OBSERVATIONAL QUANTIFICATION OF THE ENERGY DISSIPATED BY ALFVéN WAVES IN A POLAR CORONAL HOLE: EVIDENCE THAT WAVES DRIVE THE FAST SOLAR WIND

摘要

We present a measurement of the energy carried and dissipated by Alfvén waves in a polar coronal hole. Alfvén waves have been proposed as the energy source that heats the corona and drives the solar wind. Previous work has shown that line widths decrease with height in coronal holes, which is a signature of wave damping, but have been unable to quantify the energy lost by the waves. This is because line widths depend on both the non-thermal velocity v nt and the ion temperature T i. We have implemented a means to separate the T i and v nt contributions using the observation that at low heights the waves are undamped and the ion temperatures do not change with height. This enables us to determine the amount of energy carried by the waves at low heights, which is proportional to v nt. We find the initial energy flux density present was 6.7 ± 0.7 × 105 erg cm–2 s–1, which is sufficient to heat the coronal hole and accelerate the solar wind during the 2007-2009 solar minimum. Additionally, we find that about 85% of this energy is dissipated below 1.5 R ☉, sufficiently low that thermal conduction can transport the energy throughout the coronal hole, heating it and driving the fast solar wind. The remaining energy is roughly consistent with what models show is needed to provide the extended heating above the sonic point for the fast solar wind. We have also studied T i, which we found to be in the range of 1-2 MK, depending on the ion species.