Atmospheric electron–ion and ion–ion recombination processes

The electron–ion and ion–ion recombination processes of importance in the upper atmosphere are considered, and available laboratory experimental and theoretical information concerning the relevant processes is discussed. For atomic ions the principal electron–ion recombination process is radiative, with theory indicating that the two-body coefficient at ∼200 °K is ∼10 −11  cm 3 /s and decreases with increasing electron temperature. Microwave afterglow/mass spectrometer studies of diatomic ionospheric ions (e.g. NO + , O 2 + , and N 2 + ) show a loss by dissociative recombination with a coefficient substantially in excess of 10 −7  cm 3 /s at 250 °K and decreasing with increasing electron and ion temperature. There is some evidence from flame studies that H 3 O + ions exhibit a very large coefficient (10 −6 –10 −5  cm 3 /s) at 300 °K. Ion–ion recombination evidently proceeds by mutual neutralization, with laboratory studies of ions such as NO + and NO 2 − indicating a two-body coefficient of the order of 10 −7  cm 3 /s at 300 °K. In the lower D region, three-body Thomson recombination may be important, since laboratory studies of "air" ions indicate a three-body coefficient of ∼2 × 10 −25  cm 6 /s at 300 °K.