Solar off-limb line widths with SUMER: revised value of the non-thermal velocity and new results

Abstract

Abstract. 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 and in the upper corona. In the lower corona, one can use the line profiles to infer the ion temperatures. But the value of the so-called "non-thermal" (or "unresolved") velocity, potentially related to the amplitude of Alfvén waves propagating in the corona, is critical in firmly identifying ion-cyclotron preferential heating. In a previous paper, we proposed a method to constrain both the Alfvén wave amplitude and the preferential heating, above a polar coronal hole observed with the SUMER/SOHO spectrometer. Taking into account the effect of instrumental stray light before analysing the line profiles, we ruled out any direct evidence of damping of the Alfvén waves and showed that ions with the lowest charge-to-mass ratios were preferentially heated. We re-analyse these data here to correct the derived non-thermal velocity, and we discuss the consequences on the main results. We also include a measure of the FeVIII 1442.56Å line width (second order), thus extending the charge-to-mass ratio domain towards ions more likely to experience cyclotron resonance.

Spectroscopic Signature of Alfvén Waves Damping in a Polar Coronal Hole Up to 0.4 Solar Radii

“…Moreover, Dolla & Solomon (2008) reported detection of Alfvén waves from the study of the variation of line widths in coronal hole off-limb region by analyzing SUMER/SOHO data, but they claim no evidence for damping of the Alfvén waves because they found that the effect of stray light explained the observed decrease with height in the width of several spectral lines, starting about 0.1-0.2 R ⊙ above the limb. This result was also confirmed from a later study by Dolla & Solomon (2009).…”

A study of line widths and kinetic parameters of ions in the solar corona

“…The present paper, however, do not consider the role of ion-cyclotron heating and assumes that all ions have a common temperature at a given height. Perhaps, it is worthwhile to note that the temperature derived in this paper is in line with that of the ions with high chargeto-mass ratio obtained by Dolla and Solomon (2009, their Figure 5). The derived non-thermal velocities at heights of 57 ′′ and 102 ′′ are also equal within the error bars in both papers.…”

“…According to the study by Dolla and Solomon (2008), it was found that the temperature of Fe X is particularly larger than others. A similar result was also obtained by the authors for Fe VIII in the following paper (Dolla and Solomon 2009). The authors suggested that this result constitutes a clue to ion-cyclotron heating, since the study of ion-cyclotron heating shows that the ions with the lowest charge-to-mass ratios always absorb a major portion of the wave energy and leave nothing to appreciably heat the other ions (e.g., Vocks and Marsch 2002).…”

“…The derived non-thermal velocities at heights of 57 ′′ and 102 ′′ are also equal within the error bars in both papers. For instance, the non-thermal velocity at height of 57 ′′ obtained by Dolla and Solomon (2009) is 20.5 ± 2 km s −1 and this velocity is 21.6 ± 1.5 km s −1 in the present paper. In addition, we should also indicate that, when we keep the data points associated with the Fe X lines in the fit, the value of non-thermal velocity becomes slightly higher, since the intercept of regression line (not shown) is a little larger than that without consideration of the Fe X lines.…”