The Effect of Ion Flow Shear on Electrostatic Ion-Cyclotron Waves

Summary form only given. Plasma waves near the ion gyrofrequency, propagating nearly perpendicular to the Earth´s magnetic field, have often been observed on satellites in the auroral region. The excitation of these waves has traditionally been attributed to an electrostatic ion-cyclotron (EIC) instability driven by magnetic field-aligned currents. One problem with this scenario has been that often the waves were observed in association with currents that were below the theoretical excitation threshold. Also, while the wave spectrum was expected to be narrowband and structured around a few cyclotron harmonics, a broadband frequency spectrum was often reported. Ganguli and his associates suggested that transverse gradients in the ion flow along the magnetic field (ion flow shear) could provide a mechanism for EIC wave generation, either in addition to the field-aligned currents, or in the absence of field-aligned currents. They showed further, that since the critical shear required to excite EIC waves was approximately independent of the cyclotron wave harmonic number, ion flow shear could generate higher cyclotron harmonics, a common signature of the auroral observations. In the work presented here, the effect of ion flow shear on the excitation of EIC waves was studied experimentally in a double-ended Q-machine. An annular region of ion flow shear was produced by using a ring+disk configuration, as described by Kim et al. Waves with frequencies near the ion gyrofrequency and multiple harmonics were launched from an antenna into a plasma with no field-aligned electron current. As the EIC waves propagated across the magnetic field through a region with ion flow shear, the waves were observed to grow in amplitude. The simultaneous excitation of a multi-harmonic EIC spectrum was also observed when a broadband signal from a white noise source was applied to the antenna.