Statistical Characteristics of MF/HF Auroral Radio Emissions Emanating from the Topside Ionosphere

The terrestrial auroral ionosphere is a natural emitter of electromagnetic waves in the MF/HF ranges (up to 6 MHz) as well as well-known intense auroral kilometric radiation (AKR) and auroral hiss in the VLF/LF ranges. We report on the statistical properties of Terrestrial Hectometric Radiation (THR), MF/HF auroral radio emissions emanating from the topside ionosphere, using a long-term data set obtained from the Plasma Waves and Sounder (PWS) experiment mounted on the Akebono satellite during 2 solar cycles. 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. Statistical studies using the Akebono/PWS data reveal clear bimodality in the frequency distribution of THR with two broad peaks near 1.6 MHz and 3.6 MHz and the spatial distribution of occurrence rate of THR-L (lower than 2.5 MHz) and THR-H (higher than 2.5 MHz). In the morning to postnoon sectors (3h-15h MLT), the spatial distribution of both types of THR is confined to magnetic latitudes higher than 70 deg, while during nighttime (15h-3h MLT) it spreads to lower magnetic latitudes (~ 30 deg) at higher altitudes. The explanation of this distribution is that THR is generated in the night-side auroral latitudes near 1000-km altitude and propagation effect makes an emission cone. Occurrence rate of THR-L is higher than that of THR-H. The long-term Akebono/PWS data also show clear solar activity dependence and seasonal variations of THR appearance; THR occurrence rate drops from a few percent during solar maxima to 0.1 percent or less during solar minima and is the highest in summer and the lowest in winter.