Inhomogeneous Ionosphere - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 282

Fig. 9.3. Spatial distribution of the equivalent current intensity and its vectors
induced by the incident Alfven wave with the 'bell-like' coordinate dependence of
its magnetic field intensity ∝ exp( −x 2 / 2 L 2 ). x is the southward coordinate; L ∼
100 km. The azimuthal extension of the source was chosen to be 20 ◦ . The beam of
Alfven waves is incident on the high-latitude ionosphere near the noon meridian.
Equinox (a) and Summer in the Northern hemisphere (b)

Contrary to the above case, the even disturbances are associated with field-
aligned currents of the same direction in the conjugate hemispheres.
Let us assume, for example, that there is a geomagnetic pulsations maxi-
mum at high latitudes ( 67 ◦ ). Let the latitude dependence of these pulsations

intensity be described by the 'bell'-like function

∼

x 2 / 2 L 2 ) , where x is
the southward coordinate, and L is the characteristic half-width of the 'bell'.
Using such a function, it is possible to approximate an amplitude distribution
of many types of oscillations, for example Pi 2. This distribution of the mag-
netic field in an Alfven wave corresponds to a pair of field-aligned currents of
opposite directions extended along a parallel, and separated by a space scale
L in latitude. The respective system of ionospheric equivalent currents Ψ sol is
presented in left-hand panel in Fig. 9.3 for equinox. In this case, the source
(field-aligned current) is localized on the noon meridian in both hemispheres.
When the disturbances are small-scale along a parallel, a spatial distribu-
tion of the solenoidal current system Ψ sol is dipole-like. Such a current system
makes the equivalent current and magnetic field dependent on the distance,
like R − 2 at long distances from its ionospheric source, which is a projection of
the Alfven magnetospheric source on the ionospheric surface in the case under
consideration. In the middle and high latitudes, the current's stream line (line
of equal current) aligns along the terminator, and two current vortices arise
in the middle and low latitudes in both hemispheres. One of these vortices
captures almost all the sunlit part of the ionosphere and spreads to the night-
side. The other vortex is localized in the middle latitudes and has a focus at
colatitude

∝ exp( −

75 ◦ at local time 15 LT (225 ◦ ). At the equator, as well as in the
case of a localized point source, there are no terminator effects. Terminator
influence becomes more or less significant beginning at colatitude

≈

60 ◦ . ≈