Propagation Effects in Quiet Sun Observations at Meter Wavelengths. (arXiv:2009.10604v2 [astro-ph.SR] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Sharma_R/0/1/0/all/0/1">Rohit Sharma</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Oberoi_D/0/1/0/all/0/1">Divya Oberoi</a>
Quiet sun meterwave emission arises from thermal bremsstrahlung in the MK
corona, and can potentially be a rich source of coronal diagnostics. On its way
to the observer, it gets modified substantially due to the propagation effects
– primarily refraction and scattering – through the magnetized and turbulent
coronal medium, leading to the redistribution of the intensity in the image
plane. By comparing the full-disk meterwave solar maps during a quiet solar
period and the modelled thermal bremsstrahlung emission, we characterise these
propagation effects. The solar radio maps between 100 and 240 MHz come from the
Murchison Widefield Array. FORWARD package is used to simulate thermal
bremsstrahlung images using the self-consistent Magnetohydrodynamic Algorithm
outside a Sphere coronal model. The FORWARD model does not include propagation
effects. The differences between the observed and modelled maps are interpreted
to arise due to scattering and refraction. There is a good general
correspondence between the predicted and observed brightness distributions,
though significant differences are also observed. We find clear evidence for
the presence of significant propagation effects, including anisotropic
scattering. The observed radio size of the Sun is 25–30% larger in area. The
emission peak corresponding to the only visible active region shifts by 8′–11′
and its size increases by 35–40%. Our simple models suggest that the fraction
of scattered flux density is always larger than a few tens of percent, and
varies significantly between different regions. We estimate density
inhomogeneities to be in the range 1–10%.
Quiet sun meterwave emission arises from thermal bremsstrahlung in the MK
corona, and can potentially be a rich source of coronal diagnostics. On its way
to the observer, it gets modified substantially due to the propagation effects
– primarily refraction and scattering – through the magnetized and turbulent
coronal medium, leading to the redistribution of the intensity in the image
plane. By comparing the full-disk meterwave solar maps during a quiet solar
period and the modelled thermal bremsstrahlung emission, we characterise these
propagation effects. The solar radio maps between 100 and 240 MHz come from the
Murchison Widefield Array. FORWARD package is used to simulate thermal
bremsstrahlung images using the self-consistent Magnetohydrodynamic Algorithm
outside a Sphere coronal model. The FORWARD model does not include propagation
effects. The differences between the observed and modelled maps are interpreted
to arise due to scattering and refraction. There is a good general
correspondence between the predicted and observed brightness distributions,
though significant differences are also observed. We find clear evidence for
the presence of significant propagation effects, including anisotropic
scattering. The observed radio size of the Sun is 25–30% larger in area. The
emission peak corresponding to the only visible active region shifts by 8′–11′
and its size increases by 35–40%. Our simple models suggest that the fraction
of scattered flux density is always larger than a few tens of percent, and
varies significantly between different regions. We estimate density
inhomogeneities to be in the range 1–10%.
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