The Effect of Magnetic Turbulence Energy Spectra and Pickup Ions on the Heating of the Solar Wind

TL;DR

This study compares Kolmogorov and Iroshnikov-Kraichnan turbulence models in solar wind heating, highlighting how pickup ions influence turbulence and temperature predictions at large distances.

Contribution

It demonstrates that the IK turbulence spectrum can match observations as well as Kolmogorov, with less impact from pickup ions, providing insights into turbulence spectra effects on solar wind heating.

Findings

IK cascade rate produces good agreement with observations.

Pickup ions have less effect in the IK model due to shallower spectrum.

Shallower spectrum generates fewer waves and turbulence.

Abstract

In recent years, a phenomenological solar wind heating model based on a turbulent energy cascade prescribed by the Kolmogorov theory has produced reasonably good agreement with observations on proton temperatures out to distances around 70 AU, provided the effect of turbulence generation due to pickup ions is included in the model. In a recent study [Ng et al., J. Geophys. Res., 115, A02101 (2010)], we have incorporated in the heating model the energy cascade rate based on Iroshnikov-Kraichnan (IK) scaling. We showed that the IK cascade rate can also produce good agreement with observations, with or without the inclusion of pickup ions. This effect was confirmed both by integrating the model using average boundary conditions at 1 AU, and by applying a method [Smith et al., Astrophys. J., 638, 508 (2006)] that uses directly observed values as boundary conditions. The effects due to pickup ions is found to be less important for the IK spectrum, which is shallower than the Kolmogorov spectrum. In this paper, we will present calculations of the pickup ions effect in more details, and discuss the physical reason why a shallower spectrum generates less waves and turbulence.