Plasma diagnostic in eruptive prominences from SDO/AIA observations at 304 Å

Nicolas Labrosse and Kristopher McGlinchey

University of Glasgow, Scotland, UK

Motivation

Theoretical calculations have shown that when solar prominences

move away from the surface of the Sun, their radiative output is

affected via the Doppler dimming or brightening effects. Hyder & Lites (1970), Heinzel & Rompolt (1987), Gontikakis et al (1997), Labrosse et al (2007, 2008)

This is due to their strong sensitivity to the incoming radiation from the solar disc.

He II resonance lines are mostly formed by scattering of the incident radiation under

typical quiescent prominence conditions.

Can we find observational signatures of the changes in the radiative

output of eruptive prominences in EUV observations at 304 Å?

What are the plasma parameters in eruptive prominences?

We analyse SDO/AIA observations and compare them with new non-

LTE radiative transfer calculations of the He II 304 Å line intensity in

eruptive prominences.

The prominence 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)

SE, RT for other elements: He I (29

levels) + He II (4 levels)

The prominence model

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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 (resonant scattering, thermal processes)

–details of incident radiation (strength, emission/absorption)

See Labrosse et al (2010)

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)

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He II 304 Å line sensitive to Doppler dimming

line mostly formed by scattering of incident radiation coming from the Sun

Labrosse et al. (2007)

Plasma motions in prominences

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Results (5)

Doppler dimming effect on Helium resonance lines stronger if:

Cool plasma

Not too dense

Large temperature gradient in PCTR

NB: Plasma parameters are kept constant

Increasing column mass with all other parameters kept constant means more hot material → collisional component becomes more important ⇒ the line is less sensitive to Doppler dimming

Initial results

Question

Is Doppler dimming observed in eruptive prominences?

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2011-06-10

2010-09-08

SDO/AIA observations

Summary of observations

Variation of He II 304 intensity with radial velocity

Decreases in some cases, increases in others

How is this related to the pure Doppler dimming effect?

Need new computations

Allow plasma parameters to vary during eruption

Randomly chosen input parameters (within limits)

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New computations

Reference model

T=8800 K

M=4.8 10-5 g cm-2

Solid line

Effect of Doppler

dimming alone (no

variation of plasma

parameters)

Equal proportion of models showing computed intensities in the He II

line either greater or lower than the intensity of the reference (static) model.

Summary

•Observations show either a decrease or an increase of intensity with

radial velocity.

•New non-LTE models explain the different behaviour of the

intensities by changes in the plasma parameters inside the

prominence, in particular the column mass of the plasma and its

temperature.

•These new non-LTE models are more realistic than what was used

in previous calculations as they allow all plasma parameters to vary.

•They are able to reproduce qualitatively the range of observations

from SDO/AIA.

Labrosse & McGlinchey (accepted in A&A)

Velocity vs. time

Model parameters