Group velocity of whistlers in a two‐ion plasma

Computations of the group velocity of whistlers in a two‐ion plasma show that a quasi‐longitudinal (QL) right‐handed polarized wave (electron wave) has a minimum group velocity at the so‐called crossover frequency; a QL left‐handed polarized wave has a maximum group velocity at the same frequency. A quasi‐transverse (QT) electron wave has a minimum group velocity at the ion hybrid frequency. In an oxygen/hydrogen plasma this minimum occurs only for almost perpendicular waves, whereas in a helium/hydrogen plasma it occurs also for waves propagating off perpendicular to the magnetic field. We also find that a small concentration of helium in a hydrogen plasma introduces a ‘stop band’ for quasi‐transverse waves with frequencies just above the helium gyrofrequency. The theoretical results are used to interpret satellite observations of quasi‐transverse low‐frequency whistlers.