Ion kinetic energy conservation and magnetic field strength constancy in multi-fluid solar wind Alfv\'enic turbulence

TL;DR

This paper shows that in Alfvénic turbulence of the solar wind, proton kinetic energy is conserved in a specific frame, which explains the observed constancy of magnetic field strength in the spacecraft frame.

Contribution

It introduces a frame of reference based on alpha particles where proton kinetic energy is conserved, linking this to magnetic field constancy in solar wind turbulence.

Findings

Protons conserve total kinetic energy in a frame comoving with fluctuations.

In this frame, plasma velocity magnitude is constant and aligned with magnetic field.

Magnetic field constancy is a signature of energy conservation in the turbulence frame.

Abstract

We investigate properties of the plasma fluid motion in the large amplitude low frequency fluctuations of highly Alfv\'enic fast solar wind. We show that protons locally conserve total kinetic energy when observed from an effective frame of reference comoving with the fluctuations. For typical properties of the fast wind, this frame can be reasonably identified by alpha particles, which, owing to their drift with respect to protons at about the Alfv\'en speed along the magnetic field, do not partake in the fluid low frequency fluctuations. Using their velocity to transform proton velocity into the frame of Alfv\'enic turbulence, we demonstrate that the resulting plasma motion is characterized by a constant absolute value of the velocity, zero electric fields, and aligned velocity and magnetic field vectors as expected for unidirectional Alfv\'enic fluctuations in equilibrium. We propose that this constraint, via the correlation between velocity and magnetic field in Alfv\'enic turbulence, is at the origin of the observed constancy of the magnetic field: while the constant velocity corresponding to constant energy can be only observed in the frame of the fluctuations, the correspondingly constant total magnetic field, invariant for Galilean transformations, remains the observational signature, in the spacecraft frame, of the constant total energy in the Alfv\'en turbulence frame.