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The Radio Refraction Mind-bender

posted Mar 2, 2020, 2:15 PM by Jeremy Drake
Refraction is a physical phenomenon that spans the entire electromagnetic spectrum.  It is typically seen in optical terms as the bending of light as it passes between materials with different optical properties - glass and air for the example of eyeglasses.  Radio waves experience refraction too, especially when passing through a plasma, like that of the solar corona and wind.  The plasma refractive index  - the ratio of the speed of light in a vacuum to the speed of the radio waves in the plasma - increases with decreasing radio frequency, such that refraction is more important for low radio frequencies than high frequencies.  It also increases with the plasma density, and there is a critical density at which radio waves can no longer penetrate when they are essentially reflected instead.  The radio "photosphere" of a star, or its apparent emitting surface, then actually increases in size as radio frequency decreases.

Radio frequencies provide powerful probes of the solar corona and wind and can be observed directly from the ground, unlike the ultraviolet and X-rays. However, at "low" frequencies - of the order of a GHz and below - refraction must be taken into account in order to infer the true location or shape of the emitting source as the radio waves are bent in their path to the telescope. This is highly non-trivial as it requires detailed 3D knowledge of the plasma density. This can generally only be achieved in computer simulations. 

A paper lead by Smithsonian Astrophysical Observatory postdoc Sofia Moschou and published in the 2018 November 1 edition of the Astrophysical Journal presents a new sophisticated synthetic radio imaging tool that calculates and visualizes the bremsstrahlung radio emission from the corona and wind, following the path of radio waves from each tiny piece of space within the 3D simulation as they propagate outward along curved trajectories. Test model results are in good agreement with radio observations of the solar disk. The development promises to be a valuable new means of interpreting and understanding radio observations of the Sun. But it is in observations of stars, whose disks cannot be resolved spatially, where it could be even more important and simulations are the only means to infer what is actually going on.