4. Case Study

Figure 5

[64]  We apply our method to the backscatter Doppler velocities observed on the night of 25 July 2001 with the MU radar over Shigaraki, Japan (34.9^oN, 136.1^oE; geomagnetic latitude 25.0^oN). Backscatter from field-aligned irregularities was observed throughout almost the entire night. In Figure 5 we present a portion of these data for the time period 0220-0330 LT. In Figure 5 we show the line-of-sight Doppler velocity V _Dop as a function of local time for different altitudes shown in different colors. Here positive Doppler velocities are away from the radar, i.e., in the -y direction. The sudden splashes in the Doppler velocity at about 0252 LT near 94.8 km altitude and at about 0258 LT near 107.2 km altitude (see also the two top panels in Figure 4) are most probably caused by meteors. The Doppler velocity was mainly away from the radar except for the time period from about 0240 LT until about 0308 LT when it was toward the radar. To demonstrate the method, we chose 14 consecutive time data sets during the period of the "inverse" Doppler velocity from 0250:06 until 0259:40 LT (the phase velocity as a function of local time and altitude is presented in the two top panels in Figure 4). We analyzed only the data for which the signal-to-noise ratio (SNR) exceeds 3 dB. The major part of the data has SNR  >21 dB.

[65]  In Figure 4 we plot together the observed phase velocities of the 3.2-m irregularities and the derived polarization electric field (written in terms of the current velocity) and neutral wind. The left column presents the altitude profiles of the phase velocity (upper panel), the reconstructed current velocity (middle panel) and the reconstructed meridional neutral wind (lower panel) for different local times, and the right column shows the time history of the phase velocity (upper panel), the reconstructed current velocity (middle panel) and the reconstructed meridional neutral wind (lower panel) at different altitudes. To show the trend we connect consecutive data points by smooth lines for both the phase velocity and reconstructed parameters.

[66]  The altitude dependence of the reconstructed zonal polarization electric field in the middle left panel of Figure 4 shows sinusoidal behavior for 0250:50.0 LT between 96.2 and 98.2 km altitudes and for 0253:02.9 LT between 100.2 and 102.2 km altitudes with the same height period of approximately 670 m. The zonal polarization electric field at these altitudes is due to ion drag by the zonal neutral wind and electron drift in the crossed background meridional electric and geomagnetic fields (equation (12)). Since the background electric field by definition has a much larger typical scale length, the observed sinusoidal altitude dependence is due to either the zonal wind or the plasma density fluctuation in the primary wave, which we have assumed constant (equations (18) and (19)).

[67]  The reconstructed altitude profiles at consecutive times and time histories at consecutive altitudes for the meridional neutral wind are plotted in the bottom panels of Figure 4. The splashes of the wind velocity near 0251 LT are due to distortion of the Doppler velocities by meteors.

[68]  Note that our method allows the reconstruction of the meridional wind velocity up to about 100 km altitude (in some cases up to 102-103 km). Then, with increasing altitude the coefficient of the wind velocity decreases rapidly (see Figure 2) and at 108-110 km becomes so small that observed winds could not provide any noticable contribution to the phase velocity in equation (18).

[69]  The large-scale electric field may be reconstructed for altitudes above 100 km. We found that the magnitude of the background zonal electric field was about - (0.2-0.5) mV m^-1. During 0250:05.9-0252:18.6 LT the reconstruction shows a zonal electric field of up to about -4 mV m^-1 in the 100-104 km altitude range. This large-scale (about 4 km) electric field is most probably a polarization electric field for the primary waves caused by a large-scale ionization cloud which appeared in the radar field of view.