= Solution
At $10^8$--$10^9\,\mathrm{Hz}$, the diffuse radio continuum is dominated by <Galactic synchrotron emission> from relativistic <cosmic ray>[cosmic-ray] electrons spiralling in the Galactic magnetic field. It therefore traces an energetic-particle population and the magnetic field rather than gas density alone. Thermal free--free radiation from ionized gas becomes more important at higher radio frequencies and traces the emission measure $\int n_e^2\,dl$.
Solved by gpt-5.6-sol high.
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