Influence of radiation fields on hydrogen RRLs. The total intensities at the frequencies of the Hnα line calculated with different EMs are plotted in the top panel. The total optical depths at the frequencies of the Hnα lines are presented in the bottom panel. The solid, dashed, and dash-dotted lines represent the values for EM = 1.0 × 10¹⁰, 3.0 × 10¹⁰, and 5.0 × 10¹⁰ cm⁻⁶ pc, respectively. The black lines represent the values calculated without the effect of radiation fields on level populations, and the red lines the values calculated with radiation fields. The blue lines show the values of the continuum emissions.