Dynamics and Electrodynamics of the Mesosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 355

Detrended electron density from MAC/SINE 15.245

400
200

0

200
400

85.05 85.1 85.15 85.2 85.25 85.3 85.35

Normalized wavelet scalogram

306.9

76.7

19.2

4.8

1.2

0.3

85.05 85.1 85.15 85.2 85.25 85.3 85.35

Altitude (km)

1

0 1

Figure 7.11 Detrended electron density data and wavelet scalogram from a 300m
section centered at 85.2 km during the MAC/SINE 15.245 rocket flight.

[After

Alcala et al. (2001). Reproduced with permission of the American Geophysical

Union.]

where is the kinematic viscosity of the neutrals and D A is the ambipolar plasma

diffusion coefficient. Usually S c ≈

ν

1 and in the turbulence case the passive scalar
electrons have the same spectrum as the neutral gas with an inner scale of tens
of m in the mesosphere. Batchelor (1959) showed that a passive scalar (plasma
in this case) can have structure at scales much smaller than the neutral gas if
S c is large. Figure 7.13 shows a comparison of the electron fluctuation spectra
measured using rocket probes at three locations. The equatorial and nonpo-
lar summer cases show the usual neutral turbulence-like inner scale (tens of
meters) but the polar summer spectrum extends well into the submeter scale
sizes. Some 5-7 orders of magnitude of higher spectral density are found at
VHF Bragg scales, which directly translate into 50-70 db of increased radar echo
strength.
A high Schmidt number is caused by a low diffusion coefficient, which in turn
is caused by heavy charged aerosols. An analogy by J. Cho likens electrons to
flies buzzing around individual cows slowly lumbering around a field. Once the
heavy aerosols tie up around half the charge, D A drops dramatically (Cho et al.,
1992a). The simple model by these authors has been extended by Hill et al.
(1999) and Rapp and Lübken (2004a, b), which show that the evolution of D A