Fig. 14. Observed and predicted variation of the apparent fast direction at HRV. Observations are shown by triangles with error bars. A subset of "robust" data points (circled) was chosen so that the error in the splitting time was less than one third of the splitting time. Clearly, "robust" and "poor" data points follow the same pattern. Crosses show values of fast direction reported for HRV by Barruol et al. [1997] (as given in electronic supplement table 2). Thick lines show patterns predicted by our orthorhombic (solid) and hexagonal (dotted) models, thin lines show predictions for equivalent 2-layer splitting operators [Silver and Savage, 1994]. While all models capture the periodicity of the pattern, the spread of values and the deviations from the 2 pi pattern are not matched by the splitting operator predictions. (Postscript version)