Comparison of models and data at Millstone Hillduring the 5–11 June 1991 storm

Abstract We compare measurements of the ionospheric F region at Millstone Hillduring the severe geomagnetic disturbances of 5–11 June 1991 with results from the IZMIRANand FLIP time-dependent mathematical models of the Earths ionosphere and plasmasphere. Somecomparisons are also made with the Millstone Hill semi-empirical model which was previouslyused to model this storm. New rate coefficients from recent laboratory measurements of the O + +N 2 and O + +O 2 loss rates are included in theIZMIRAN and Millstone Hill models. The laboratory measurements show that vibrationallyexcited N 2 and O 2 (N 2 ( v ) and O 2 ( v )) are both important at high temperatures such as found in the thermosphere during disturbedconditions at summer solar maximum. Increases in the O + +N 2 loss ratedue to N 2 ( v ) result in a factor ∼2 reduction in the daytime F 2 peak electron density. On some days inclusion of N 2 ( v ) improves theagreement between the models and the data, and on other days it worsens it. In the present workwe show for the first time the significant effect that the increase in the O + recombination rate due to O 2 ( v ) may have on the calculated NmF 2 . There are considerable uncertainties in the model calculations during the unusual,extremely disturbed conditions found during the daytime on 6 June. The results illustratedifficulties involved and the current state of the art in modelling severe disturbances, and thusprovide a benchmark against which future progress can be gauged.