Absence of stopbands in a multicomponent plasma

It is shown that thermal motion and longitudinal magnetic-field nonuniformity lead to the absence of stopbands for Alfven ion-cyclotron waves for a sufficiently low concentration of cold heavy ions. Specifically, it is found that, in the magnetospheric plasma, cold helium ions with a temperature of the order of several eV do not form stopbands if their concentrations are equal to or less than 1 percent.