Helium lines in solar
prominences
Nicolas Labrosse
University of Glasgow, Scotland
Collaborators:
Pierre Gouttebroze, Jean-Claude Vial: Institut d'Astrophysique Spatiale
Petr Heinzel: Ondrejov Observatory
Brigitte Schmieder: Observatoire de Paris
Plasma parameters
Temperature, density, ionisation, filling factor, ...
Accurate measurements are
crucial to construct realistic models of prominences
difficult to obtain
prominence plasma not in local thermodynamical equilibrium
(non-LTE) because of strong incident radiation coming from
the Sun
Large span of measured values
depending on the observed structure
depending on the technique used
Non-LTE radiative transfer modelling of prominence
plasma
sheds light on line formation mechanisms
helps to interpret spectroscopic observations / imaging
See forthcoming review paper in Space Science Reviews on Spectral Diagnostics and
Non-LTE Modelling
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A prominence
The
prominence model
model
•1D plane-parallel vertical slab
Free parameters
Gas pressure
Temperature
Column mass
Height above the limb
Radial velocity
Equations to solve
Pressure equilibrium, ionisation
and statistical equilibria (SE),
radiative transfer (RT) for H (20
levels) – Gouttebroze et al 1993
SE, RT for other elements: He I
(29 levels) + He II (4 levels) –
Labrosse & Gouttebroze 2001
Anzer & Heinzel (1999)
Prominence-corona transition region
(PCTR)
Temperature inside the prominence slab for γ=2 (extended PCTR), γ=10, and
γ=20 (narrow PCTR). The column mass is M = 5×10−6 g cm−2 and the central
temperature is 9000 K.
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Hα vs He II 304
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Influence of PCTR on line profiles
H Lyman α
He I 584 Å
model without
transition region
models with
transition region
Labrosse et al (2002)
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Prominence diagnostic with SUMER
BBSO Hα
MEDOC campaign #13,
15–16/6/2004
Observed profiles compared
with grid of 4720 computed
models (T, n, ...)
⇩
Ly-β, Ly-ε, and
He I 584 Å
observed by
SUMER/SOHO
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Prominence diagnostic with SUMER
●
Prominence model: 1D plane-parallel slab
ne = 6 108 cm-3 (surface)
ne = 5 109 cm-3 (center)
Labrosse, Vial, & Gouttebroze (2006)
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Conclusions
Importance of taking into account PCTR
–Already shown by Heinzel et al (2001) for Lyman lines
–Also true for He I triplet lines
Calculations provide constraints for determination of
–Opacities
–Ionisation degree
– Variations
in ionisation degree along LOS can be important
–Radiative losses for energy balance calculations
Models must be constrained by comparing with
several lines (H+He)
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Effects
Effects of
of radial
radial motions
motions
•For a simple 2-level atom with photo-excitation
–Doppler dimming if the incident line is in emission
–Doppler brightening if the incident line is in absorption
•If coupling between several atomic levels
–situation gets more complex: dimming and brightening
–e.g. coupling between first two excited levels of H
•Main factors determining effects of radial
motions
–line formation mechanism
–details of incident radiation (strength, emission/absorption)
See Heinzel & Rompolt (1987), Gontikakis et al (1997), Labrosse et al (2007)
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V=0 km s-1
V=80 km s-1
T = 8000 K
T = 15000 K
V=200 km s-1
V=400 km s-1
He I 584
He II 304
He I 10830
Labrosse et al. (2007)
Plasma motions in prominences
●
He II 304 Å line sensitive to Doppler dimming due to
radial motion of prominence plasma
Labrosse et al. (2007)
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Results
(5)
Results
Effects on Helium
resonance lines
(Same trend as
H Lyman lines)
Doppler dimming
Cool
plasma
Not too dense
Large temperature
gradient in PCTR
Effects on Helium subordinate lines
10830, D3, ... are less sensitive to Doppler dimming/brightening
due to weak incident radiation
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(erg s1 cm-2 sr-1 Å-1 )
E(He II 304) vs. radial velocity
Labrosse et al (2008)
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DiagnosticHe
of velocity
I model fields
atom
●
Imaging measurements
–
●
Doppler shifts in prominence spectra
–
●
apparent motion of structure in plane-of-sky
velocity along line-of-sight
Doppler dimming / brightening
–
varies with radial velocity
The full velocity vector may be inferred, but
requires at least the radial velocity.
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2D cylindrical threads
ionization ratios
temperature
H
He I
He II
electron-tohydrogen ratio
H and He
ionization
neutral He
Variations of temperature T and population ratios with
the distance to the axis (r) at the foot of the loop
Gouttebroze & Labrosse (2009)
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Ionization ratio of Helium
50000 K
30000 K
65000 K
80000 K
20000 K
15000 K
6000 K
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He I 10830
He I 584
He I 5876
He II 304
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Integrated intensities
He I 10830
He II 304
He I 5876
He I 584
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Future plans
•Model filament eruptions
•Test different models of filaments and prominences
giving the thermodynamical parameters
–take into account variation of plasma thermodynamical
parameters during heating and eruption
•Compare computations with observations
–integrated intensities or full line profiles
• development of a prominence catalogue to facilitate
statistical studies with large samples
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Diagnostics with Hinode/EIS
Prominence observed on 25 and 26 April 2007
EIS: fil_rast_s2 study
2’’
slit makes a raster of 240’’x256’’
10
narrow spectral windows and
1 large spectral window
SOT Hα and Ca II K
TRACE
SOHO: CDS, SUMER, EIT
Ground-based observatories
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SOT /NB Hα
A
B
D
E
C
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EIS rasters
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EIS observation of prominence
Prominence seen in emission
He II 256.32 (log T=4.7)
What
exactly do we see?
Fe VIII 185.21, Fe VIII 186.60, Si VII 275.35 (log T=5.8)
Prominence
to corona transition region
Prominence seen in absorption
Fe XII 195.21
Expected:
Absorption from H and He of background coronal radiation in H and
He resonance continua + emissivity blocking
Mg VI 270.40
Unexpected
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Line profiles
Mg VI
270.40
Fe XIV 270.51
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He II 256 profiles in region A
Si X 256.37
Fe XII 256.41
Fe XIII 256.42
He II 256.32
He II 256.32 is not the only contributor
to the total integrated emission!
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Conclusions
EIS opens new window for solar prominence diagnostics
Different
parts of the prominence are probed with lines formed between
log T=4.7 and log T=6.3
Detailed
investigation of line profiles necessary to interpret raster images
– Dark
absorption features in Mg VI 270.40 explained by contribution from Fe XIV
270.51
– He
II 256.32 line contributes 20-60% of total integrated emission in this window
– Preliminary
comparisons with non-LTE radiative transfer calculations reveal that ~
50% of He II line emission comes from scattering of incident radiation
Cool parts (log T~4) of prominences emit in He II as well!
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