Ion flow driven by low frequency Alfvén waves in a low-beta plasma

Abstract

In a low-beta plasma, the ion flow in the parallel direction along the background magnetic field is investigated when ions are accelerated by low-frequency polarized Alfvén waves with the finite amplitude propagating along the magnetic field due to nonresonant interactions. The results indicate that the magnitude of the ion flow is closely related to not only the amplitude but also the frequency and the polarization of the wave, which is verified by a test particle simulation. The ion flows driven by the nondispersive and dispersive Alfvén waves are proportional to vAα21±ε2 and vAα21±ε3/2, respectively, where α is the ratio of the magnetic field component of the Alfvén wave to the background magnetic field, ε is the ratio of the wave frequency to the ion cyclotron frequency, and the positive sign corresponds to the right-handed Alfvén wave and the negative sign corresponds to the left-handed Alfvén wave, respectively. For a wave with finite frequency, the ion flow is different from different polarized waves in low beta plasmas. In particular, the saturation value of ion flow has a minimum threshold for the left-handed polarized Alfvén wave and a maximum threshold for the right-handed polarized Alfvén wave. If the frequency is less than 1/10 of the ion cyclotron frequency, the Alfvén wave can be seen as propagating in nondispersive medium. When the frequency of the Alfvén wave is far less than the frequency of the ion cyclotron, the ion flows driven by the left-handed and right-handed polarized Alfvén waves with and without wave dispersion tend to be the same.