Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 284

concluded that plasma instabilities must create an anomalously large D A , which
then limits the sharpness of the gradients and the thickness of the layer.
Finally, we note that a wind shear is not necessary for all layer formation.
A westward Northern Hemispheric wind will drive ions downward but into
an ever increasing atmosphere. Collisions (low κ i ) cause the vertical velocity to

decrease with decreasing altitude and a layer forms. A southward electric field
has the same effect. As seen in the various examples of sporadic E layers, as their
altitude decreases, they seem to come nearly to rest below 95 km. In this height
range, ion chemical reactions by metal ions become much more frequent and
eventually they are destroyed chemically.
These metallic ions have their origin in the ablation of meteors. This source also
leads to copious metallic atoms in the 90-100 km altitude range. The existence
of sodium, iron, and potassium atoms allows the use of resonant lidar scatter
to study this important height range. In the case of sodium, Cox and Plane
(1998) first seriously studied sodium ion chemistry and attempted to explain the
atom layers. A slightly modified version of their chemical model is illustrated
in Fig. 6.10. A crucial reaction rate was incorrect in the earlier study, and the
initial results did not fit the data very well. In conjunction with new data taken
at Arecibo and a revised reaction rate for the NaO + + Na + N 2 + N 2 → O and

NaO + + Na + + O 3 branches, Collins et al. (2002) have produced much

better agreement. Through such interactions, sodium ions can maintain sodium
atom layers (see Fig. 6.7).

O 2 →

Na 1

CO 2 ( 1 M)

O, O 2

N 2

( 1 M)

O

CO 2

Na 1 ? CO 2 Na 1 ? N 2

NaO 1

N 2

e 2

e 2

e 2

Na

Figure 6.10 Ion chemical reactions involving sodium. [After Collins et al. (2002). Figure
reproduced with permission of Elsevier Science Ltd.]