Fluctuation-phase relation between positive and negative ions on pair-plasma electrostatic waves

Three kinds of electrostatic modes are experimentally observed to propagate along magnetic-field lines for the first time in the pair-ion plasma consisting of only positive and negative fullerene ions with an equal mass. It is found that phase lags between the density fluctuations of positive and negative ions vary from 0 to $\pi$ depending on the frequency and is fixed at $\pi$ in the cases of ion acoustic and ion plasma waves, respectively. In addition, a new mode with the phase lag about $\pi$ appears in an intermediate-frequency band between the acoustic and plasma waves.