helium line emission - its relation to atmospheric structure
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1From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
Helium line emission - Its relation to atmospheric structure
V. AndrettaINAF - Osservatorio Astronomico di Capodimonte, Italy
2From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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.
3From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
4From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
Actually, that was not the first paper on the subject...
5From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
6From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
In the beginning...
7From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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.
8From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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)
9From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
A paradigm emerges (some numbers)
10From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
Coronal holes
11From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
Coronal holes
(From spectra-spectroheliograms taken at the Kitt Peak Vacuum Tower Telescope)
12From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
An alternative view
C. Jordan (1975): Mixing of He0, He+ atoms/ions with “hotter” electrons could enhance the observed (EUV) lines through sensitivity to T
e 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 gradient
13From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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”
14From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
From the '80s to the '90s
15From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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...
16From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
A more detailed, complex picture emerging
Pure Photoionization-Recombination [PR] (τ
504~1)
Collisional formation (T>20000 K)
Mixed formation
17From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
The case of the (nearly) optically thin He I 10830 line [Andretta & Jones 1997]
A more detailed, complex picture emerging
(Further quantitative determinations of the role of EUV coronal back-radiation, e.g.: Centeno et al. 2008)
PR contribution
Collisional contribution
18From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
Coronal hole boundaries
19From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
Coronal hole boundaries
20From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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.”)
Model FAL C(QS)
AR (Plage)
QS
21From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
Even more refined models...Figures from FAL 2002 (“Energy balance in the solar
transition region. IV. Hydrogen and helium mass flows with diffusion”)
Hydrogen
Helium
22From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
Related issues...
FIP effect
Fig. 1 from Geiss (1998)
23From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
...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”
Chromosphere
24From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
Interlude: A housewarming party
25From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
26From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
“Helium Line Formation in a Dynamical Solar Atmosphere”, Naples, April 2000
27From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
The last ten years (or so)
28From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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”
29From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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 line alone emits more photons than all the corona below 228 Å.
Note: The CDS radiometric calibration has been recently revised (Del Zanna et al. 2010, Del Zanna & Andretta, 2011), but the above test remains valid, if somewhat less stringent.
30From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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 possibly
31From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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
AR
32From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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.
33From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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.”
34From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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. ...”
35From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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]
36From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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”
37From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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”
38From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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 & D
2
39From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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%
40From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
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%
Old C
DS cal
ibra
tion!
41From Atoms to Stars:The Impact of Spectroscopy on AstrophysicsOxford, UK, 26th July 2011
Conclusions
No conclusion yet... because we don't fully understand the solar atmosphere yet. But: The helium spectrum is an excellent “stress test” for our understanding of the chromosphere and transition region...
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