic waves transportkinetic energy from the stellar surface upward, and thenanoflare model, in which dissipation of magnetic energy
occurs at localized current sheets. To investigate whether
the heating mechanism is common between the Sun andSun-like stars, we examined the scaling relations ofmagnetic flux vs. irradiances over a broad spectral range
from X-rays to radio waves [1].
Using the long-term Sun-as-a-star observation data overthe last 10 years, we created scatter plots of magnetic fluxand irradiances of different spectral lines, which aresensitive to the plasmas with the formation temperaturesfrom log(T [K]) = 6-7 (corona) to ~3.8 (chromosphere).
Figure 1 shows the double logarithmic scatter plots, inwhich the colored dots represent the solar data. In eachpanel, we fitted a straight line to the data points. The
obtained scaling laws were then compared with theobservational data of G-type main-sequence stars of avariety of activity level with ages from 50 Myr to 4.5 Gyr.
It was found that the stellar data points are located at theextensions of solar power laws, even though theexponents very for different formation temperatures. This
striking consistency indicates that the plasma heating
mechanism is universal to the Sun and Sun-like stars,

regardless of stellar ages and activity levels.
The above results further motivated us to model thespectra of Sun-like stars, specifically of the ultraviolet(UV) and X-ray range [2]. By measuring the power-law
scaling between the irradiance and magnetic flux for eachwavelength bin of UV/X-ray, we reconstructed the spectra
for the active Sun-like stars. As shown in Figure 2, weconfirmed that the reconstructed spectra are in good
agreement with the actual observations, indicating that ourempirical reconstructions can be used for, e.g., input to thephotochemical calculation of exoplanetary atmospheres,
which may provide clue to understanding the habitability
on close-in exoplanets around the active stars.

Figure 2: Comparison between the reconstructed spectrum(green) and actual stellar data (black) for the active young
Sun-like star k1 Ceti (600 Myr). Figure adopted from [2].
References[1] S. Toriumi & V. S. Airapetian, The AstrophysicalJournal, 927, 179 (2022)[2] K. Namekata, S. Toriumi, V. S. Airapetian, et al., The
Astrophysical Journal, 945, 147 (2023)

Figure 1: Mmagnetic flux and irradiances of five spectral lines for the Sun and Sun-like stars. In each panel, the straightline indicates the power-law fitting to the solar data (colored dots), which are compared to the stellar data (diamonds).