The Sun at Radio Frequencies
Joachim Köppen -- Kiel -- Sept 2019

Some brief explanations

This simulation shows how the Sun looks in various radio frequencies. The solar atmosphere consists of several layers of plasma with different temperatures and densities: The lowest is the photosphere, a thin layer of about 400 km thickness and with a temperature near 6000 K which produces the visible light. It is the deepest layer we can inspect with electromagnetic waves, and may thus be called the surface of the Sun. Above it lies the chromosphere, about 1000 km thick and with a temperature of about 10000 K. Above a few 1000 km there lies the corona with a few million K temperature which extents out to a few solar radii. Radio emission of different frequencies is produced by these plasma regions in different amounts. Since the radio waves travel through a charged plasma - notably electrons - their paths do not follow straight lines, but are bent because of the refraction caused by the electrons. This is especially strong at frequencies below about a few MHz.
For the temperature and density stratification in the solar atmosphere one of several models can be chosen: S.F.Smerd was the first person to compute the radio emission from the Sun and how radio waves travel through the solar atmosphere. This model is quite simple but it captures the essential elements of the solar atmosphere. The results which we may obtain with this schematic model should not be overinterpreted. Temperature and density can be shown in the left hand plot as a function of distance from the Sun's centre as well as a function of the height above the photosphere.
The radio brightness temperatures can be computed for a given position on the solar disk, offset from the disc centre. This is done by following a line of sight which starts with the observer, which in the sideways view of the Sun (right hand plot) would be far away to the right. At every part of the line of sight - which may be bent because of refraction - the simulation adds up the emission produced by each location. The resulting sum is displayed in the plot on the left hand side.
There are additional plots of opacity and optical depths (for the specialists :-)

The controls are:
Solar model:
chromosphere T [kK]
chr/cor height [km]
corona T [kK]


frequency [GHz]

single offset [Rsun]

no. of lines of sight
offset range [Rsun]

Mouse position:

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