A reconciliation of propagation modes of auroral kilometric radiation

The propagation modes of auroral kilometric radiation (AKR) are theoretically investigated by three‐dimensional ray tracing using an electron density model based on the auroral plasma cavity. Both right‐hand extraordinary (R‐X) mode and left‐hand ordinary (L‐O) mode observations are explained in a consistent manner. The following assumptions, which are based on observations near the source, are used for the ray tracing. (1) The radiation is generated in both R‐X and L‐O modes. (2) The ratios of the wave frequency to the R‐X mode cutoff frequency f/f R range from 1.01 to 1.1. (3) AKR waves are generated at wave normal angles almost perpendicular to the geomagnetic field. (4) AKR is generated in the auroral plasma cavity along a nightside field line with an invariant latitude of 70°. L‐O mode waves can propagate within a wider region than R‐X mode waves. Observed characteristics of AKR can be well explained under the following two additional assumptions, which are consistent with the cyclotron maser mechanism. (1) The R‐X mode is stronger than the L‐O mode. (2) Predominantly upgoing waves are generated at the source. The R‐X mode is dominant at high latitudes because it is stronger. The L‐O mode is received at lower latitudes which the R‐X mode waves cannot reach because of refraction and reflection mainly in the vicinity of the source and the plasmapause. Previous papers which claim the R‐X mode radiation are reconsidered. The present interpretation consistently explains the results of their observations without contradiction with the existence of the L‐O mode. It is clarified that they observed both modes.