To determine the polarization of various radio emissions crossed loop antennas and associated electronics were installed at Churchill, Manitoba in March 1997. The modified antenna system consists of two vertical loop antennas oriented at right angles to each other in an approximately N-S/E-W position. In the polarization detector, a 90° phase lag is introduced into the signal from the N-S loop. On alternate sweeps this signal is inverted, effectively shifting the phase of the N-S loop signal from -90° to +90° relative to the E-W loop. The input to the receiver is the sum of the shifted N-S signal and the E-W signal. If the original signals induced in the antenna loops are equal in amplitude but differ in phase by exactly 90°, as would be expected for vertically incident right- or left-circularly polarized waves, the input signal to the receiver alternates between zero and twice the induced signal strength. On the other hand, a linearly polarized signal induces signals in the antenna loops which are in phase with each other, resulting in a constant input signal to the receiver, since there is no difference between shifting the N-S signal ±90° in this case. In order to determine polarization using this technique, the measured signals must be relatively constant in amplitude and polarization during two consecutive meaurements (~2 s); signals whose amplitude varies faster than that may register a false or indeterminant polarization. The sense of polarization (right or left) is determined by noting the relative signal strengths of the two sweeps and comparing that to the direction of the phase shift in the N-S loop signal.
All wave polarizations are measured with respect to the local magnetic field. The electric field vector of a right-handed circularly polarized (RCP) wave rotates clockwise in time as viewed in the direction of the magnetic field. For a receiver in the Northern Hemisphere the electric field vector rotates clockwise as seen by an observer looking down on the antennas. This definition is standard in plasma physics [e.g., Chen 1984] and is used in previous auoral radio emission polarization studies [Tanaka et al., 1976].
Figure 1a (top panel) is a spectrogram showing three types of auroral radio emissions recorded 0445-0459 UT on April 4, 1997. Auroral hiss occurs below ~500 kHz beginning near 0452 UT lasting for ~5 minutes and again during the last ~30 seconds of the record. MF-burst is the broadband (~2 MHz) emission above ~1.5 MHz and correlated with the auroral hiss. Auroral roar is the relatively narrowband (~200 kHz) emission at ~3 MHz beginning near 0448 UT and ending near 0453 UT. Horizontal dark bands are fixed-frequency transmissions; the band from 550-1600 kHz is the AM broadcast band. A sweep identification marker is at 1-1.25 MHz. Clearly, alternate sweeps are light or dark, implying that they are elliptically polarized. Careful inspection shows that the MF-burst and auroral roar are LP and auroral hiss is RP. This result is consistent with previous auroral hiss polarization measurements using several techniques [Tanaka et al., 1976]. Also, auroral hiss is believed to propagate in the whistler mode in the ionosphere, which implies RP, consistent with the observation. This observation of auroral hiss provides a natural calibration of the polarization detector.
Figure 1b (bottom panel) shows the polarization of the signals as a grayscale. In this display, white and black pixels correspond to LCP and RCP waves respectively. Elliptically polarized waves are represented as gray pixels between the two extremes, with linear polarization being at the middle of the gray band (half way between white and black). As expected, the auroral hiss shows up as dark pixels implying right-hand polarization. In contrast, both MF-burst and auroral roar are left-hand polarized. There are two sweeps at the onset of an auroral substorm near 0453 UT during which the polarization measurement momentarily implies that the auroral roar is RP, but at this time the auroral roar amplitude is probably highly time variable, as is known from fine structure measurements [e.g., LaBelle et al., 1997; Shepherd et al., 1997], and under such conditions the polarization measurement cannot be trusted.
As mentioned, in the case of auroral roar the polarization measurement has significant implications for theories. Several authors have calculated that for realistic loss-cone distribution functions the X-mode cyclotron maser instability operating at F-region altitudes can have a growth rate exceeding electron-neutral collision frequencies [Weatherwax et al., 1995; Yoon et al., 1996] However, Yoon et al. [1996] point out that only the X-mode will reach the ground from this mechanism, because the O-mode remains trapped in the ionosphere. Hence this mechanism predicts that the waves should be right-hand elliptically polarized. The measurement of LP for auroral roar emissions excludes this mechanism. The cyclotron instability also excites trapped Z-mode waves with high growth rates at locations where the upper hybrid frequency matches the cyclotron harmonics [e.g., Kaufmann, 1980;Yoon et al., 1997], and it has been suggested that these waves may convert by a variety of mechanisms to either L-O mode or R-X mode electromagnetic waves [Gough and Urban, 1983; Weatherwax et al., 1995]. This mechanism therefore predicts either LP or RP radiation depending on the conversion mechansism and hence is not excluded by the polarization measurements presented above. However, the polarization measurements place a constraint on the conversion mechanisms which may be considered in that theory.
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