Velocity Distributions and Proton Beam Production in the Solar Wind

Helios, Ulysses, and Wind spacecraft have observed the velocity distribution functions (VDFs) of solar wind particles deviating significantly from Maxwellians. We review recent models using different approximations and mechanisms that determine various observed characteristics of the VDFs for the electrons, protons and minor ions. A new generation mechanism is proposed for super‐Alfvénic proton beams and tails that are often observed in the fast solar wind. The mechanism is based on the proton trapping and acceleration by kinetic Alfvén waves (KAWs), which carry a field‐aligned potential well propagating with super‐Alfvén velocities.