Spatial profiles of magnetic field in the near-Earth plasma sheet during substorm growth phase: THEMIS five-point analysis

We present five-point measurements of the near-Earth plasma sheet to assess a geometrical structure during substorm growth phase. During two intervals of dipolarization events on 8 April 2009 and 12 April 2009, all five THEMIS probes were located at a radial distance of 10 Re in the premidnight sector. In the magnetotail under the highly stretched configuration, which is expected during growth phase and was observed, the difference in the magnetic field from different probes are mostly attributed to a latitudinal (north-south direction) profile. It is found that the value of Bz increased with a distance away from the magnetic equatorial plane, which differs substantially from a conventional simple magnetic field model. Instead, the observation can be explained by a magnetic field model that is not represented by a monotonically decreasing function of a distance but has a minimum in the near-Earth tail, as required from the force-balanced magnetotail models. In order to see this, we performed an analysis using equations for two dimensional magnetic field; the field lines are linearly extrapolated to the magnetic equator to estimate a gradient of the radial profile of the magnetic field from the observed latitudinal profile. Our analysis shows that the equatorial magnetic field in the inner magnetotail exhibited a broad minimum during extended intervals prior to the diopolarization onset (20 and 30 min for 8 and 12 April 2009 events, respectively).