Geotail-Polar Observation of Substorm-Time Field Increase in the Tail and the Polar Magnetosphere

We present a case study of substorm-time magnetic field perturbations in high-altitude polar lobe and in the tail lobe, simultaneously observed by the POLAR and GEOTAIL satellites, together with ground-based CANOPUS observations and WIND solar-wind observations. During the growth phase, the magnetic field strength (B_total) increased both in the polar lobe (observed by POLAR) and in the tail lobe (observed by GEOTAIL); this is ascribed to the pileup of dayside-reconnected field lines over the magnetopause. On the other hand, while B_totalat GEOTAIL decreased during the expansion phase, B_totalat POLAR did not start decreasing until ~35 min after the expansion onset. This absence of the field-decrease signature in the polar lobe for ~35 min could be attributed to dipolarization/compression of closed field line region of the nightside inner magnetosphere. This compression could offset the effect of the magnetic field decrease in the polar lobe, and would be absent in the open field line region of the tail lobe allowing GEOTAIL to detect the decrease in B_totalduring this ~35 min period.