Ionospheric observations of magnetospheric low‐latitude boundary layer waves on August 4, 1991

Abstract

We report on a coordinated set of observations from Greenland magnetometers, Sondrestrom incoherent scatter radar, DMSP F9 and F10 satellite data, and IMP 8 measurements of the interplanetary magnetic field (IMF) and solar wind plasma. These data were acquired during an interval (August 4, 1991, 1100 ‐ 1400 UT) which is characterized by magnetic ULF pulsations observed on the ground which occur in a limited band between 73° and 78° magnetic latitude. The spatial characteristics of the ground ULF magnetic pulsation disturbances are consistent with a continuous train of ionospheric convection vortices moving westward across local magnetic noon sector with a phase velocity of 1.3 km/s. The vortices occur at the ionospheric convection reversal boundary. Defense Meterological Satellite Program (DMSP) drift meter data are consistent with the incoherent scatter convection measurements, and DMSP particle data indicate that the ionospheric reversal boundary maps to the low‐latitude boundary. Using magnetometer stations across Greenland from the east coast, central Greenland, and the west coast, we can determine that individual vortices are long lived, lasting over 15 min. Cross correlations of the pulsation wave forms at stations separated longitudinally by about 500 km produce coefficients between 0.65 and 0.75. Cross correlations between stations separated longitudinally by 1000 km produce coefficients of only 0.39. This indicates that while individual vortices are long lived, they are evolving significantly in shape, latitudinal location, and intensity, as they move distances of 1000 km across the ionosphere. Using a field‐aligned current filament model to fit the data, we find the strength (50 to 100 kA) of these currents to be larger than typical currents previously reported. We find solar wind pressure variations an unlikely source for the generation of these pulsations and suggest that they are the ionospheric signature of either a Kelvin‐Helmholtz mechanism or perhaps magnetic merging during an interval of IMF oriented predominantly in the By direction.