Estimation of magnetosphere-ionosphere mapping accuracy using isotropy boundary and THEMIS observations

摘要

It is difficult to establish the degree to which global magnetospheric mapping models are accurate, because there exists no definitive, independent method of validating such models. Toward that end we use the isotropy boundary (IB) of precipitation of energetic particles, as determined by low-altitude spacecraft. These particles are observed at ionospheric altitudes but their precipitation is governed by the magnetic field near the equator. Precipitating and trapped fluxes measured at the ionosphere can thus be used to determine the equatorial field strength, which can in turn be compared with predictions of magnetospheric models. By using hundreds of IB observations at the ionosphere during THEMIS major tail conjunctions in 2008 we report on the mapping accuracy obtained using three models: T96, AM-01, and AM-02. The first model is driven by the simultaneous solar wind and Dst measurements, whereas the latter two are obtained by fitting model data to THEMIS observations. The AM-02 and T96 models show comparable agreement with proton IB locations, with error estimates of about 1° in latitude. However, the AM-02 outperforms T96 in predicting electron IB locations. Mapping errors increase with magnetic activity and have significant magnetic local time dependence. We conclude that event-based magnetospheric models can be as good as or better than solar wind-based models, provided that a number of distributed magnetotail spacecraft are used to constrain model parameters.