The effects of beam proportion on electromagnetic proton/proton instability and associated ion heating: 2D hybrid simulation

In this paper, the effects of beam proportion on the electromagnetic proton/proton instability and involved proton dynamics have been investigated with a two-dimensional hybrid simulation model. The beam proportion is defined as the number density ratio between the beam and all protons. The simulation results show that the saturation amplitude of excited oblique Alfvén waves becomes larger with the increasing proportion of beam protons. Besides, the polarization of oblique Alfvén waves generated by beam protons with a higher proportion tends to be more linear. Moreover, with the increase in the beam proportion, the peak frequencies of excited waves will move toward higher values even exceed the proton gyrofrequency. This will result in more efficient heating of core protons in the perpendicular direction through the cyclotron resonance with Alfvén waves. Our study may provide some new insights into the heating mechanism of the solar wind.