Empirical recombination rates in the lower ionosphere

The number density of free electrons in the atmosphere is the result of a balance between ionisation and electron loss. The former is reasonably predictable at non-auroral latitudes, but at high latitudes - where ionisation by energetic particles dominates - one aims to infer the ionising particle spectra from the resulting electron densities. For this procedure, as well as for the verification of ion-chemical models, a sound knowledge of the recombination rate is crucial, notably in the D-region where variations of the recombination rate by orders of magnitude can be expected. Empirically one obtains this rate by forming the ratio of the ionisation rate to the square of the electron density. Therefore, both parameters have to be known with good accuracy. The number of cases is thus essentially limited to those reliable electron densities for which all important ionisation processes are also known. From less than 50 such cases we attempt to deduce representative recombination rate profiles for day- and night-time conditions. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.