MHD-Waves in Layered Media - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 183

7
MHD-Waves in Layered Media
7.1 Introduction
The main properties of MHD-waves in the magnetosphere have been discussed
in the previous chapters wherein the general features of these waves traveling
from a magnetospheric source towards the ionosphere have been established.
Moreover, we have constructed the shape of the MHD-wave caused by the
FLR not only in the magnetosphere but also on the upper boundary of the
ionosphere. In the present and forthcoming chapters, we will consider penetra-
tion of the MHD-waves incident to the ionosphere from the magnetosphere to
the ground observer. We shall find the ionospheric frequency response function
in the wide-scale size of the ULF-range.
We begin by constructing models for the wave and the system ground-
atmosphere-ionosphere-magnetosphere. Consider a layered model of the
ionosphere. Here, the horizontal scale size of change of the ionospheric con-
ductivity is much more than the horizontal scale size of the magnetospheric
wave coming to the ionosphere. An Alfven wave activated close to the equa-
torial magnetospheric plane is trapped by the geomagnetic field and guided
along the field-lines. It results in the mapping of the place of generation onto
the ionosphere. Due to the convergency of the field-lines, the horizontal scale
of the wave above the ionosphere becomes essentially less than the Earth's
radius. It is valid for the latitudinal dependency of the wave.
Conductivities of the dayside and nightside ionospheres depend weakly on
the latitude and longitude. And only within specific regions like the boundary
between dayside and nightside ionospheres and in the equatorial and polar
regions, the horizontal scale of the ionospheric change becomes comparable
with the horizontal scale of the Alfven wave.
The FMS-waves undergo a strong refraction in the magnetosphere and
usually their reflection points are located far away from the ionosphere. The
amplitude of these waves decreases exponentially as they propagate towards
the ionosphere and their intensity becomes smaller compared to that of Alfven
waves. In this way, the polarization of the waves reaching the ionosphere can
be considered to be basically corresponding to that of Alfven type waves.