Substorm-time magnetic field perturbations in the polar magnetosphere: POLAR observations

We present a case study of substorm-time magnetic field perturbations in the high-altitude polar magnetosphere, corresponding to the polar lobe, using data from the POLAR spacecraft together with ground-based CANOPUS observations and WIND solar-wind observations. The substorm of this paper had a clear growth phase and one clear major expansion onset, enabling an unmistakable comparison of the substorm timings and those of the magnetic field perturbations observed by POLAR. During the growth phase, the magnetic field strength (B_total) increased and the flaring angle of the field line decreased at POLAR: They are ascribed to the pileup of magnetic field lines over the polar magnetosphere and to the braking effect of the ionosphere on the tailward motion of the piled-up field lines, respectively. For ∼28 min after the expansion onset, B_total at POLAR did not decrease toward its pre-substorm value, in contrast to past tail-lobe observations where B_total decreased during the expansion phase. This absence of the field-decrease signature in the polar lobe for ∼28 min, reported for the first time in this paper, could be ascribed to dipolarization/ compression of the inner magnetosphere during the expansion phase, cancelling the field-decreasing effect.