Alfven-wave turbulence and perpendicular ion temperatures in coronal holes


Low-frequency Alfven-wave turbulence causes ion trajectories to become chaotic, or "stochastic," when the turbulence amplitude is sufficiently large. Stochastic orbits enable ions to absorb energy from the turbulence, increasing the perpendicular ion temperature T(perpendicular to i) even when the fluctuation frequencies are too small for a cyclotron resonance to occur. In this paper, an analytic expression for the stochastic heating rate is used in conjunction with an observationally constrained turbulence model to obtain an analytic formula for T(perpendicular to i) as a function of heliocentric distance r, ion mass, and ion charge in coronal holes at 2 R(circle dot) less than or similar to r less than or similar to 15 R(circle dot). The resulting temperature profiles provide a good ;fit to observations of protons and O(+5) ions at 2 R(circle dot) less than or similar to r less than or similar to 3 R(circle dot) from the Ultraviolet Coronagraph Spectrometer (UVCS). Stochastic heating also offers a natural explanation for several detailed features of the UVCS observations, including the preferential and anisotropic heating of minor ions, the rapid radial increase in the O(+5) temperature between 1.6 R(circle dot) and 1.9 R(circle dot), and the abrupt flattening of the O(+5) temperature profile as r increases above 1.9 R(circle dot).

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