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  1. OA@INAF
  2. PRODOTTI RICERCA INAF
  3. 1 CONTRIBUTI IN RIVISTE (Journal articles)
  4. 1.01 Articoli in rivista
Please use this identifier to cite or link to this item: http://hdl.handle.net/20.500.12386/26982
Title: Investigating the behaviour of neutral hydrogen Lyα spectral line width in polar coronal holes at solar minimum
Authors: SPADARO, Daniele 
SUSINO, ROBERTO 
DOLEI, Sergio 
VENTURA, Rita 
Antonucci, E.
Issue Date: 2017
Journal: ASTRONOMY & ASTROPHYSICS 
Number: 603
First Page: A35
Abstract: We investigate the behaviour of the H I Lyα spectral line widths measured by UVCS/SOHO in polar coronal holes at minimum of solar magnetic activity. The line widths are reported to significantly increase up to 3 R☉, while above 3 R☉ there is observational evidence of either nearly constant or slightly decreasing values. We adopt empirical models of polar coronal holes at solar activity minimum reported in the literature and calculate the characteristic timescales relevant to different processes coupling neutral hydrogen atoms and protons, which are heated and accelerated in the outflowing plasma. This analysis leads us to believe that the progressive decoupling of the two sets of particles below 10 R☉, caused by the decrease of the plasma density due to the rapid expansion of the wind, cannot explain the behaviour of the Lyα line profile observed in polar coronal holes. We also synthesise the intensity and profile of the Lyα line as a function of heliocentric distance from the coronal hole models, adopting H I densities computed in non-equilibrium ionisation with the aim of satisfactorily reproducing the UVCS Lyα observations reported in the literature. Our analysis shows that the coronal Lyα emission decreases with heliocentric distance, down to values below the interplanetary Lyα emission, owing to the decrease of the plasma density and to non-equilibrium ionisation effects in the expanding plasma. This can lead to the predominance of the interplanetary emission, which is characterised by H I velocity distributions corresponding to temperatures about one order of magnitude lower than the coronal temperatures, and to the narrowing of the resulting coronal profile at higher heliocentric distances. This scenario can be a plausible explanation for the behaviour of the Lyα line profile with height observed in polar coronal holes at solar activity minimum.
URI: http://hdl.handle.net/20.500.12386/26982
URL: https://www.aanda.org/articles/aa/abs/2017/07/aa29663-16/aa29663-16.html
ISSN: 0004-6361
DOI: 10.1051/0004-6361/201629663
Bibcode ADS: 2017A&A...603A..35S
Fulltext: open
Appears in Collections:1.01 Articoli in rivista

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