Direct Observation of L-X Mode of Auroral Kilometric Radiation in the Lower Latitude Magnetosphere by the Arase Satellite

Previous studies have shown that auroral kilometric radiation (AKR) can play an important role in the magnetosphere-atmosphere coupling and has the right-handed extraordinary (R-X), left-handed ordinary (L-O) and left-handed extraordinary (L-X) modes. However, the L-X mode has not been directly observed in the lower latitude magnetosphere yet, probably because of its very limited frequency range. Here, using observations of the Arase satellite on 6 September 2018, we present an AKR event with two distinct bands (8-20 and 300-1000 kHz) around the location: L = 8 and latitude = -37 degrees. The low (high) band is identified as the L-X (R-X) mode based on the polarization and frequency ranges. Simulations of 3-D ray tracing show that most of ray paths with 14 (11 and 18) kHz pass (miss) the location of Arase, basically consistent with observations. Our study provides direct evidence that the L-X mode can propagate from high latitudes downward to lower latitudes. Auroral kilometric radiation (AKR) is a widely existing radio emission with kilometric wavelength at the Earth, contributing to the magnetosphere-atmosphere coupling. Similar emissions have been observed on all magnetic planets of the solar system. Previous studies have shown that AKR primarily occurs in the R-X mode, with a small contribution in the L-O and L-X modes. The L-X mode at lower latitudes has not been directly observed so far, most likely due to its extremely limited frequency range. Here, we present an L-X mode (peak frequency similar to 14 kHz) in the lower latitude magnetosphere observed by the Arase satellite. Using the 3-D ray tracing method, we simulate ray paths with different initial wave parameters and source locations. Simulations show that ray paths with 14 (11 and 18) kHz pass (miss) the location of the Arase satellite and are highly dependent on initial wave parameters and the location of source. Our results provide a direct evidence that the L-X mode from high latitude source regions can propagate downward to lower latitudes under suitable conditions. This study enriches the understanding of AKR propagation characteristics in the magnetosphere. An auroral kilometric radiation (AKR) event with two distinct bands (8-20 kHz and 300-1000 kHz) is observed around the location: L = 8 and latitude = -37 degrees Based on the polarization and frequency ranges, the low (high) band AKR is identified as the L-X (R-X) mode 3-D ray tracing simulations show that L-X mode can propagate downward to lower latitudes, basically consistent with observations