NIGHTSIDE ION FRICTIONAL HEATING - ATOMIC AND MOLECULAR ION TEMPERATURE ANISOTROPY AND ION COMPOSITION CHANGES

An ion frictional heating event was observed by the EISCAT facility at 22:00 UT on 5th September 1989. The Sheffield University plasmasphere and ionosphere model has been used to investigate this event. A close d subauroral tube of plasma is considered in the model which has been developed to include temperature anisotropy in the NO+ as well as in t he O+ ion populations. During the event the O+ temperature distributio n becomes more anisotropic than the NO+. This means that in the F regi on the temperature of the NO+ ions parallel to the magnetic field is s ubstantially greater than the temperature of the OC ions parallel to t he magnetic field. The model predicts that the ion composition in the F region becomes more molecular during the event. At an altitude of 30 0 km the composition changes from almost 100%, O+ ions to 70% NO+ ions . To account for the change in ion composition which is not considered in the standard EISCAT analysis the EISCAT data were reanalysed using the ion composition predicted by the model. To compare with the measu red ion temperature an average ion temperature was calculated from the model O+ and NO+ distributions. An additional heat source was introdu ced to enhance the model electron temperatures. There was then reasona ble agreement between the modelled and measured parameters. Similar ob servational and model results were obtained for the ion frictional hea ting event observed at 21:00 UT on 9th May 1982. (C) 1997 Elsevier Sci ence Ltd.