Night-time sudden commencements observed by CHAMP and ground-based magnetometers and their relationship to solar wind parameters

We have studied 41 Sudden Commencements (SC) using simultaneous magnetic field data from the CHAMP satellite and ground stations of the years 2000-2007. They are all night time events, since the influence of ionospheric currents on the SC is supposed to be minimal at night. This is confirmed by our study for geomagnetic latitudes below +/- 40 degrees. We further found that the onset times of the SC signature at satellite altitude and on the ground are the same within an uncertainty of 10 s and that the slopes of the corresponding magnetic field variation are very similar. For magnetic latitudes poleward of +/- 40 degrees the amplitude of SCs increases both at the satellite and on ground, probably a consequence of field-aligned currents. CHAMP sometimes records small-scale magnetic variations different from the ground, which can be explained by local ionospheric currents. We also studied the relationship between the SC amplitude seen by CHAMP and the corresponding abrupt solar wind dynamic pressure change, using ACE data. Our results are compared with earlier studies using ground-based data and with theoretical expectations. It turns out that the induction effect in the Earth is quite small at low latitudes. Another important result is that the magnetic signature near the Earth is over-proportionally reduced for weak SC events. A discussion of accuracy and the uncertainty of our results completes the paper.