Heating the magnetically open ambient background corona of the Sun by Alfven waves

Observations of UVCS/SOHO of very high emission line widths in the outer corona suggest that the open field regions there are heated by ion-cyclotron resonance absorption of Alfven waves resulting in the ions being much hotter than the electrons. In the lower corona it is usually assumed that the effective Coulomb-interactions ensure equal temperatures of ions and electrons. However, recent models have shown that in regions of strong magnetic field gradients the ion-cyclotron absorption can be so efficient that the ion temperature overcomes the electron temperature. In this paper we will present new observational results from SUMER/SOHO showing that the lines of O V and S VI have very large line widths just above the limb. The peak line width occurs at about 10 0 0 above the limb and corresponds to ion temperatures of more than 3 x 10(6) K. We compare these observational results to new models in which plasma in coronal funnels is heated and accelerated by means of ion-cyclotron absorption of high-frequency waves. As our model is in good qualitative agreement with the observations we come to the conclusion that the open corona in coronal funnels could well be heated by an ion-cyclotron absorption mechanism, even close to the Sun in the low corona.