From AMS Glossary
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(Also called light ion, fast ion.) An atmospheric ion of the type that has the greatest mobility and hence, collectively, is the principal agent of atmospheric conduction.
The exact physical nature of the small ion has never been fully clarified, but much evidence indicates that each is a singly charged atmospheric molecule (or, rarely, an atom) about which a few other neutral molecules are held by the electrical attraction of the central ionized molecule. Estimates of the number of satellite molecules are as high as twelve. When freshly formed by any of several atmospheric ionization processes, small ions are probably singly charged molecules, but after a number of collisions with neutral molecules they acquire (actually, in a fraction of a second) their cluster of satellites. Even with these satellites clustering about the central charged molecules, the ion mobility of the resulting complex is of the order of 104 times greater than that of large ions. Negative small ions exhibit slightly greater mobilities than positive small ions, 1.9×10-4 m s-1 per volt cm-1 being typical of negative, and 1.4×10-4 m s-1 per volt m-1 being typical of positive small ions in dry air at sea level. Small ions may disappear either by direct recombination with oppositely charged small ions or by combination with neutral Aitken nuclei to form new large ions, or by combination with large ions of opposite sign. The concentration of small ions near sea level is typically about 5×10-4 of each sign per m3, both over the oceans and over land. This concentration increases with altitude, and at 18 km it is about 10-3 per m3.