Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 332

6.7.8 The Wavelength Limiting Effect and Small-Scale Instabilities
If a meridional electric field causes the instability or if the layer is subject to a
zonal wind of only one sign, then only one side of a layer is unstable. When this
occurs a very interesting phenomenon occurs. The outer scale of the instability is
limited to waves which do not map to the other side of the layer where damping
occurs. This is due to the Farley mapping effect. That is, if

1 / 2 is greater

λ ⊥ (σ 0 /σ ρ )

than the layer thickness, the wave cannot exist.
A very nice example of this effect is presented in Fig. 6.42. The wind was
westward during the event and waves were found only on the top of the layer.
Furthermore, the outer scale was 30m, exactly as predicted by the mapping
effect. This type of release of free energy in scales of 1 to 30mmay be the primary
way that m-scales are created from plasma patches and billows. Urbina et al.

Arecibo radar HF34 downleg

94 94

93 93

92 92

91 91

24 0 1 2 3 22 0 2 4

Electron density (10 4 /cc) (mV/m)

Figure 6.42 Sporadic E layer (left-hand panel) along with fluctuating electric fields.
[After Kelley and Gelinas (2000). Reproduced with permission of

the American

Geophysical Union.]