An analysis of the accuracy of magnetopause reconnection rate variations deduced from cusp ion dispersion characteristics

We present an analysis of the accuracy of themethod introduced by Lockwood et al. (1994) for the determination of themagnetopause reconnection rate from the dispersion of precipitating ions in theionospheric cusp region. Tests are made by applying the method to synthesiseddata. The simulated cusp ion precipitation data are produced by an analyticmodel of the evolution of newly-opened field lines, along which magnetosheathions are firstly injected across the magnetopause and then dispersed as theypropagate into the ionosphere. The rate at which these newly opened field linesare generated by reconnection can be varied. The derived reconnection rateestimates are then compared with the input variation to the model and theaccuracy of the method assessed. Results are presented for steady-statereconnection, for continuous reconnection showing a sine-wave variation in rateand for reconnection which only occurs in square wave pulses. It is found thatthe method always yields the total flux reconnected (per unit length of theopen-closed field-line boundary) to within an accuracy of better than 5%, butthat pulses tend to be smoothed so that the peak reconnection rate within thepulse is underestimated and the pulse length is overestimated. This smoothing isreduced if the separation between energy channels of the instrument is reduced;however this also acts to increase the experimental uncertainty in theestimates, an effect which can be countered by improving the time resolution ofthe observations. The limited time resolution of the data is shown to set aminimum reconnection rate below which the method gives spurious short-periodoscillations about the true value. Various examples of reconnection ratevariations derived from cusp observations are discussed in the light of thisanalysis