Geoscience Reference
In-Depth Information
Chapter 5
Ionospheric Alfvén Resonator (IAR)
Abstract
The topic of this chapter is the ionospheric Alfvén resonator (IAR) which
has been the subject of a great deal of research during recent years. The IAR
resonance cavity occupies a space between the conducting E layer and the topside
ionosphere where there occurs the strong gradient of Alfvén velocity. The IAR
accumulates the Alfvén wave energy in the ULF/ELF frequency range, typically
between 0.5 and 7 Hz. In this chapter, the structure, models, and possible physical
mechanisms for the IAR excitation are studied. Dispersion relation and the IAR
resonance spectra at night and daytime conditions are calculated.
Keywords
Dispersion relation E layer Exosphere Hall and Pedersen con-
ductivities Ionospheric Alfvén resonator (IAR)
5.1
Structure and Models of IAR
Analysis of the plasma wave propagation in the low frequency range made in Sect.
I
has shown that there are two kinds of plasma waves: the shear Alfvén wave and
the compressional wave which has been also termed the fast magnetosonic (FMS)
wave. According to Eqs. (
1.59
) and (
1.61
) the phase velocity of the shear Alfvén
waves in a homogeneous plasma depends on the angle included between the velocity
vector and the Earth magnetic field, whereas the group velocity, V
A
, is a constant
and parallel to the ambient magnetic field
B
0
. By contrast, the compressional wave
propagates at constant velocity V
A
in all directions. This means that the shear Alfvén
wave predominantly propagates along the geomagnetic field lines, whereas the
compressional mode propagates isotropically. As the shear Alfvén wave propagates
along the geomagnetic field lines, both the wave modes have the same velocity, V
A
,
the so-called Alfvén speed, which depends on B
0
Dj
B
0
j
and on the plasma density
0
, according to Eq. (
1.60
).
Typical Alfvén speed profiles are sketched in Figs.
5.1
and
5.2
, as a function
of altitude for nighttime and daytime conditions and for different sunspot activity
Search WWH ::
Custom Search