Increasing exposure of geosynchronous orbit in solar wind due to decay of Earth's dipole field

The Earth's dipole moment has been decaying over the past 1.5 centuries. The magnetosphere thus has been shrinking and the chance of geosynchronous magnetopause crossings has been increasing. We quantitatively evaluate the increasing exposure of geosynchronous orbit in the solar wind caused by the decay of dipole moment and the variation of solar wind condition and study the possible situation if such decay persists for several more centuries. The results show that the average subsolar magnetopause distance would move earthward by similar to 0.3 R-E per century, assuming the linear decreasing of the Earth's dipole moment at present rate. The minimum solar wind dynamic pressure required for geosynchronous magnetopause crossings will decrease by similar to 4 nPa (2 nPa) in the next 100 years under northward (southward) interplanetary magnetic field. Under normal solar wind conditions, the noon region of the geosynchronous orbit will be exposed to the solar wind in the next few centuries. These results suggest that the secular variations of geomagnetic field are of paramount importance for our understanding of space climate.