Helium line emission - Its relation to atmospheric structure

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


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.



Actually, that was not the first paper on the subject...



In the beginning...


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.


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)


A paradigm emerges (some numbers)


Coronal holes


Coronal holes

(From spectra-spectroheliograms taken at the Kitt Peak Vacuum Tower Telescope)


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


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”


From the '80s to the '90s


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...


A more detailed, complex picture emerging

Pure Photoionization-Recombination [PR] (τ

504~1)

Collisional formation (T>20000 K)

Mixed formation


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


Coronal hole boundaries


Coronal hole boundaries


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


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


Related issues...

FIP effect

Fig. 1 from Geiss (1998)


...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


Interlude: A housewarming party



“Helium Line Formation in a Dynamical Solar Atmosphere”, Naples, April 2000


The last ten years (or so)


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”



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.



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


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


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.


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.”


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. ...”


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]


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”



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”



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



Result: First spectroscopic measurement of AHe

in the

solar chromosphere:

6.5%<AHe

<8.5%



Result: First spectroscopic measurement of AHe

in the

solar chromosphere:

6.5%<AHe

<8.5%

Old C

DS cal

ibra

tion!


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...

Technology

Helium line emission - Its relation to atmospheric structure