Figure 3 displays the radial electric field component () and an azimuthal magnetic field component () measured by POLAR instruments on January 10, 1997, 21-24 UT, while moving from the northern polar cap toward the equator at 05:30 MLT. At 21:53 UT, POLAR began to observe B-field oscillations whereas E-field oscillations were not observed before 22:10 UT presumably due to the unfavorable orientation of the E-field long booms. The B-field perturbations appear mainly in the azimuthal direction (), and the electric field oscillates in the radial direction (); wave signals in other components are weak. The background magnetic field is oriented along the z-axis, and therefore we conclude the observed wave is a toroidal mode. Both components are observed at L shells from 9.0 to 6.6. The observed wave periods vary in the 150-250 s range, corresponding to wave frequencies of 4-7 mHz.
Figure 4 presents bandpass-filtered time series data for the 21:50-23:10 UT interval. Here we have used a Butterworth band-pass filter at 52 mHz to remove noise from the data shown in Figure 3. The E- and B-field components are clearly 90 out of phase, indicating that the observed mode is a standing wave.
© 1998 AGU