IAS 20 June 2013

Celebrating the achievements of Alan Gabriel

Laboratory spectroscopyExploring the process of dielectronic

recombinationS. Volonte

IAS 20 June 2013

Laboratory spectroscopy

1969 -1973• Under Alan’s supervision development of an electromagnetic T-tube

• Study of high density effect in He plasmas

• Plasma Polarization Shift (PPS) of lines from ions due to partial screening from neighbouring perturbing electrons

• Studies by Griem et al

• Controversial theoretical and experimental results

IAS 20 June 2013

Laboratory spectroscopy

• First shift measurements of He II resonance series up to n=4 • Blue shifts measured and predicted ~n4 scaling confirmed

• Shift measurements in overall quantitative agreement only with quantum estimate (Griem 1970) based on hydrogenic continuum wavefunctions for perturbing electrons

• Study showed that if perturbing electrons are assumed as bound in high orbitals the effect corresponds to emission of high dielectronic

satellites and is negligible in high density plasmas • Conclusion: only free perturbers produce measurable PPS (Volonte PhD

1972 , Gabriel and Volonte 1973a,b)

IAS 20 June 2013

Exploring dielectronic recombination

• In same period Alan’s main laboratory activity-production of highly ionized spectra with theta-pinch machine for simulation of spectra of solar sources

• In parallel, research work on satellite lines-faint features on red side of

resonance lines of H-like and He-like ions (Edlen and Tyren 1939)

• With co-workers (a.o C. Jordan) he showed that production process was dielectronic recombination of free electron bound to parent ion in excited state to form an auto-ionzing doubly excited state of recombined ion from which satellites are emitted

• In developing the theory of dielectronic satellites he derived the intensity ratio

of the satellite to the resonance line as given by Is/Ir ~ Ar/Te exp((E0-Es)/kTe) with Ar~Z4

The ratio varies only with temperature and the n scaling shows that satellites are much stronger in heavy ions (factor ~100 from O to Fe)

IAS 20 June 2013

Exploring dielectronic recombination

• The theory was further developed for inner-shell satellites produced by collisional excitation of inner-shell transition in the recombined ion.

• For inner-shell satellites the intensity ratio to resonance line is given by

Is’/Ir~ (NHe/NLi)(C’/C)F(A)

Here the intensity ratio varies with the relative abundancies (Te dependant) which depend on the ionisation equilibrium of the source (i.e. wether ionising, recombining or steady state)

• These satellite intensity ratios give independent access to the electron

temperature and the ionization state of the source in coronal plasmas.

IAS 20 June 2013

Exploring dielectronic recombination

• This early work was devoted to the red shifted prominent satellites produced from doubly-excited states with n=2 for the most solar abundant He-like ions (O to Fe)

• Confirmation of these calculations was obtained from the first ever

observations of solar flare iron spectra (Fe XXV near 1.85 A) observed by the Intercosmos-4 satellite (1972).

• Best fit between the observed and computed spectra provided the electron temperature and the ionization state of the flaring plasma thus demonstrating the diagnostic capabilities of the satellite line theory

IAS 20 June 2013

Exploring dielectronic recombination

1974 -1986 • The next step undertaken by Alan was to improve the diagnostic

capability of the method by extending the work to include satellites emitted from auto-ionizing states with n>2 for H-like and He-like ions

• This entailed calculation of a very large number of atomic parameters

• This work was realised through an extended collaboration with the atomic physics groups at the Nice observatory (F. Bely-Dubau, P. Faucher, L. Steeman-Clark) and Meudon (J. Dubau, M. loulergue, M. Cornille) and myself at Mons

IAS 20 June 2013

Exploring dielectronic recombination

• A series of workshops were held regularly mainly at Nice (see

photos) but also at Meudon and Culham and occasionally at Mons to discuss progress of the work

• The early results of the collaboration concerned the high n satellite lines of Fe XXV (Bely-Dubau, Gabriel and Volonte 1979)

• The results showed the importance of including these lines to improve agreement between observed and calculated spectra thus enhancing the accuracy of the diagnostics.

IAS 20 June 2013

Exploring dielectronic recombination

• A series of workshops were held regularly mainly at Nice (see

photos) but also at Meudon and Culham and occasionally at Mons to discuss progress of the work

• The early results of the collaboration concerned the high n satellite lines of Fe XXV (Bely-Dubau, Gabriel and Volonte 1979)

• The results showed the importance of including these lines to improve agreement between observed and calculated spectra thus enhancing the accuracy of the diagnostics.

IAS 20 June 2013

Nice workshop

IAS 20 June 2013

Nice workshop

IAS 20 June 2013

Exploring dielectronic recombination

• The collaboration worked actively on the atomic parameters needed

in preparation of NASA’s Solar Maximum Mission (SMM 1980)

• Satellite line data up to n=5 were obtained for the most solar abundant H-like and He-like

• Examples of solar flare spectra from the BCS spectrometer on board SMM are shown below. The calculated spectra have adjusted to provide the best fit with the observations. Note the Te values and the ionization diagnostics obtained from the satellite line intensities.

IAS 20 June 2013

Exploring dielectronic recombination

• The work on satellite spectra was eventually tested on

spectra obtained in fusion machines (a.o. Princeton and Frascati Tokamak)

• Calculated and observed spectra were in good agreement

• The temperatures derived from the satellite intensities showed excellent agreement (few %) with independent measurements from Thomson scattering provided high satellites were accounted for

• This was a further confirmation of the powerful diagnostic

capabilities of the satellite lines

IAS 20 June 2013

Exploring dielectronic recombination

IAS 20 June 2013

Exploring dielectronic recombination

IAS 20 June 2013

Conclusion Through his leadership, scientific insight and as

an instigator of international collaboration Alan‘s work in laboratory spectroscopy, theory and interpretation of spectra has provided not only important data but also new diagnostics methods (e.g. satellite lines) which had not been available earlier. These research topics are among the many other important contributions that Alan has provided throughout his scientific career.