Constraining low-frequency Alfvnic 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 low-frequency, oblique, Alfvén-wave turbulence. Because small-scale oblique Alfvén waves (kinetic Alfvén waves, KAWs) are compressive, the measured density fluctuations in the solar wind place an upper limit on the amplitude of KAWs and hence an upper limit on the rate at which the solar wind can be heated by low-frequency, Alfvénic turbulence. We evaluate this upper limit for both coronal holes at 5 R _⊙ and the near-Earth solar wind. At both locations, 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 is the primary mechanism for heating the near-Earth solar wind, KAWs 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 coronal holes are heated by non-turbulent high-frequency waves ("sweeping"), but are compatible with heating by low-frequency Alfvénic turbulence.