The Galileo Earth encounter: Magnetometer and allied measurements
Authors: Kivelson M. G. , Kennel C. F. , McPherron R. L. , Russell C. T. , Southwood D. J. , Walker R. J. , Khurana K. K. , Coleman P. J. , Hammond C. M. , Angelopoulos V. , Lazarus A. J. , Lepping R. P. , Hughes T. J. 
Published: February 2008

The Galileo spacecraft flew by Earth on December 8, 1990, at high speed along a trajectory that traversed the magnetotail and the near Earth magnetosphere. Galileo's orbit through a region of the magnetotail from which limited data are available provided a unique opportunity to study a number of substorm‐related phenomena. Several groups cooperated in collecting correlative data in order to take advantage of this special opportunity. Fortunately, geomagnetic conditions were rather disturbed during the entire day, and an interplanetary shock passed Earth when the spacecraft was in the magnetotail at about 30 RE geocentric distance. In this first report we provide an overview of the Galileo magnetometer observations from the crossing of the tail magnetopause at an antisolar distance of close to 100 RE through exit into the solar wind on the dayside. We link these measurements with correlative data from ground stations and from IMP 8 which was ideally located to serve as a monitor of the solar wind upstream of the bow shock. Based on our analysis, we present a time line of the important geomagnetic events of the day that we believe provides a framework for the full multi‐instrument analysis of the flyby data. In this paper we use the observations to investigate aspects of the relationship between magnetotail dynamics and the separate intensifications of a multiple onset substorm inferred from ground‐based data. The spacecraft spent 6 h downstream of lunar orbit, of which more than 4 h were spent outside of the plasma sheet in regions where traveling compressional regions (TCRs) should have been apparent. Although six substorm intensifications were recorded on the ground during this interval, we did not observe a detectable TCR or plasmoid for every intensification. Our interpretation has important implications for the description of substorm dynamics in the tail. We propose that the signatures associated with individual substorm intensifications are localized in dawn‐to‐dusk extent even at remote locations in the magnetotail, just as they are in the ionosphere, and that the tail disturbances associated with successive substorm intensifications step across the tail towards the dusk flank. This latter interpretation is appealing as it can explain the failure of Galileo to observe a signature associated with each intensification without invalidating the conclusion of ISEE 3 investigators that in the same region of the magnetotail at least one signature can be associated with each substorm viewed as a collection of individual intensifications. Plasmoidlike signatures with strong axial fields along the GSM y axis and parallel to the By of the interplanetary magnetic field (IMF) were present when the spacecraft was embedded close to the center of the plasma sheet. We interpret these signatures as flux ropes, that is, twisted magnetic structures with one end possibly tied to the ionosphere. The modeled structure yields j × B/jB ≪ 1 which suggests that the flux ropes are magnetically force free to within the limitations of the model. We point out that plasmoids and flux ropes form a continuum of structures distinguished by the magnitude of By. Our observations lend additional support to the view that bipolar Bz signatures in the magnetotail may often be better described as flux ropes than as disconnected plasmoids. Our other principal results are only summarized in this paper; they will be discussed in greater detail elsewhere. They include (1) additional evidence that the IMF By controls the lobe magnetic field only in the quadrants that are magnetically linked to the solar wind, and (2) evidence that the low‐frequency response (the classic “sudden impulse” or SI signature) to a solar wind shock can be absent in the magnetic signature obtained within a high β plasma sheet. We believe that these observations will provide insight useful for improving phenomenological models of substorms.