PLASMA-FIELD COUPLING at SMALL LENGTH SCALES in SOLAR WIND NEAR 1 au

In collisionless plasmas such as the solar wind, the coupling between plasma constituents and the embedded magnetic field occurs on various temporal and spatial scales, and is primarily responsible for the transfer of energy between waves and particles. Recently, it was shown that the transfer of energy between solar wind plasma particles and waves is governed by a new and unique relationship: the ratio between the magnetosonic energy and the plasma frequency is constant, E _ms/ω _pl ∼ ℏ_∗. This paper examines the variability and substantial departure of this ratio from ℏ_∗ observed at ∼1 au, which is caused by a dispersion of fast magnetosonic (FMS) waves. In contrast to the efficiently transferred energy in the fast solar wind, the lower efficiency of the slow solar wind can be caused by this dispersion, whose relation and characteristics are derived and studied. In summary, we show that (i) the ratio E _ms/ω _pl transitions continuously from the slow to the fast solar wind, tending toward the constant ℏ_∗; (ii) the transition is more efficient for larger thermal, Alfvén, or FMS speeds; (iii) the fast solar wind is almost dispersionless, characterized by quasi-constant values of the FMS speed, while the slow wind is subject to dispersion that is less effective for larger wind or magnetosonic speeds; and (iv) the constant ℏ_∗ is estimated with the best known precision, ℏ_∗ ≈ (1.160 ± 0.083) × 10^-22 Js.