The CM4 model prediction of ground variation of the geomagnetic diurnal field away from quiet time

• Geomagnetic diurnal variation for non- quiet time and the CM4 model prediction.
• CM4 produced more reasonable predictions of the field away from quiet time.
• Limitations of CM4 in predicting diurnal field for days away from quiet time is shown.
• Fit between CM4 & obs data increases as spherical harmonic degree increases.
• External field description in CM4 for active days not sufficient to explain field.

We analyse the geomagnetic diurnal variation for days away from quiet time to see how well the comprehensive model (CM4) can reasonably predict ground variation of the diurnal field. To do this, we compared ground observatory hourly means to predictions given by the CM4 model for days away from quiet time. Our results show that, away from quiet time, the CM4 model is producing more reasonable predictions than expected, despite the lack of active data in the original model dataset. However, the CM4 model is not doing so well predicting short term features during period of rapid variations, especially for the X component of the geomagnetic diurnal variation field. When comparing the different modelled diurnal variation field maps of the CM4 model and the observatory data, our results show that the model to data fitness increases as we increase the spherical harmonic degrees. From our results we could see that the inability of the CM4 model to accurately predict the geomagnetic diurnal field for days away from quiet time, during time of rapid variations, may be due to the fact that the external field descriptions included in the CM4 model could not sufficiently explain the field contributions for days away from quiet time. This is seen in the low coherence, agreement and correlation in the comparison and cross correlation coefficient between the X component of the observatory data and the Dst index (which allows CM4 model to respond to active conditions outside of the original geomagnetic activity remit).