Cumulative instabilities of anisotropic protons and electrons in the solar wind: New insights from a quasilinear approach

In collision-poor space plasmas the main physical processes are governed by fluctuations and their interactions with plasma particles. An important source of waves and coherent fluctuations are kinetic instabilities driven by, e.g., protons and electrons exhibiting temperature anisotropies. Unfortunately, such instabilities are generally investigated independently of each other, thereby ignoring their interplay and preventing a realistic treatment of their implications. Here we present the first results of an extended quasilinear approach, which not only confirms linear predictions but also unveils new regimes triggered by cumulative effects of the proton and electron instabilities (e.g., electromagnetic cyclotron, firehose). By comparison to individual excitations combined proton- and electron-induced fluctuations grow and saturate at different intensities as well as different temporal scales in the quasilinear phase. Moreover, the enhanced wave fluctuations can markedly stimulate or inhibit the relaxation of temperature anisotropies, this way highly conditioning the evolution and saturation of instabilities.