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Constraining Low-Frequency Alfvenic Turbulence in the Solar Wind Using Density Fluctuation Measurements
Journal article   Open access   Peer reviewed

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

Benjamin D. G Chandran, Eliot Quataert, Gregory G Howes, Qian Xia and Peera Pongkitiwanichakul
The Astrophysical journal, Vol.707(2), pp.1668-1675
08/05/2009
DOI: 10.1088/0004-637X/707/2/1668
url
https://doi.org/10.1088/0004-637X/707/2/1668View
Published (Version of record) Open Access

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.

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