Dynamics and Electrodynamics of the Mesosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 359

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Figure 7.14b The lidar-observedNLC are dark regions and the radar observed PMSE are
the white contour lines. [After von Zahn and Bremer (1999). Reproduced with permission
of the American Geophysical Union.]

7.6 The Role of Charged Ice

In Appendix A, a discussion of Langmuir probes describes how an isolated metal
sphere charges negatively in a plasma. The higher thermal velocity of the elec-
trons leads to a high flux and negative voltage results, which repel most of the
electrons until the positive and negative currents match. The absolute value of
this voltage is a few times k B T e e , the electron temperature in electron volts.

Charged mesospheric ice particles behave similarly except they are so small that
only a small charge can be accommodated. For mesospheric temperatures, a
metallic sphere would have to have a diameter as large as 30 nm to accommo-
date even one electron on average. The particles are far from spherical, however,
and it is thought that even smaller particles can hold an electron but in any case,
the number of electrons that can be accommodated is small.
This simple idea provides some insight but not a quantitative model dealing
with how ice charges. Reid (1990) has explored this problem in some detail.
His results are summarized in Figs. 7.15a and b. In the former, a variety of
species number densities are accounted for during daytime conditions for a par-
ticle radius of 1 and 10 nm. The balance equations have been solved for various
ionization rates. Reid (1990) quotes 10 cm − 3 s − 1 as a typical ionization rate for
daytime mesospheric heights. In both cases he finds significant electron deple-
tions. Charged dust thus can cause an electron bite-out as observed (Pedersen
et al., 1970; Ulwick et al., 1988).
The scenario suggested by Reid (1997) is that charged nanometer-sized dust
or molecular ions nucleate ice formation in the 90 km height range. As the
dust/ice particles grow they begin to fall, growing ever larger, becoming the
10 nm particles in Fig. 7.15b. As the temperature rises below 85 km, the particles
begin to sublimate and all but the dust disappears. Since the radar echoes

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