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12
12
12
608
608
608
708
708
708
808
808
18
06
18
06
18
06
2
00
B
z
,26.9 nT
00
27.0 nT ,
B
z
,24.9 nT
00
25.0 nT <
B
z
< 22.9 nT
12
12
12
608
608
608
708
708
708
06
18
06
18
06
18
00
2
3.0 n
T ,
B
z
,2
0.9 nT
00
21.0 nT ,
B
z
,11.0 nT
00
10.9 nT <
B
z
< 1 3.0 nT
12
12
12
608
608
608
708
708
708
06
18
06
18
06
18
00
12.9 nT ,
B
z
,15.0 nT
00
14.9 nT ,
B
z
,17.0 nT
00
17.0 nT #
B
z
Figure 10.11a
Mass plots of arcs in corrected geomagnetic coordinates for different
values of
B
z
. [After Lassen and Danielsen (1978). Reproduced with permission of the
American Geophysical Union.]
auroral emission. The entire feature is referred to as a theta aurora. The particle
energies associated with these auroras are in the “soft” category (
100 eV), and
both the plasma production and auroral airglow emissions therefore occur at
lower F-region or upper E-region altitudes.
Other experiments show that sun-aligned arcs seem to occur in a region of
shear in the plasma flow. In fact, the data suggest that electrons precipitate in
a region where the flow vorticity has a positive sign (Burke et al., 1982). This
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