A method of approximate analytical evaluation of ionic off-resonances terms in the dispersion equation for ion-cyclotron waves, appropriate for lower-order harmonics and for Td/Ti<or=10, is presented. The significance of these terms for the quasi-perpendicular electrostatic ion-cyclotron modes and instabilities in the longwave range, in a weakly-ionized Maxwellian plasma with one and two ion species and with electrons drifting along magnetic lines of forces, is demonstrated. It is shown that the inclusion of the off-resonance terms alters the forms of spectra, lowers the threshold drifts and augments the critical wavelengths at which the instabilities set in. Moreover, for some values of Te/Ti of the order of excitation of harmonics is reversed, as the second harmonic is found to have a smaller threshold drift than the first one.
Observations of parametric drift wave instabilities in an electron cyclotron resonance ion source