Electrodynamic Properties of Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 32

dependencies of the effective ν in ( h ). In the ionospheric E -layer, ν in is defined
by the elastic collisions of NO + and O + ions with molecules, and in the F -layer
by the intercharge of O + .

E -Layer

The most important region determining the reflection and dissipation of mag-
netospheric MHD-waves as well as the effectiveness of their penetration into
the Earth is the highly-conductive E -layer. While higher ionosphere layers
and the magnetosphere are anisotropic dielectrics in the ULF-range, the E -
layer is similar in its properties to a conductive anisotropic medium. The
geomagnetic field is so strong for the ionospheric electrons that they become
strongly magnetized, i.e. the dimensionless ratio of their cyclotron frequency
ω ce to collision frequency is much larger than one. An electric field applied
across the geomagnetic field causes an electron drift in a direction perpen-
dicular to both the ambient magnetic and the applied electric fields. Ions are
non-magnetized at these altitudes because they collide with neutrals so often
that their collision frequency ν in begins to noticeably exceed the cyclotron
frequency ω ci .
The transversal electric conductivities in the E -layer, determined by both
electrons and ions, become anisotropic. Therefore, the electric field pro-
duces a Pedersen current directed along the field as well as a current di-
rected perpendicularly to both the electric and geomagnetic fields, called
the Hall current. The ionosphere and the magnetosphere can be considered
to be magnetoactive media. In the coordinate system with the z -axis par-
allel to the magnetic field B 0 , the components of the electric current are
j z = σ zz E z ,j x ±

iE y ). Relations between σ xx ( ω ),
σ xy ( ω ) , and respectively σ P , σ H are given in (1.92). The conductivities
in terms of collision and cyclotron frequencies are (1.86)-(1.88). Figure 2.4
presents altitude dependencies σ zz = σ , σ P and σ H for dayside and nightside
models.
The electron and ion magnetization, β e = ω ce /ν e and β i = ω ci /ν i ,are
given in (1.93) and (1.94), respectively. Both σ P and σ H consist of two parts
- electron and ion. Figure 2.5 shows the height distribution of the ion β i and
electron β e magnetization parameters. At heights of less than 60 km, both β e
and β i vanishandwehave

ij y =( σ xx ∓ iσ xy )( E x ±

σ P = Ne 2

mν e ,

H =0 .

Above 70 km electrons rotate around the magnetic field rarely colliding
with neutral particles. Parameter β e increases from 1 in the D -layer and
reaches 10 4 at 200 km. It is evident that the electron part of σ H is larger
than the ion part at heights between 70 and 140 km where β i

1 . One can
say that the Hall conductivity in the D -and E -layers is determined by elec-
trons. As to the higher levels ( F -layer), σ H vanishes because both electrons