Fig. 4

a Spectrogram of intensity of MFB emissions observed at Toolik Lake on October 19, 2017; b altitude profiles of key frequencies (\(f_{\mathrm{ce}}\), \(f_{\mathrm{pe}}\), \(f_{\mathrm{uh}}\), and \(f_{\mathrm{L}}\)) above Toolik Lake for the date and time of the observations in a, based on IGRF and IRI model values of magnetic field and electron density; c altitude profiles of key frequencies calculated by scaling the IRI profile such that the maximum L-cutoff frequency lies below all observed left-polarized MFB, and plasma frequency profile (solid green trace) obtained by adding to the scaled-IRI model an enhancement approximating that expected from impact ionization due to low-energy auroral electrons