Investigation of a secular variation impulse using satellite data: The 2003 geomagnetic jerk

Observatory monthly means provide an excellent opportunity to study the temporal changes of the magnetic field at a given location. Unfortunately, the uneven distribution of the present observatory network makes it difficult to determine the global field change pattern. Recently, we have developed an approach to extract satellite monthly means at a regular network of "virtual observatories" at 400 km altitude, based on CHAMP magnetic measurements. Using monthly means for 2001-2005 from those "virtual observatories" we investigate the space-time structure of the short-period variation of the Earth's magnetic field by means of a Spherical Harmonic Expansion, followed by a separation into external (magnetospheric) and internal part. This allows, for the first time, to study the secular variation globally and directly from satellite magnetic data. Analyzing the time series of the magnetic field at the "virtual observatories" as well as those of the spherical harmonic expansion coefficients, we detect a secular variation impulse (an abrupt jump in the second time derivative of the magnetic field) in the CHAMP satellite data during the first months of the year 2003. The jerk occurred simultaneously in the northern and southern hemispheres in a rather limited area near 90 degrees E, with maximumjerk strength at about 301 latitude, a region also characterized by a strong secular acceleration (second time derivative of the magnetic field). We show that the 2003 geomagnetic jerk is not worldwide in occurrence and that there is an evidence for this event in the length-of-day variation.