Observations of magnetic flux ropes and associated currents in Earth's magnetotail with the Galileo spacecraft

Abstract

On December 8, 1990, during its first flyby of the Earth, Galileo recorded signatures of several flux rope type structures in the Earth's magnetotail. One of the prominent structures had a negative/positive polarity in the Bz component and was moving earthward, whereas another of the prominent structures had a positive/negative polarity in Bz and was observed to be traveling away from the Earth. Both of these flux ropes had strong core fields (≥10 nT) aligned in the positive YGSM direction. We have performed an analysis of combined field and particle data obtained within these flux ropes. We show that the length scale of a flux rope can be determined quite accurately from magnetic data if an independent measure of either the current density or the convection velocity is available. A surprising result that emerges from this analysis is that even though temporal durations (5–10 min) and magnetic field perturbations of the two flux ropes are similar, the scale sizes of the two structures are very different. The earthward moving flux rope with a radius ≤1000 km was traveling very slowly (υx < 10 km/s). The tailward moving flux rope with a radius of ∼8 RE had a speed of ∼150 km/s. Even though the earthward moving flux rope was small in size, the current density in its core was well above that of the tail current sheet and was detected directly by the plasma instrument. We believe that this is the first time that measurements of field aligned current in a flux rope core have been made directly by a plasma instrument. We discuss the details of the distribution functions of the current‐carrying particles. We also speculate on the conditions that led to the vast differences in the scale sizes of the two flux ropes.