Helium line emission - Its relation to atmospheric structure

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Invited talk presented by V. Andretta at the symposium From Atoms to Stars:the impact of Spectroscopy on Astrophysics, 26th-28th July 2011, Oxford, UK.

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Helium line emission - Its relation to atmospheric structure

  1. 1. Helium line emission - Its relation to atmospheric structure V. Andretta INAF - Osservatorio Astronomico di Capodimonte, ItalyOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 1
  2. 2. Helium line emission - Its relation to atmospheric structure Abstract: The title of this talk reprises the title of a 1980 paper by Carole Jordan on the anomalously high intensities of helium lines when compared with lines of other ions formed at similar temperatures. From that starting point, I will give a historical overview of an apparently marginal riddle in Solar Physics, a riddle nonetheless that has intrigued many in the course of several decades, inspiring some interesting ideas on the structure and dynamics of the solar atmosphere.Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 2
  3. 3. Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 3
  4. 4. Actually, that was not the first paper on the subject...Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 4
  5. 5. Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 5
  6. 6. In the beginning...Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 6
  7. 7. Early studies Goldberg (1939) postulated “...an excess of ultraviolet radiation in the 500 A region;...” to explain the 1932 eclipse observation of the helium spectrum Hirayama (1971) analyzed prominence spectra taken during the 1966 eclipse, concluding that “...the intensity of neutral helium can be explained in terms of ionization due to UV radiation even if the kinetic temperature is as low as 5000 K.” On the other hand, calculations based on “standard” collisional excitation at temperatures T>20000 K (e.g.: Milkey, Heasley, Beebe 1973) typically failed to reproduce both He and other TR lines.Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 7
  8. 8. A paradigm emerges Zirin (1975, “The helium chromosphere, coronal holes, and stellar X-rays”, ApJ 199, L63) : “...We demonstrate how the D3 [He I 5876 A] emission, as well as the other He I and He II lines, can be explained quantitatively by photoionization by coronal back-radiation. A Chapman layer with N(He)H=5x1017 [cm-2] is formed near τ=1 in the He I and He II continua. The chromospheric He emission or absorption is weak in coronal holes because there is no coronal back-radiation...” (Note: The relevant photoionization thresholds are at 504 Å for He I, and 228 Å for He II)Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 8
  9. 9. A paradigm emerges (some numbers)Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 9
  10. 10. Coronal holesOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 10
  11. 11. Coronal holes (From spectra- spectroheliograms taken at the Kitt Peak Vacuum Tower Telescope)Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 11
  12. 12. An alternative view C. Jordan (1975): Mixing of He0, He+ atoms/ions with “hotter” electrons could enhance the observed (EUV) lines through sensitivity to Te of the excitation rates (~exp(-ΔE/kTe)). C. Jordan (1980) provided the list of ingredients for the “mixing” recipe (in a certain class of processes), mainly: – Non-thermal motions – Long ionization/recombination times – Temperature gradientOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 12
  13. 13. An alternative view From the 1980 paper: “...One can think either of the non-thermal motions carrying the ions up the steep temperature gradient or of an intermittent penetration down of hotter electrons. ...” An intrinsically “dynamical view”Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 13
  14. 14. From the 80s to the 90sOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 14
  15. 15. Observations, and more observations... Daily synoptic observations of magnetic field, He I 10830 from Kitt Peak (spectra and images): 1992- 2003 (then SOLIS, 1993 to the present). Rocket flights (e.g.: SERTS): EUV line profiles. SOHO: SUMER, CDS, EIT...Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 15
  16. 16. A more detailed, complex picture emerging Pure Photoionization- Collisional formation Recombination [PR] (τ504~1) (T>20000 K) Mixed formationOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 16
  17. 17. A more detailed, complex picture emerging The case of the (nearly) optically thin He I 10830 line [Andretta & Jones 1997] PR contribution Collisional contribution (Further quantitative determinations of the role of EUV coronal back-radiation, e.g.: Centeno et al. 2008)Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 17
  18. 18. Coronal hole boundariesOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 18
  19. 19. Coronal hole boundariesOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 19
  20. 20. More models... The FAL (Fontenla, Avrett, and Loeser) series of papers: diffusion effects (ambipolar diffusion). Figures from FAL 1993 (“Energy balance in the solar transition region. III. Helium emission in hydrostatic, constant-abundance models with diffusion.”) AR (Plage) Model FAL C (QS) QSOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 20
  21. 21. Even more refined models... Figures from FAL 2002 (“Energy balance in the solar transition region. IV. Hydrogen and helium mass flows with diffusion”) Helium HydrogenOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 21
  22. 22. Related issues... FIP effect Fig. 1 from Geiss (1998)Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 22
  23. 23. ...and more models... Multifluid models, and the solar wind helium abundance: Hansteen, Leer, and Holtzer. (1997): “The role of helium in the solar outer atmosphere” Killie, Lie-Svendsen, and Leer (2005): “The helium abundance of quiescent coronal loops” ChromosphereOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 23
  24. 24. Interlude: A housewarming partyOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 24
  25. 25. Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 25
  26. 26. “Helium Line Formation in a Dynamical Solar Atmosphere”, Naples, April 2000Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 26
  27. 27. The last ten years (or so)Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 27
  28. 28. Quantifying the extent of the problem How enhanced are EUV He lines with respect to rest of the TR spectrum? Macpherson & Jordan 1999: “The anomalous intensities of helium lines in the quiet solar transition region” Jordan et al. 2001: “The anomalous intensities of helium lines in a coronal hole”Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 28
  29. 29. Quantifying the extent of the problem Testing the PR mechanism: Andretta, Del Zanna, S. Jordan (2003): “The EUV helium spectrum in the quiet Sun: A by-product of coronal emission?” Result: the He II 304 Å Note: The CDS radiometric line alone emits more calibration has been recently photons than all the revised (Del Zanna et al. 2010, Del Zanna & Andretta, 2011), but corona below 228 Å. the above test remains valid, if somewhat less stringent.Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 29
  30. 30. Quantifying the extent of the problem Reduced network contrast in the He lines: photon scattering in the cell centers from the boundaries: Jordan, Smith, and Houdebine (2005), MNRAS 362, 411: “Photon scattering in the solar ultraviolet lines of He I and He II” Result: quite possiblyOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 30
  31. 31. A slightly more detailed version of the mechanism proposed in the1980 paper [Andretta et al. 2000] The relevant scaling parameter (the so-called “velocity redistribution parameter”): He II 304 Å O III 600 Å QS AROxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 31
  32. 32. Yet another generalization From G. R. Smith and C. Jordan, 2002, “Enhancement of the helium resonance lines in the solar atmosphere by suprathermal electron excitation – I. Non-thermal transport of helium ions”: In this formulation, though, the distinction between quiet Sun and coronal holes is somewhat lost/hidden.Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 32
  33. 33. The other mixing mechanism From G. R. Smith, 2003, “Enhancement of the helium resonance lines in the solar atmosphere by suprathermal electron excitation – II. Non- Maxwellian electron distributions”: “...Enhancements of the helium resonance line intensities are found, but many of the predictions of the models regarding line ratios are inconsistent with observations. These results suggest that any such departures from Maxwellian electron distributions are not responsible for the helium resonance line intensities.”Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 33
  34. 34. More ways to mix helium and “hot” electrons Pietarila & Judge (2004): “On the formation of the resonance lines of helium in the Sun” Judge & Pietarila (2004): “On the formation of the resonance lines of helium in the Sun: Analysis of SOHO data”: “...We propose a new enhancement mechanism [...] in which predominantly neutral species such as helium diffuse across magnetic field lines into regions of hot coronal plasma, but charged ions do not. ...”Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 34
  35. 35. More ways to mix helium and “hot” electrons Feldman, Ralchenko, and Doschek (2010): “The effect of hot coronal electrons on EUV spectral lines of He II emitted by solar TR plasmas”: “...We show that although the influence of a fraction as small as 10-4-10-3 of hot electrons on the intensities of the C and O lines is noticeable, the effect on the intensities of the He lines is much larger...” [Note: “hot”, coronal electrons are modelled here by a second Maxwellian]Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 35
  36. 36. Where the formation of the helium spectrum is not a (big) problem Prominces/filaments, for instance. Examples: Labrosse & Gouttebroze (2001), A&A 380, 323: “Formation of helium spectrum in solar quiescent prominences” Labrosse et al., (2010), Space Sci. Rev. 151, 243: “Physics of Solar Prominences: I – Spectra Diagnostics and Non-LTE Modelling”Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 36
  37. 37. Where the formation of the helium spectrum is not a (big) problem Active regions? Maybe so, if the enhancement mechanism is “velocity redistribution”: lower “turbulent” velocities, higher pressures than in the QS. Example: Andretta et al. (2008), ApJ 681, 650: “Helium line formation and abundance during a C- class flare”Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 37
  38. 38. Where the formation of the helium spectrum is not a (big) problem Data set includes: – From SOHO/CDS: • He I 584 Å and He II 304 Å (2nd order) • Various TR and coronal lines – From the Horizontal Spectrograph at the NSO/DST (Dunn Solar Telescope) at Sacramento Peak: • He I 5876 Å and He I 10830 Å • Ca II H &K • Hα • Na D1 & D2Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 38
  39. 39. Where the formation of the helium spectrum is not a (big) problem Result: First spectroscopic measurement of AHe in the solar chromosphere: 6.5%<AHe<8.5%Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 39
  40. 40. Where the formation of the helium spectrum is not a (big) problem Result: First spectroscopic measurement of AHe in the n! io solar chromosphere: t ra lib ca 6.5%<AHe<8.5% S D C ldOxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 40 O
  41. 41. Conclusions No conclusion yet... because we dont fully understand the solar atmosphere yet. But: The helium spectrum is an excellent “stress test” for our understanding of the chromosphere and transition region...Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 41

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