Extraordinary and Z-Mode wave propagation in the auroral electron density cavity*

Numerous satellites observed the electron density cavity in which the electron plasma frequency (f_pe) is much less than the cyclotron frequency (f_ce) in the auroral region. The density cavity is known as a source region of auroral kilometric radiation (AKR) since the low densities favor wave growth in the extraordinary wave mode. The AKR is generated by the electron cyclotron-maser instability, which excites extraordinary (X) modes, ordinary (O) modes, and Z-modes. Therefore, understanding wave properties near the density cavity is critical to understand AKR propagation characteristics. This presentation demonstrates the plasma wave dispersion relation near the auroral electron density cavity and boundaries by adopting an empirical ionospheric density model such as the international reference ionosphere (IRI) with a density depletion effect. Because the dispersion relation of X- and Z-mode dramatically changes along with f_pe/f_ce; thus, we mainly focus on X- and Z mode wave propagation near the boundaries. The results provide the cutoff and resonance condition of each wave mode and predict wave properties detected spacecraft observations when waves traverse through such cavity boundaries.