EDP Sciences
Free access
Volume 483, Number 1, May III 2008
Page(s) 271 - 283
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20077903
Published online 19 March 2008

A&A 483, 271-283 (2008)
DOI: 10.1051/0004-6361:20077903

Solar off-limb line widths: Alfvén waves, ion-cyclotron waves, and preferential heating

L. Dolla1 and J. Solomon1, 2

1  Institut d'Astrophysique Spatiale, UMR8617, Université Paris-Sud, 91405 Orsay, France
    e-mail: [laurent.dolla;jacques.solomon]@ias.u-psud.fr
2  CNRS, Orsay 91405, France

(Received 16 May 2007 / Accepted 18 January 2008)

Context. Alfvén waves and ion-cyclotron absorption of high-frequency waves are frequently brought into models devoted to coronal heating and fast solar-wind acceleration. Signatures of ion-cyclotron resonance have already been observed in situ in the solar wind (HELIOS spacecrafts) and, recently, in the upper corona (UVCS/SOHO remote-sensing results).
Aims. We propose a method to constrain both the Alfvén wave amplitude and the preferential heating induced by ion-cyclotron resonance, above a partially developed polar coronal hole observed with the SUMER/SOHO spectrometer.
Methods. The instrumental stray light contribution is first substracted from the spectra. By supposing that the non-thermal velocity is related to the Alfvén wave amplitude, it is constrained through a density diagnostic and the gradient of the width of the $\ion{Mg}{x}$ 625 Å line. The temperatures of several coronal ions, as functions of the distance above the limb, are then determined by substracting the non-thermal component to the observed line widths.
Results. The effect of stray light explains the apparent decrease with height in the width of several spectral lines, this decrease usually starting about 0.1-0.2 $R_\odot$ above the limb. This result rules out any direct evidence of damping of the Alfvén waves, often suggested by other authors. We also find that the ions with the smallest charge-to-mass ratios are the hottest ones at a fixed altitude and that they are subject to a stronger heating, as compared to the others, between 57´´ and 102´´ above the limb. This constitutes a serious clue to ion-cyclotron preferential heating.

Key words: Sun: corona -- Sun: UV radiation -- line: profiles

© ESO 2008