Geomagnetic field fluctuations during the passage at the Earth’s orbit of the tail of the 15–16 July 2000 ejecta

Abstract. In this work we present the analysis of the geomagnetic field fluctuations observed at different ground stations (approximately along two latitudinal arrays, separated by several hours in local time) during the passage at the Earth’s orbit of the tail of the 15–16 July 2000 coronal ejecta. The time interval of interest is characterized by northward interplanetary magnetic field conditions and several changes in the solar wind dynamic pressure. We found at all stations, both in the local morning and in the local evening, simultaneous and highly coherent waves at the same discrete frequencies (~ 1.8 and ~ 3.6 mHz) and suggest a possible interpretation in terms of global compressional modes driven by an impulsive variation of the solar wind pressure. Along the array situated in the morning sector, at the highest latitudes, the higher frequency mode seems to couple with the local field line resonance; on the other hand, along the array situated in the evening sector, the characteristics of the observed fluctuations suggest that the highest latitude station could be located at the footprint of open field lines. Our results also show that solar wind pressure variations observed during the recovery phase of the storm do not find correspondence in the geomagnetic field variations, regardless of local time and latitude; conversely, some hours later continuous solar wind pressure variations find a close correspondence in the geomagnetic field variations at all stations.

Key words. Magnetospheric physics (solar wind-magnetosphere interaction; MHD waves and instabilities)