Electromagnetic Proton Beam Instabilities in the Inner Heliosphere: Energy Transfer Rate, Radial Distribution, and Effective Excitation

Differential flows among different ion species are often observed in the solar wind, and such ion differential flows can provide the free energy to drive Alfven/ion-cyclotron and fast-magnetosonic/whistler instabilities. Previous works mainly focused on the ion beam instability under the parameters representative of the solar wind nearby 1 au. In this paper we further study the proton beam instability using the radial models of the magnetic field and plasma parameters in the inner heliosphere. We explore a comprehensive distribution of the proton beam instability as functions of the heliocentric distance and the beam speed. We also perform a detailed analysis of the energy transfer between unstable waves and particles and quantify how much the free energy of the proton beam flows into unstable waves and other kinds of particle species (i.e., proton core, alpha particle and electron). This work clarifies that both parallel and perpendicular electric field are responsible for the excitation of oblique Alfven/ion-cyclotron and oblique fast-magnetosonic/whistler instabilities. Moreover, this work proposes an effective growth length to estimate whether the instability is efficiently excited or not. It shows that the oblique Alfven/ion-cyclotron instability, oblique fast-magnetosonic/whistler instability and oblique Alfven/ion-beam instability can be efficiently driven by proton beams drifting at the speed $\sim 600-1300$ km/s in the solar atmosphere. In particular, oblique Alfven/ion-cyclotron waves driven in the solar atmosphere can be significantly damped therein, leading to the solar corona heating. These results are helpful for understanding the proton beam dynamics in the inner heliosphere and can be verified through in situ satellite measurements.