Solar cycle dependence of quiet-time magnetospheric currents and a model of their near-Earth magnetic fields

Satellite measurements from high-resolution magnetic field mapping missions over almost a complete solar cycle have been used to investigate the variability of major magnetospheric currents during quiet times. As judged from near-Earth magnetic field observations, the magnetotail current system exhibits no long-term trend. Activity-dependent enhancements decay within hours. A suitable quantity to parameterize the temporal variation is the merging electric field, delayed by 60 minutes with respect to its value at the bow shock. Conversely, the ring current strength shows pronounced solar cycle dependences. We show for the first time that a solar cycle dependent bias has to be added to D-ST to represent the actual ring current strength. It ranges from -15 nT at solar maximum to zero at solar minimum. The scaled solar flux index, F-10.7, delayed by 20 months, is found to be a suitable proxy for predicting the bias. This empirical result reflects no physical relation between F-10.7 and ring current bias. A spherical harmonic model valid at Earth's surface is presented that reflects the quiet-time magnetospheric fields.