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

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 (z) over cap) electric field E(x) (x) over cap in a laboratory plasma to excite inhomogeneous energy-density driven (IEDD) waves and by exploiting the k(y)v(E) dependence of the IEDD mode frequency, where k(y) and v(E) are the components of the IEDD wavevector and plasma drift speed, respectively, along the E x 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.