Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 483

600

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100

27 February 1980
09:32 to 09:45 UT

0

61 62 63 64 65 66

Invariant latitude — degrees

67 68 69 70 71

(a)

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50

40

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0

0.01
Spatial frequency — km 21

0.1

(b)

Figure 10.10 (a) Altitude/latitude variation of electron density in the midnight sector
auroral zone measured by the Chatanika radar. (b) Spectrum of electron density irreg-
ularities obtained by analyzing the radar measurements of the latitudinal variations of
electron density at 350 km altitude shown in (a). In this plot the x -axis corresponds to 1/ λ,

not 2 , as is usual in this text. [After Kelley et al. (1982a). Reproduced with permission

of the American Geophysical Union.]

π/λ

We turn now to a discussion of polar cap aurora. When B z is northward, the
auroral oval contracts and particle precipitation are much reduced in the auroral
zone. However, auroral activity actually increases in the polar cap region during
B z north conditions as illustrated in Fig. 10.11a. There seem to be two basic states
of the polar cap when B z is north, an ordered multicell flow pattern and a chaotic
flow (see Chapter 8 for details). In the ordered case, a very long sun-aligned
auroral arc can form in the polar cap. An example is shown in Fig. 10.11b,
which is a photograph of the polar region taken on the Dynamics Explorer 1
satellite. The circular auroral oval is linked from day to night by a sun-aligned