Propagation of medium frequency (1-4 MHz) auroral radio waves to the ground via the Z-mode radio window

Recent ground-based observations of auroral radiowaves have identified narr owband emissions near 2 and 3 times the lower ionospheric electron cyclotro n frequency (f(ce)) known as auroral roars. In this paper the propagation o f these waves in the auroral ionosphere is investigated by means of a ray-t racing technique. We model one particular scenario in which a large-scale ( tens of kilometers) horizontal density structure, based on density structur es observed with the Sondrestrom radar at times of auroral roar emissions, plays a crucial role in both guiding the waves to the ground and enabling m ode conversion. The location and the mode characteristics of the initial wa ves are determined on the basis of local stability properties, which sugges ts that Z-mode wave excitation is favored near 2f(ce). However, since Z-mod e cannot propagate to the ground they must first undergo a mode conversion to one of the free-space modes (X and O). It is found that for a narrow ran ge of frequencies and initial wave phase angles the trapped Z mode can be c onverted to O mode via the Ellis radio window. This finding is consistent w ith the fact that auroral roar emissions are nearly 100% O-mode polarized. However, it is important to note that the evaluation of the damping of the Z-mode waves along the ray path is not considered within the context of thi s preliminary study and will be critical for eventually determining the exa ct physical scenario of the auroral roar generation mechanism.