The refractive index of the ionosphere is

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(C) The ionosphere is the upper part of the Earth's atmosphere,which contains a high concentration of free electrons and ions. When an electromagnetic

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less than one

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(C) The ionosphere is the upper part of the Earth's atmosphere,which contains a high concentration of free electrons and ions. When an electromagnetic wave travels through the ionosphere,the phase velocity of the wave becomes greater than the speed of light in a vacuum $(c)$. The refractive index $(n)$ is defined as the ratio of the speed of light in a vacuum $(c)$ to the phase velocity of the wave in the medium $(v)$,i.e.,$n = c/v$. Since $v > c$ in the ionosphere,the refractive index $n$ is less than $1$.

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