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A new type of terrestrial line spectra has been detected by the Arase satellite. These spectra are called hectometric line spectra. They consist primarily of constant frequency narrowband components at frequencies between 525 and 1,700 kHz, which originate and are sometimes amplified from AM broadcasting waves and other generated emissions. These line spectra can be amplified or generated by the odd-harmonic electrostatic cyclotron instability while in the Z mode. Hectometric line spectra need to be mode-converted to the L-O mode to be received at higher altitudes because the Z mode can only exist when its frequency is lower than the local upper hybrid resonance frequency. Wave penetration into the ionosphere and mode conversion are enabled by low-density regions and steep density gradients in ionospheric irregularities. Note that the broadcasting waves passed through the ionosphere with frequencies lower than the maximum plasma frequency along their paths.

Plain Language Summary A new type of terrestrial line spectra that consists primarily of constant frequency narrowband emissions at frequencies of 525 to 1,700 kHz and additional emissions with changing frequencies has been detected by the Arase (ERG) satellite. The former originate and are amplified from AM broadcasting waves in a similar frequency band. The latter are naturally generated emissions or emissions stimulated by broadcasting waves. These spectra are called hectometric line spectra because the wavelengths of the spectra are between 176 and 571 m. Broadcasting waves whose frequencies are lower than the maximum plasma frequency along their paths can penetrate into the ionosphere in the Z mode. Then, they can be amplified or generated by the odd-harmonic electrostatic cyclotron instability while in the Z mode. Hectometric line spectra need to be mode-converted to the L-O mode (the free-space mode) to be received at higher altitudes because waves in the Z mode are trapped in regions where their frequency is lower than the local upper hybrid resonance frequency.