MF/HF auroral radio emissions emanating from the topside ionosphere

This is a brief report on the first observation of polarization features and the source region of MF/HF auroral radio emissions emanating from the topside ionosphere. These emissions are called “Terrestrial Hectometric Radiation (THR)” and regarded as a counterpart of auroral roar and MF burst which are observable from the ground. THR typically occurs in either or both of two frequency bands near 1.5–2.0 MHz and 3.0–4.0 MHz, whose polarization features correspond to the L‐O and R‐X mode. The R‐X mode, which has never been reported as auroral roar and MF burst, can be attributed to nonlinear coupling of two upper hybrid waves. The Akebono satellite observation shows THR emissions merge with upper hybrid waves in a frequency‐time diagram under the matching condition fUH ∼ 2fce. This observation suggests that plasma instability enhances the upper hybrid waves under this condition, and then they are converted into MF/HF auroral radio emissions.