Comparison of chromospheric diagnostics in a 3D model atmosphere: Hα linewidth and mm continua
arxiv(2023)
摘要
The Ha line, one of the most studied chromospheric diagnostics, is a tracer
for magnetic field structures, while its line core intensity provides an
estimate of the mass density. The brightness temperatures from Atacama Large
Millimetre-submm Array (ALMA) observations provide a complementary view of the
activity and the thermal structure of stellar atmospheres. These two
diagnostics together can provide insights into the physical properties of
stellar atmospheres. In this paper, we present a comparative study between
synthetic continuum brightness temperature maps for mm wavelengths (0.3 mm to
8.5 mm) and the width of the Ha 6565Å line. The 3D radiative transfer codes
Multi3D and Advanced Radiative Transfer (ART) are used to calculate synthetic
spectra for the Ha line and the mm continua respectively, from an enhanced
network atmosphere model with non-equilibrium hydrogen ionisation generated
with the state-of-the-art 3D rMHD code Bifrost. We use Gaussian Point Spread
Function (PSF) for simulating the effect of ALMA's limited spatial resolution
and calculate the Ha vs. mm continuum correlations and slopes of scatter plots
for the original and degraded resolution of the whole box, quiet sun and
enhanced network patches separately. The Ha linewidth and mm brightness
temperatures are highly correlated and the correlation is highest at a
wavelength 0.8 mm i.e. ALMA Band 7. The correlation increases with decreased
resolution. On the other hand, the slopes decrease with increasing wavelength.
The degradation of resolution does not have a significant impact on the
calculated slopes. With decreasing spatial resolution the standard deviations
of the observables, Ha linewidth and brightness temperatures decrease and the
correlations between them increase, but the slopes do not change significantly.
Hence, these relations may prove useful to calibrate the mm continuum maps
observed with ALMA.
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