A new approach to directly determine the secular variation from magnetic satellite observations

Observatory monthly means provide an excellent opportunity to study the temporal changes of the geomagnetic field at a given location. Unfortunately, determination of the global pattern of changes using observatory data is hampered by their uneven distribution. Satellite data provide excellent global coverage, but the spacecraft movement makes direct comparisons of satellite and observatory data difficult. To investigate short-period secular variation in observatory and satellite data, we developed an approach to extract satellite monthly means for virtual observatories'' at 400 km altitude, using CHAMP magnetic measurements. Comparison of these virtual observatory monthly means with the corresponding ground values shows a remarkably well-correlated signal at time-scales of months to years, which is beyond the temporal resolution limit of recent global models. Here, we describe this newly developed approach, its validation, and discuss how it can be used to understand short-period changes of the recent geomagnetic field.