Field line resonant pulsations associated with a strong dayside ionospheric shear convection flow reversal

Abstract

We discuss a set of coordinated observations from the arrays of Greenland ground magnetometers, Sondrestrom incoherent scatter radar, and the DMSP, GOES 7, and IMP 8 satellites during 1000–1400 UT on August 4, 1991. The work presented here follows work presented by Clauer and Ridley [1995]. In the previous work we show that this particular interval is characterized by a large positive interplanetary magnetic field (IMF) By and near‐zero IMF Bz components. Associated with these conditions is a very strong ionospheric convection shear reversal boundary in the dayside noon and prenoon sector in the northern hemisphere. The convection reversal boundary is observed to be very dynamic, showing wave‐like displacements of several degrees in invariant latitude with a tailward phase propagation. These variations are associated with magnetic pulsations with a period of about 34 min. We have suggested that these waves are produced by a Kelvin‐Helmholtz instability at the shear convection reversal boundary. We present here a more detailed analysis of the magnetic variations from the Greenland arrays of magnetometers. Our new findings show that equatorward of the convection reversal boundary, there is power in the pulsation spectra in bands with periods of about 34, 17, 12, and 8 min. Examination of the northward component of the pulsations along a meridional chain of stations ranging from 76.23° to 66.86° invariant latitude shows high coherence between stations and a change in relative phase of about 160° over the observed latitude range for the 8‐min. pulsations. These observations are consistent with the expected signature of a field line resonance. We suggest that the source of the resonance is the disturbance generated at the shear convection reversal boundary which then produces resonances on nearby, equatorward closed field lines. In this case we suggest that the source is the Kelvin‐Helmholtz wave established at the east‐west convection shear which develops near local noon and prenoon regions during large IMF By positive conditions. This is possibly the first observation of the field line resonance associated with such a source.