Residual energy in magnetohydrodynamic turbulence and in the solar wind

TL;DR

This paper explains why magnetic energy exceeds kinetic energy in solar wind turbulence by using weak turbulence theory to show nonlinear Alfvén wave interactions break equipartition.

Contribution

It introduces a weak turbulence model demonstrating how nonlinear Alfvén wave interactions generate magnetic energy more efficiently, explaining solar wind observations.

Findings

Magnetic energy is more energetic than kinetic energy in the inertial range.

Nonlinear interactions of Alfvén waves break the equipartition.

Magnetic fluctuations have steeper spectra than velocity fluctuations.

Abstract

Recent observations indicate that kinetic and magnetic energies are not in equipartition in the solar wind turbulence. Rather, magnetic fluctuations are more energetic and have somewhat steeper energy spectrum compared to the velocity fluctuations. This leads to the presence of the so-called residual energy E_r=E_v-E_b in the inertial interval of turbulence. This puzzling effect is addressed in the present paper in the framework of weak turbulence theory. Using a simple model of weakly colliding Alfv\'en waves, we demonstrate that the kinetic-magnetic equipartition indeed gets broken as a result of nonlinear interaction of Alfv\'en waves. We establish that magnetic energy is indeed generated more efficiently as a result of these interactions, which proposes an explanation for the solar wind observations.