Geoscience Reference
In-Depth Information
6.2
Field-Line Resonance (FLR)
6.2.1
MHD Box Model
We now take up one more viewpoint on the magnetospheric resonances. To study
the MHD-waves coupling in a little more detail, however, we need to consider
a simple approximate model of the magnetosphere sketched in Fig.
6.1
.Inthe
model the dipole geomagnetic field lines are replaced by straightened field lines
in such a way that the area marked in Fig.
6.1
with Mis transformed into the
parallelepiped/box magnetosphere shown in the bottom of Fig.
6.1
.They axis
which was originally in the west-east direction is now transformed into a straight
line that is infinite in length. In this picture the y-coordinate in the box model
corresponds to the azimuthal direction/coordinate ' in the reference frame fixed
to the Earth spin axis.
The box contains a cold magnetized plasma immersed in a straight magnetic
field,
B
0
D
B
0
.x/
O
z
, which is a function of x. Both the plasma mass density, ,
and the Alfvén velocity, V
A
, also depend on only x, which plays a role of radial
coordinate in the equatorial plane. The magnetic field lines are finite in length in the
z
direction and there are boundary conditions at the ends of lines. The box surfaces
z
D
0 and
z
D
l
1
correspond to the southern and northern ionospheres. The box
surface x
D
0 represents the equatorial region of the ionosphere while the plane
x
D
l
2
corresponds to the outer boundary of the magnetosphere. This model was
originally suggested by Radoski (
1966
,
1967a
,
b
) and has been termed the MHD box.
Ionosphere
N
M
B
0
Magnetopause
S
Plasmapause
z
Ionosphere
l
1
B
0
l
2
0
x
y
Ionosphere
Fig. 6.1
Sketch of MHD-box model of the magnetosphere. The figure is partly adapted from
Southwood and Kivelson (
1982
)
Search WWH ::
Custom Search