  Kepler Space Telescope and Transiting Exoplanet Survey Satellite unveiled
that Sun-like stars frequently host exoplanets. These exoplanets are subject to
fluxes of ionizing radiation in the form of X-ray and extreme-ultraviolet (EUV)
radiation that may cause changes in their atmospheric dynamics and chemistry.
While X-ray fluxes can be observed directly, EUV fluxes cannot be observed
because of severe interstellar medium absorption. Here, we present a new
empirical method to estimate the whole stellar XUV (X-ray plus EUV) and FUV
spectra as a function of total unsigned magnetic fluxes of stars. The response
of the solar XUV and FUV spectrum (0.1-180 nm) to the solar total unsigned
magnetic flux is investigated by using the long-term Sun-as-a-star dataset over
10 yrs, and the power-law relation is obtained for each wavelength with a
spectral resolution of 0.1-1 nm. We applied the scaling relations to active
young Sun-like stars (G-dwarfs), EK Dra (G1.5V), $\pi^1$ Uma (G1.5V) and
$\kappa^1$ Ceti (G5V), and found that the observed spectra (except for the
unobservable longward EUV wavelength) are roughly consistent with the extension
of the derived power-law relations with errors of an order of magnitude. This
suggests that our model is a valuable method to derive the XUV/FUV fluxes of
Sun-like stars including the EUV band mostly absorbed at wavelengths longward
of 36 nm. We also discuss differences between the solar extensions and stellar
observations at the wavelength in the 2-30 nm band and concluded that
simultaneous observations of magnetic and XUV/FUV fluxes are necessary for
further validations.
