Dispersion Measurements of Electrostatic Ion Waves in a Uniform Magnetic Field

The phase and group velocities of electrostatic ion waves in a uniform magnetic field have been studied experimentally, both as a function of propagation angle $\mathrm{(\theta )}$ with respect to the field, and as a function of wave frequency. For ${\mathrm{\omega \hspace{0.17em}\gg \hspace{0.17em}\omega }}_{\mathrm{ci}}$ , the ion cyclotron frequency, the phase velocity is found to be identical to the ion acoustic velocity ${\mathrm{(KT}}_e{\mathrm{/M)}}^{\text{1/2}}$ . For ${\mathrm{\omega \hspace{0.17em}\gtrsim \hspace{0.17em}\omega }}_{\mathrm{ci}}$ , the electrostatic ion cyclotron mode has been observed. The slow mode ${\mathrm{(\omega \hspace{0.17em}<\hspace{0.17em}\omega }}_{\mathrm{ci}}\mathrm{cos\; \theta )}$ has been observed to have a constant phase velocity ${\mathrm{(KT}}_e{\mathrm{/M)}}^{\text{1/2}}\mathrm{\hspace{0.17em}cos\hspace{0.17em}\theta }$ , in apparent contradiction to theory. The frequency gap ${\mathrm{(ohgr;}}_{\mathrm{ci}}{\mathrm{\hspace{0.17em}cos\hspace{0.17em}\theta \hspace{0.17em}<\hspace{0.17em}\omega \hspace{0.17em}<\hspace{0.17em}\omega }}_{\mathrm{ci}})$ , where waves theoretically do not propagate, has been demonstrated.