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20
10
50
Frequency (H
Z)
100
Figure 10.21
Detection of a pure oxygen cyclotron wave near the edge of an auroral
arc. [After Kelley et al. (1975). Reproduced with permission of the American Institute of
Physics.]
an intense upward field-aligned current. Hydrogen ion cyclotron waves are com-
monly observed above 5000 km (Kintner et al., 1978) but observations of oxygen
cyclotron waves such as these are more rare.
Kelley and Carlson (1977) also showed evidence for less intense and more
broadband electrostatic waves in regions of intense velocity shear and field-
aligned currents at the edge of the same auroral arc. The velocity shear measured
by the dc electric field detector in the same region was 20 s
−
1
, which is very high
and implies a change in the plasma flow velocity comparable to the sound speed
in one ion gyroradius. Less intense irregularities in the same wavelength regime
were observed equatorward of the arc but not above the arc itself or poleward
of it. The dc electric field was such that the ionospheric plasma convection had
an equatorward component in the whole region probed, which suggests that
the structures may have been formed at the arc boundary and were transported
equatorward. Similar spectra have been reported in the dayside auroral oval by
Earle (1988) and Earle and Kelley (1993).
The possible origins of ionospheric structure with scales in the range of a
few meters (
r
gi
) to a few tens of meters are quite numerous and no unifying
mechanism is likely to explain all of the observations. It is very likely that some
energy cascades into this wavelength range from larger scales in a manner similar
to neutral turbulence. In a plasma the situation is made much more complicated
by the possible generation of wave modes which compete with or modify the
cascade concept.
Of the several possible free energy sources available, most theoretical effort
has gone into calculations of the generation of electrostatic ion cyclotron waves
≈
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