31st Annual European Meeting on Atmospheric Studies by Optical Methods, Ambleside, Lake District, U.K., 22-28 August 2004

Local enhancement of neutral nitric oxide during afternoon absorption spike events

E. Turunen1, Th. Ulich1, C.-F. Enell1, P. Verronen2, F. Honary3

1Sodankylä Geophysical Observatory, Sodankylä, Finland
2Finnish Meteorological Institute, Geophysical Research Division, Helsinki, Finland
3Dept of Communication Systems, Lancaster University, U.K.


High energy particle precipitation events are known to affect local neutral chemistry in the mesosphere-lower thermosphere region. Significant momentary production of odd nitrogen through dissociation, ionisation and subsequent ion-chemical processes during solar proton events is a topic of several recent investigations. Less attention was paid on detailed quantifications of the chemical forcing by high energy electron precipitation. Recent theoretical results from the Sodankylä Ion-Chemistry Model show how nitric oxide is produced and sustained in the high-latitude ionosphere in response to energetic auroral electron precipitation. Dramatic local increases in nitric oxide above background levels seem to persist into the dayside after auroral activity, and the persistence of nitric oxide leads to elevated electron densities long after the activity has subsided.

In this paper we present theoretical estimates on local effects of afternoon absorption spike events on background concentration of nitric oxide. We use the Sodankylä Ion Chemistry model, which is a 1-dimensional time-dependent model of the lower ionosphere. It solves for concentrations of 36 positive ions, 25 negative ions, and 11 neutral species. The altitude range is from 50 to 150 km, with 1 km resolution. The model includes several hundred chemical reactions, photoionisation/dissociation of N2, O2, O2(1Dg), O, NO, NO2, O3, and H2O, as well as vertical transport in the form of eddy and molecular diffusion.

The short lasting spike events are often seen in the afternoon in the IRIS imaging riometer data. Isolated spike events would serve as an excellent experimental opportunity to verify theoretical predictions. A short discussion is given on how supporting observations by EISCAT incoherent scatter radars and optical instruments could be used for verifications.