Ion composition and effective ion recombination rate in the nighttime auroral lower ionosphere

We present the results of a combined study applying the Lancaster-Sodankyla model and EISCAT radar observations of the lower ionosphere during evening to early morning hours. The purpose is to derive the spectrum of incoming energetic electrons which produce the ionisation, and to determine the effective recombination coefficient and the concentrations of the positive and negative ions over the height range 70 to 120 km. This was done by combining a semiempirical 'profile-inversion' technique to estimate the spectrum of the precipitating electrons from the electron density profiles measured by the EISCAT radars, and the 35-ion Sodankyla: model of the chemistry of the D and E regions. Both procedures assume chemical equilibrium conditions. It is found that, below 85 km, precipitation reduces the average ion mass at a given height by changing the relative proportions of the various ions, with the simpler molecular ions becoming dominant over the complex and hydrated ions. The same reactions occur as in the undisturbed nighttime mesosphere, and the concentrations of most cluster ions as well as most negative ions are increased by precipitation, though by less than those of the molecular ions. The effective electron-ion recombination rate is very sensitive to the energetic charged particle spectrum, and is strongly reduced by any increase of the precipitation flux at the higher energies. Above a certain threshold flux of harder precipitation (exceeding 120-150 keV), the profile of the effective recombination coefficient appears to change very little with flux, reaching values comparable to that for daytime conditions. (C) 1997 Elsevier Science Ltd.