Regions of negative Bz in the Tsyganenko 1989 Model Neutral Sheet

We point out a disturbing feature of the Tsyganenko (1989) model magnetic field, namely the occurrence of negative Bz in the model neutral sheet. On the basis of observations of Bz in the neutral sheet we conclude that this is an artifact of the model and not a real effect. This feature of the model should be considered when the model is used either to infer mappings from the ionosphere to the vicinity of the neutral sheet or as a tool in theoretical studies. We propose that in the development of future models, it would be useful for the distribution of Bz in the neutral sheet to be imposed as a constraint on the model.