Constraining Low-Frequency Alfvenic Turbulence in the Solar Wind Using Density Fluctuation Measurements

TL;DR

This paper constrains the role of low-frequency Alfvenic turbulence in solar wind heating by analyzing density fluctuations, finding that such turbulence could plausibly account for observed heating rates near Earth.

Contribution

It provides the first quantitative upper limits on kinetic Alfven wave amplitudes from density fluctuations, supporting low-frequency turbulence as a viable heating mechanism.

Findings

Upper limits on turbulence are consistent with solar wind heating models.

Density fluctuations at 1 AU are compatible with low-frequency Alfvenic turbulence.

Models involving high-frequency waves are likely ruled out by density fluctuation measurements.

Abstract

One proposed mechanism for heating the solar wind, from close to the sun to beyond 10 AU, invokes low-frequency, oblique, Alfven-wave turbulence. Because small-scale oblique Alfven waves (kinetic Alfven waves) are compressive, the measured density fluctuations in the solar wind place an upper limit on the amplitude of kinetic Alfven waves and hence an upper limit on the rate at which the solar wind can be heated by low-frequency, Alfvenic turbulence. We evaluate this upper limit for both coronal holes at 5 solar radii and in the near-Earth solar wind. At both radii, the upper limit we find is consistent with models in which the solar wind is heated by low-frequency Alfvenic turbulence. At 1 AU, the upper limit on the turbulent heating rate derived from the measured density fluctuations is within a factor of 2 of the measured solar wind heating rate. Thus if low-frequency Alfvenic turbulence contributes to heating the near-Earth solar wind, kinetic Alfven waves must be one of the dominant sources of solar wind density fluctuations at frequencies of order 1 Hz. We also present a simple argument for why density fluctuation measurements do appear to rule out models in which the solar wind is heated by non-turbulent high-frequency waves ``sweeping'' through the ion-cyclotron resonance, but are compatible with heating by low-frequency Alfvenic turbulence.