formation of helium lines in prominences

Formation of Helium lines in solar prominences

Nicolas LabrosseUniversity of Glasgow, Scotland, UK

22 November 2011 Presentation to Solar Physics Group at Purple Mountain Observatory

Outline

•Introduction on solar prominences

•Radiative transfer modelling

–Description of the models

–Influence of the prominence-to-corona transition region (PCTR) on line profiles and intensities

–Influence of the radial motions of the plasma on line profiles and intensities

•Conclusions


SOHO/EIT

coronaT≥1-2 MKn~108 cm-3

prominencebody

T~8000 Kn~1010 cm-3

Solar prominences


•How do prominences form?

–What is the magnetic configuration of filament channels, and how is this highly sheared structure created?

–Where does their dense material originate, and how is it maintained?

–How do prominences reach and maintain energy balance with the ambient corona?

–How are the magnetic structure and the plasma dynamics linked?

Puzzles

Labrosse et al. (2010), Mackay et al. (2010)


•Prominence fine structure and diagnostics

–What are their detailed thermal and magnetic structures?

–How can we use existing (SOHO, Hinode, STEREO, SDO) and future (Solar Orbiter) space missions to obtain the best information on solar prominences?

–Can we construct a prominence model that reproduces the observed emission in optically thin and optically thick lines?

Puzzles



•Prominence disappearance

–What can observations of heating and activation in prominences tell us about their disappearance?

–Why do filament channels generate the most energetic solar eruptions?

–What tools can we develop to forecast prominence eruptions in a reliable way?

Puzzles



Patsourakos & Vial (2002), Labrosse et al. (2010).

Physical parameters


Temperature, density, ionisation, filling factor, ...Accurate measurements are

crucial to construct realistic models of prominencesdifficult to obtain

prominence plasma not in local thermodynamical equilibrium (non-LTE) because of strong incident radiation coming from the Sun

Large span of measured valuesdepending on the observed structuredepending on the technique used

Non-LTE radiative transfer modelling of prominences sheds light on line formation mechanismshelps to interpret spectroscopic observations / imaging

Plasma parameters


•Line formation–observations are difficult to understand•Necessity to solve equations–Statistical equilibrium–Radiative transfer including optically thick lines and continua•Non linear and non local coupling between matter and radiation

Non-LTE radiative transfer


Lyman lines of hydrogen form in different parts of the prominence (Heinzel 2007)Optically thick core reveals fine structure close to prominence boundariesOptically thin wings result from integration of several elements along LOSSame for He I and He II resonance linesHe I 584 Å, He I 537 Å, He II 304 Å, He II 256 ÅPlasma out of local thermodynamic equilibrium (LTE)Plasma diagnostics is complexNon-LTE radiative transfer calculations with velocity fields are needed to build realistic prominence models

H and He EUV resonance lines

The prominence model

•1D plane-parallel vertical slabFree parametersGas pressureTemperatureColumn massHeight above the limbRadial velocity

Equations to solvePressure equilibrium, ionisation and statistical equilibria (SE), radiative transfer (RT) for H (20 levels)SE, RT for other elements: He I (29 levels) + He II (4 levels)

Anzer & Heinzel (1999)

The prominence model


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.

Prominence-corona transition region (PCTR)

He I: 29 energy levelsHe II: 4 energy levels

76 bound-bound transitions and 33 bound-free transitions561 transitions overall

We can now calculate the emergent radiation.

He I model atom


He I triplet line intensity ratio depends on prominence altitude

Labrosse & Gouttebroze (2004)

E(10830)/E(D3) vs height above the limb

Intensities and physical parameters


Labrosse et al (2002)

model without transition region

models with transition region

Influence of PCTR on line profiles

H Lyman α He I 584 Å


T<6000 KT<6000 K

T>16000 KT>16000 K

E(10830)/E(D3) vs optical depth at 504 Å

Labrosse & Gouttebroze (2004)

•PCTR affects formation mechanisms of lines formed in cool parts of the prominence–statistical equilibrium of He I atomic states

Influence of PCTR on He I triplet lines


MEDOC campaign #13, 15–16/6/2004Observed profiles compared

with grid of 4720 computed models (T, n, ...)

⇩

Ly-β, Ly-ε, and He I 584 Å observed by

SUMER/SOHO

Prominence diagnostic with SUMERBBSO Hα


ne = 6 108 cm-3 (surface)

ne = 5 109 cm-3 (center)

● Prominence model: 1D plane-parallel slabTemperature profile inside prominence slab(Anzer & Heinzel 1999)

Labrosse, Vial, & Gouttebroze (2006)

Prominence diagnostic with SUMER


Prominence diagnostic with EIS

n(He III)/n(He)

Surface: 0.8Centre: 0

n(HeII)/n(He)

Surface: 0.20Centre: 3.3x10-5

Max = 0.99

np/nH

Surface: 1Centre: 0.94

TemperatureSurface: 105 KCentre: 104 K

See also Heinzel et al. (2008), Labrosse et al. (2011)


2D models

H ionisation only

H + He ionisation

Gouttebroze & Labrosse (2009)

Ionisation degree in cylindrical prominence


2D models

6000 K

10000 K

15000 K20000 K30000-50000 K

8000 K

Gouttebroze & Labrosse (2009)

Variation of the ionisation ratio with T


● Imaging measurements– apparent motion of structure in plane-of-sky

● Doppler shifts in prominence spectra– 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.

He I model atomHe I model atomDiagnostic of velocity fields


Effects of radial 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•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, 2008)

Effects of radial motions

V=0 km s-1

V=80 km s-1

V=200 km s-1

V=400 km s-1

He I 584 He II 304 He I 10830

T = 8000 K

T = 15000 K

Labrosse et al. (2007)


● He II 304 Å line sensitive to Doppler dimming due to radial motion of prominence plasma

Labrosse et al. (2007)

Plasma motions in prominences


Effects on Lyman αDoppler dimming ifLarge temperature

gradient in PCTRNot too denseCool plasma

Doppler dimming of Lyman α line less pronounced when PCTR is extended.increased contribution in line formation of collisional processes in

higher temperature region relative to narrow PCTR case

Results




Increasing column mass with all other parameters kept constant means more hot materialcollisional component of Ly-α becomes more important ⇒ the line is

less sensitive to Doppler dimming

Results




Increasing temperature of main prominence body increases amount of hot materialcollisional component of Ly-α becomes more important ⇒ the line is

less sensitive to Doppler dimming

Results


Results (5)

Effects on Heliumresonance lines(Same trend as Lyman lines)Doppler dimmingCool plasmaNot too denseLarge temperature

gradient in PCTR

Effects on Helium subordinate lines10830, D3, ... are less sensitive to Doppler dimming/brightening due to weak incident radiation

Results


(erg

s1 c

m-2 s

r-1 Å

-1)

(PCTR = prominence-to-corona transition region)

E(He I 584) vs. radial velocity


(erg

s1 c

m-2 s

r-1 Å

-1)

(PCTR = prominence-to-corona transition region)

E(He II 304) vs. radial velocity


2011-06-10

2010-09-08

Labrosse & McGlinchey (subm)


Comparison with observations

Labrosse & McGlinchey (subm)


Importance of taking into account PCTR–Affects plasma diagnostics from most lines in most cases

Calculations provide constraints for determination of–Opacities–Ionisation degree

– Variations in ionisation degree along LOS can be important

–Radiative losses for energy balance calculations

Compare 2D calculations with observations–Models must be constrained by using several lines (H+He)

Conclusions / Future plans

formation of helium lines in prominences

Documents

solar physics group

purple mountain observatory

prominence plasma

prominence slab

prominence eruptionsin

realistic prominence

prominences form

radiative transfer rt