Pressure anisotropy can strongly influence the dynamics of weakly collisional, high-beta plasmas, but its effects are missed by standard magnetohydrodynamics (MHD). Small changes to magnetic-field strength generate large pressure-anisotropy forces, heating plasma, driving instabilities and rearranging flows, even on scales far above particles’ gyroscales where kinetic traditionally considered m...

Cosmic rays scatter off magnetic irregularities (Alfvén waves) with which they are resonant, that is waves of wavelength comparable to their gyroradii. These waves may be generated either by the cosmic rays themselves, if they stream faster than the Alfvén speed, or by sources of MHD turbulence. Waves excited by streaming cosmic rays are ideally shaped for scattering, whereas the scattering eff...

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