Geoscience Reference
In-Depth Information
Northern Hemisphere
Southern Hemisphere
12
12
January 3, 1998
DMSP F13
Cross-track drift
~1300 UT
18
6
18
6
8
85
8
85
8
80
80
8
75
8
75
8
70
8
70
8
MLAT
8
MLAT
0 MLT
0 MLT
Figure 8.1b
An example of the cross-track plasma drift velocity measured by DMSP
satellites on consecutive polar passes. The left panel shows the drift in the northern hemi-
sphere and the right panel shows the drift in the southern hemisphere. Ten degrees of
latitude is about 1 km/s on this scale. The winter drift shows significantly more structure
than the summer drift and is at least 3-4 times the average summer drift.
B
z
as measured
by the ACE satellite was southward during this time, with a value of about
−
0
.
8 nT dur-
−
.
ing the northern pass and about
2
2 nT during the southern pass. (Figure courtesy of
M. J. Nicolls.)
for simplicity's sake). In the preceding equation,
E
sw
is expressed in mV/m,
P
sw
in nPa,
pc
in kV. As in the simple conceptual approach above,
the polar cap ionospheric electric field (proportional to
P
in S, and
pc
) can be inversely
proportional to
P
is large enough to
cause a saturation of the polar cap potential, limiting it to values of the order of
250 kV (Hairston et al., 2003).
Weimer (see Appendix B) has developed a climatological model of the cross
polar cap potential based on DMSP data. Inputs involve solar wind parame-
ters. In Fig. 8.1c, the Hill-Siscoe equation (8.10b) is compared to the Weimer
model using TIMEGCM to determine
P
. This holds true whenever
E
sw
or
p
(Weimer, 2001). The agreement is
remarkable except when
B
z
is almost exactly northward. For the high solar wind
velocity and pressure in this event during
B
z
north, a PCP of almost 50 keV was
observed.
From the viewpoint of ionospheric physics, it is important to understand how
the currents from the generator link up to the load currents. Dividing
J
into
components perpendicular
(
⊥
)
and parallel
(
||
)
to the magnetic field and setting
the divergence of
J
to zero yields
∇
·
J
=
∇
⊥
·
J
⊥
+
∂
J
||
∂
s
=
0
(8.11)
where
s
is distance along the magnetic field line. Repeating and slightly extending
the discussion in Section 2.4, we may integrate this equation over the field line
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