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. |
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