Velocity‐shear origin of low‐frequency electrostatic ion‐gyroresonant waves

Abstract

Electrostatic ion‐cyclotron waves with spectral features below the ion gyrofrequency and spaced by small fractions of the ion gyrofrequency are reported, a result not possible for waves associated with the well‐known current‐driven electrostatic ion‐cyclotron (CDEIC) instability. This is accomplished by producing a localized, transverse (to the magnetic field B ž ) electric field E ( x ) x ) in a laboratory plasma to excite inhomogeneous energy‐density driven (IEDD) waves and by exploiting the k y ν E dependence of the IEDD mode frequency, where k y and ν E are the components of the IEDD wavevector and plasma drift speed, respectively, along the E × B direction. The IEDD frequency range is shown to reach down to one‐third of the ion gyrofrequency, a range usually reserved for other types of waves, e.g. , ion‐acoustic waves. These results may be relevant to broadband ELF waves observed in the ionosphere.