Heating and Acceleration of the Solar Wind via Gravity Damping of Alfven Waves

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Abstract
In this paper we present a two-fluid model for the heating of the solar corona and acceleration of the solar wind, based on the dissipation of Alfvén waves by gravity damping. This mechanism was proposed by Khabibrakhmanov & Mullan but has not previously been applied in modeling efforts. After extending the Khabibrakhmanov & Mullan theory to give an expression for the evolution of the Alfvén wave amplitude as a function of the local parameters of the atmosphere, we show how gravity damping compares with other mechanisms that have been proposed for the dissipation of Alfvén waves. Then we introduce the system of equations that we use for the wind model: this includes, in the energy equation, a gravity dissipation term and, in the momentum equation, a different wave acceleration term from that which is usually adopted. Initial conditions for the integration of the equations are compatible with recent Ulysses measurements, and the integration proceeds from 1 AU toward the base of the solar corona and into the transition region [where T=(1-2)×105 K]. Our results show that the gravity damping of Alfvén waves heats protons in the solar plasma to several million degrees and accelerates the solar wind to 600-700 km s-1. Model predictions at low heliocentric distances compare favorably with recently acquired data. One prediction of our model is that the damping process is most effective in regions where the Alfvén speed is low. Another prediction is that although the energy is deposited mainly into protons, the deposition occurs close enough to the Sun that collisional coupling also leads to effective heating of the electrons (to Te≈106 K). We compare and contrast the present model with models based on ion-cyclotron resonant processes.