A DETERMINATION OF THE HYDROMAGNETIC-WAVES POLARIZATION FROM THEIR PERTURBATIONS ON THE TERMINATOR

The influence of the homogeneous and inhomogeneous ionosphere on the o rientation angle of the horizontal magnetic vectors of the long-time g eomagnetic pulsations is under consideration in this study. It was rea lized that this angle is small in the case of the homogeneous ionosphe re for both the Alfven and magnetosonic types of oscillations. An incr ease in the ionospheric electric held was discovered as the ionospheri c conductivity changes during the switch from day to night conditions. It is valid only for the initial Alfven wave. The ionospheric equival ent current systems excited by the initial magnetospheric waves of Alf ven and magnetosonic types as well as their behavior near the terminat or were studied for different seasons. For the Alfven source, seasonal variations of the orientation angle close to sunrise at the equator d epend on the type of source: odd or even modes of Alfven oscillations excite observable pulsations. It was found that the ionospheric two-vo rtex equivalent current system of the long-period pulsations arising i n high-latitudes in the equatorial region alters not only its directio n, but its intensity too. The largest anomaly (similar to 25% of the s ource value) would be expected near the terminator. A new experimental method was suggested to recognize the type of incident magnetospheric waves by implementing observations either at a single observatory or at a couple of observatories. In the case of a single observatory it i s proposed to study the frequency dependence of the orientation angle of their magnetic components close to sunrise. If the initial wave is magnetosonic, this angle must not be changed as a function of the loca l time within the wide frequency range of pulsations. When pulsations have an orientation angle sensitive to the presence of the terminator, they may be classified as both Alfven and magnetosonic. For the Alfve n waves no frequency dependence of the orientational angle is peculiar . On the contrary, magnetosonic waves should be determined as oscillat ions with an orientational angle proportional to the frequency. These oscillations may be revealed at observatories located on the high-resi stance cross sections. The example of the spectral: temporal analysis of pulsation at the equatorial observatory in Huancayo was demonstrate d to confirm the proposed experimental technique. A weak dependence of the orientation angle anomaly on the frequency near the terminator wa s found. The latter is evidence for the dominant contribution of the A lfven waves to low-latitude and equatorial oscillations.