Constraining Low-frequency Alfvénic Turbulence in the Solar Wind Using Density Fluctuation Measurements

One proposed mechanism for heating the solar wind, from close to the sun to beyond ∼ 10 AU, invokes lowfrequency, oblique, Alfvén-wave turbulence. Because small-scale oblique Alfvén waves (kinetic Alfvén waves) are compressive, the measured density fluctuations in the solar wind place an upper limit on the amplitude of kinetic Alfvén waves and hence an upper limit on the rate at which the solar wind can be heated by lowfrequency, Alfvénic turbulence. We evaluate this upper limit for both coronal holes at 5R⊙ and in the nearEarth solar wind. At both radii, the upper limit we find is consistent with models in which the solar wind is heated by low-frequency Alfvénic 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 Alfvénic turbulence contributes to heating the near-Earth solar wind, kinetic Alfvén waves must be one of the dominant sources of solar wind density fluctuations at frequencies ∼ 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 Alfvénic turbulence.