Abstract
In this paper, weak turbulence theory is used to investigate the nonlinear evolution of the parametric instability in 3D low-β plasmas at wavelengths much greater than the ion inertial length under the assumption that slow magnetosonic waves are strongly damped. It is shown analytically that the parametric instability leads to an inverse cascade of Alfvén wave quanta, and several exact solutions to the wave kinetic equations are presented. The main results of the paper concern the parametric decay of Alfvén waves that initially satisfy e+ ≫ e-, where e+ and e- are the frequency (f) spectra of Alfvén waves propagating in opposite directions along the magnetic field lines. If e+ initially has a peak frequency f0 (at which fe+ is maximized) and an "infrared" scaling fp at smaller f with -1 < p < 1, then e+ acquires an f-1 scaling throughout a range of frequencies that spreads out in both directions from f0. At the same time, e- acquires an f-2 scaling within this same frequency range. If the plasma parameters and infrared e+ spectrum are chosen to match conditions in the fast solar wind at a heliocentric distance of 0.3 astronomical units (AU), then the nonlinear evolution of the parametric instability leads to an e+ spectrum that matches fast-wind measurements from the Helios spacecraft at 0.3 AU, including the observed f-1 scaling at f ≳ 3 × 10-4 Hz. The results of this paper suggest that the f-1 spectrum seen by Helios in the fast solar wind at f ≳ 3 × 10-4 Hz is produced in situ by parametric decay and that the f-1 range of e+ extends over an increasingly narrow range of frequencies as r decreases below 0.3 AU. This prediction will be tested by measurements from the Parker Solar Probe.
Highlights
The origin of the solar wind is a long-standing problem (Parker 1958) that continues to receive considerable attention
Exact solutions extending over all kz In addition to the truncated solutions described in §§ 5.1 and 5.2, equation (2.11) possesses several exact solutions that extend over all kz. These solutions are unphysical, because they correspond to infinite Alfvén waves (AWs) energy and neglect dissipation and finite system size
In this paper, weak-turbulence theory is used to investigate the nonlinear evolution of the parametric instability in low-β plasmas
Summary
The origin of the solar wind is a long-standing problem (Parker 1958) that continues to receive considerable attention. These solutions are unphysical, because they correspond to infinite AW energy and neglect dissipation (which becomes important at sufficiently large kz) and finite system size (which becomes important at sufficiently small kz) They illustrate several features of the nonlinear evolution of the parametric instability, which are summarized at the end of this section. The exact solutions presented illustrate three properties of the nonlinear evolution of the parametric instability at low β when slow waves are strongly damped. 6. Nonlinear evolution of the parametric instability when most of the AWS initially propagate in the same direction This section describes a numerical solution to (2.11) in which, initially, E+ E−.
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