The Oblique Alfvén Ion Beam Instability in the Earth’s Ion Foreshock


 How ions evolve in the Earth’s ion foreshock is a basic problem in the heliospere community, and the ion beam instability is usually proposed to be one major mechanism affecting the ion dynamics therein. This work will perform the comprehensive analyses of the oblique ion beam instability in the Earth’s ion foreshock. We show that in addition to two well-known parallel instabilities (i.e., the parallel fast-magnettosonic whistler instability and the parallel Alfv´en ion cyclotron instability), the oblique Alfv´en ion beam (OA/IB) instability can be also triggered by free energy relating to the relative drift dV between the solar wind proton and reﬂected proton populations. For slow dV (e.g., dV≤2.2VA , here VA denotes the Alfven speed), it only triggers the OA/IB instability. When dV≥2.2VA , the growth rate in the OA/IB instability can be about 0.6 times of the maximum growth rate in parallel instabilities. Moreover, this work ﬁnds the existence of two type OA/IB instabilities. The ﬁrst one appears at slow dV and in the small wavenumber region at fast dV , and this instability can be described by the cold ﬂuid model. The second one arises in large wavenumber region at fast dV , and this instability only appears in warm plasmas. Furthermore, through the energy transfer rate method, we propose that the OA/IB instability is driven by the competition among the Landau and cyclotron wave-particle interactions of beam protons, the cyclotron wave-particle interaction of core protons, and the Landau wave-particle interactions of electrons. Because oblique waves can experience signiﬁcant damping, the importance of the OA/IB instability may correspond to the effective heating of ions in the Earth’s foreshock.