April 2000 magnetic storm: Solar wind driver and magnetospheric response

Abstract

On 4 April 2000, a coronal mass ejection (CME) took place close to the western limb of the Sun. The shock front of the CME hit the Earth's magnetosphere on 6 April. A strong interplanetary southward BZ event in the sheath region caused a magnetic storm that was the second strongest in the year 2000 if quantified by the peak of the Dst index. We have analyzed this sequence of events using observations of several spacecraft in the solar wind and at geostationary orbit as well as recordings from more than 80 magnetometer stations at latitudes higher than 40°N. In the sheath region behind the shock, the interplanetary magnetic field had an intense and long‐sustained southward magnetic field orientation, and the solar wind magnetic pressure was very large, which compressed the dayside magnetopause inside geostationary orbit for a period of more than 6 hours. We conclude that it was the fluctuating but strongly southward field accompanied by the high pressure that allowed for the exceptionally strong driving of magnetospheric activity. During the main phase of the storm, the magnetosphere and ionosphere were in highly perturbed states, with several activations all around the auroral region. Detailed analysis shows that many of these activations were not substorms, in the sense that they were not associated with poleward and westward electrojet/auroral enhancement or geostationary particle injections, but were directly driven perturbations due to variations in the solar wind features. In fact, it was found that the development of the entire storm was quite independent of substorm activations and injections. Instead, the ring current development was driven by the strong convection enhancements. During the storm, the geomagnetically induced currents were strongly enhanced during several periods. While some activations were associated with substorm onsets or electrojet enhancements, others were caused by extremely localized and short‐lived electrojet activations.