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In-Depth Information
Equinox
Equinox
MLT
1635- 0446
1635- 0446
1971- 1972
1971- 1972
MLT
10
5
10
5
10
4
10
3
10
2
10
1
10
4
10
3
10
2
10
1
2H
1
2
2He
1
2
O
1
O
1
10
9
10
7
10
8
10
6
16
16
10
7
10
5
1
5
1
5
10
8
10
6
1
4
1
4
10
9
13
10
7
13
12
12
1
1
1
1
0
0
45
2
1
2
1
45
55
65
75
85
75
65
55
45
55
65
75
85
75
65
55
45
Invariant latitude (deg)
Invariant latitude (deg)
ISIS-II IMS
ISIS-II IMS
Figure 9.1
The average equinox distribution of O
+
,H
+
, and He
+
at 1400 km plotted
in the upper panels shows that the reduced concentration of light ions at high latitudes is
accompanied by significant outward flow velocities. Quiet magnetic conditions pertained
in the data sets used to construct these curves. [After Hoffman and Dodson (1980).
Reproduced with permission of the American Geophysical Union.]
concentrations between 55
◦
and 65
◦
invariant latitudes to very low, almost
constant concentrations near 100 cm
−
3
at higher latitudes. In the same latitude
sector the O
+
concentration is nearly constant on the dayside and shows only
a gradual change at night. This signature in the light ion species may be seen
at all altitudes where the species H
+
or He
+
is detectable and is a persistent
feature at all local times. It is called the light ion trough. The latitude gradient
in the H
+
concentration defines the equatorward edge of the light ion trough
and the magnitude of the gradient may change significantly as a function of local
time and season. Its position is also strongly dependent on magnetic activity.
As we shall see in Section 9.2, the magnetic dependence is a straightforward
consequence of the variability of convection paths of the plasma that, in fact,
determine where the trough will be formed. This large area of depressed light
ion concentrations relative to the O
+
concentration can be produced only by
a field-aligned motion of the light ions relative to O
+
. Satellite-borne ion mass
spectrometers have observed this relative motion at altitudes below 2000 km
as well as at much higher altitudes. The light ion velocities are described as
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