Can the comprehensive model phase 4 (CM4) predict the geomagnetic diurnal field for days away from quiet time?

摘要

The most recent comprehensive model (CM4) of the geomagnetic field (Sabaka etal., 2004) has been used in conjunction with geomagnetic ground observatorystation data to analyse and study the geomagnetic diurnal variation field fordays away from quiet time and the CM4 prediction for these times. Even thoughmuch has been learnt about many components of the geomagnetic field, thediurnal variation field behaviour for days away from quiet time (moderatelydisturbed time) has not been intensively studied. Consequently, we analysethese, and the predictive ability of the CM4 for ground variations, andwhether the CM4 prediction of the diurnal variation (whether at quiet time oraway from quiet time) is valid outside the period of reference that fromwhich the data were used in modelling. In carrying out the study, we comparedthe observatory station data and the CM4 prediction directly. Using the CM4code, well-characterised internal and magnetospheric components weresubtracted from the data, plots and global maps of the residual fieldgenerated and then compared with the CM4 to see how well the model performedin predicting the data at moderately disturbed time (Kp  ≤  5). Theresults show that the CM4 is valid and produces useful predictions outsidethe period covering the timespan of the model and during moderately disturbedtime, despite the lack of active data in the original model dataset. Themodel predictability of the data increases as we move to higher sphericalharmonic degree truncation, as the model–data misfit is reduced, but withincreased roughness as a result of small-scale features incorporated. Theobserved results show that this relationship between the increase inspherical harmonic degree truncation and reduction in misfit can berestricted by data quality or quantity and global coverage orspread.