Polarization effects of the finite-size low-altitude ionosphere

Abstract

We use two-fluid or Hall effect MHD description of weakly-ionized stratified atmosphere to describe several polarization features of the MHD disturbance penetration. We employ a pair of functions for the electric and magnetic field components ratio which can be treated analytically. As an example we derive an approximation to the case of the MHD waves in the Earth's Hall ionosphere and demonstrate its different polarization responses (ellipticity and rotation) for Alfven and fast magnetosonic modes depending on the Hall region thickness. Neglecting the Hall thickness effect we derive previously obtained, well-known results for the rotation of the polarization plane of the MHD waves (Dungey, 1963; Nishida, 1964; Inoue, 1973; Hughes, 1974; Hughes and Southwood, 1976). The ionospheric effects are more essential for the polarization of the fast magnetosonic waves. The polarization changes of the magnetosonic waves are expressed as a function of i) the ratio (R) of the height-integrated Hall (ΣH) and Pedersen (Σp) conductivities (conductances) in the Hall region (85-125 km) and ii) a wave/magnetospheric parameter (A m) and the ratio A m/Σp. The wave/magnetospheric parameter Am depends on the wave frequency and the horizontal scale of the ULF waves. Using standard models IRI 90 and MSIS 86, responses of ULF magnetosonic waves to seasonal/diurnal ionospheric variations at subauroral/middle latitudes are illustrated for arbitrary, but reasonable values of the wave/magnetospheric parameter A m. The polarization plane rotation for the ULF compressional waves ranges between 0 and π/2 and reaches the classical π/2 degree only for special cases. Along with the rotation effect an ellipticity effect has also local time course. These findings suggest a new dissipative mechanism (non-resonant) of transformation of magnetosonic waves into Alfven modes in the ionosphere. In addition we suggest a physical insight for the MHD wave transformation effects by the ionosphere. These findings should be taken into account for the analysis of various polarization features of the geomagnetic pulsations observed on the ground. Sunrise effect on the polarization of the Pc 3-4 pulsations (Saka et al., 1982), the effect of transformation of pure compressional ULF disturbance in the magnetosphere into transverse wave on ground (Lanzerotti and Tartaglia, 1972) proved to be explained in terms of both the polarization rotation and the ellipticity mechanism by the ionosphere. Simultaneous measurements of the electric and magnetic field of ULF waves at ground and balloon heights have revealed polarizations of opposite handedness (Bering et al., 1995). It is shown that the polarization changes of the magnetosonic wave through a horizontally homogeneous high-latitude ionosphere continue further through the atmosphere and would result in different polarization states for the electric and magnetic fields. The northern (southern) hemisphere ionosphere causes an additional left(right)-hand polarization effect in the ionosphere/atmosphere produced mostly on the ULF wave magnetic field. The opposite handedness of the Pc5 wave polarization recorded at the South Pole by measurements of the ULF electric and magnetic field components (Bering et al., 1995) might be explained as a result of an influence of the ionosphere on the ULF waves of an initially left-hand polarization.