Abstract
Abstract. 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°. 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° 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.
Highlights
It is generally accepted that the source of a sudden commencement (SC) is a sudden increase in solar wind dynamic pressure causing a compression of the magnetosphere
We have presented a comprehensive study of sudden commencements focussing on low-Earth orbit satellite observations
Our results confirm the assumption that night-time SC signatures are not or only minimally affected by ionospheric currents
Summary
It is generally accepted that the source of a sudden commencement (SC) is a sudden increase in solar wind dynamic pressure causing a compression of the magnetosphere. All the Magsat observations were from the dawn/dusk meridian due to the orbital configuration This is a severe limitation for a general characterisation of SCs. In a recent study Han et al (2007) made use of Ørsted magnetic field data for a direct comparison of satellite observations with ground-based recordings during 23 SC events covering all local times. Recognizing the questions raised above we make use of high-resolution vector magnetic field measurements by CHAMP, a low-altitude satellite and consider ground-based recordings at 1s resolution In this initial study we focus on the low and mid-latitude effects of SCs. Since we want to establish first the comparability of satellite and ground observations, we consider here measurements of 41 SC events on the night side (18:00 h
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