= Thermal free-free emissivity
{title2=$j_\nu\propto Z^2n_en_iT^{-1/2}e^{-h\nu/(k_BT)}\bar g_{\rm ff}$}
The emission coefficient per volume, <frequency> and <solid angle> of a Maxwellian <plasma> has $j_\nu\propto n_e\sum_i Z_i^2 n_i\,T^{-1/2}e^{-h\nu/(k_BT)}\bar g_{\rm ff}$. Define the angular convention before supplying a numerical coefficient. The high-frequency cutoff measures <temperature>, while the normalization measures a density-squared emission integral. The radio spectrum is almost flat when optically thin and rises as $\nu^2$ when optically thick at fixed projected area.
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