The auroral ionsphere is an abundant source of radio emissions, some of which are detectable on the ground. At LF/MF/HF, these include auroral hiss at 1 kHz-1 MHz [e.g., Helliwell, R. A., 1965], MF-burst at ~1.4-4.5 MHz [Weatherwax, A. T. et al., 1994; LaBelle, J. et al.,1997], and 2 and 3 auroral roar at ~3 and ~4.5 MHz, respectively [Kellogg, P. J. and S. J. Monson,1979; Weatherwax, A. T. et al.,1993]. The principal generation mechanism for auroral hiss has been described; however, the generation mechanisms of MF-burst and auroral roar remain unknown. These auroral emissions are signatures of important phenomena such as wave/particle interactions and energy exchange processes.
Polarization measurements provide an important clue about the generation mechanism of these auroral radio emissions by placing constraints on the propagation mode in the ionosphere. For example, several researchers have suggested that auroral roar may be generated in the X-mode by the cyclotron maser mechanism operating at F-region altitudes [Weatherwax, A. T. et al.,1995; Yoon, P. H. et al.,1996]. This mechanism predicts that auroral roar should be right elliptically polarized. Another possible mechanism of auroral roar involves the conversion of upper hybrid waves to electromagnetic waves [Gough, M. P. and A. Urban,1983; Weatherwax, A. T. et al.,1995]. This mechanism predicts either left-elliptical polarization (LP) or right-elliptical polarization (RP), depending on the mode conversion process. It is therefore possible to eliminate some theories based on the observation of signals propagating in a mode which is forbidden by that mechanism.
To determine the polarization of various radio emissions crossed loop antennas and associated electronics were installed at Churchill, Manitoba. The first observations with this system are described below.