Dielectronic recombination and its applications in astronomy

Abstract

Dielectronic recombination is a process particularly effective in high-temperature dilute plasmas such as those observed, for example, in astrophysics. This process was first recognised for its noticeable effects on the establishment of ionisation equilibrium in the solar corona. Over the last decade, its importance as a line formation mechanism has been emphasised. It is responsible for spectral lines appearing as satellites to the long-wavelength side of the resonance lines of the highly ionised systems typical of hot plasmas. However, in transient ionising plasmas and for the heavier ions these lines can also be produced by direct excitation of an innershell electron. This review is concerned essentially with the analysis and interpretation of the satellite lines in terms of the physical parameters characterising the emitting plasma. The theory for the satellite intensities is worked out in detail and the calculation methods of the related atomic parameters (such as wavelengths, transition probabilities, autoionisation rates, etc) are described. Extensive results concerning the astrophysically important hydrogen-like and helium-like ions are summarised. These are compared with the few available high-resolution soft x-ray spectra of solar active regions and flares and show good agreement mainly in the stronger lines. Such comparisons allow both the electron temperature and the transient state of the plasma to be determined within limitations connected mainly with the quality of the present observations.

L-SHELL DIELECTRONIC SATELLITE TRANSITIONS OF Fe XVII

“…The relative intensities of lines formed by dielectronic recombination is essentially independent of electron temperature, as the excitation and autoionization energies are to first approximation the same for all n = 3 resonances. The relative intensities are in this approximation only a function of the satellite line factor F 2 (Dubau & Volonté 1980):…”

“…This has been specially true for dielectronic satellite transitions associated with the emission of the closed-shell helium-like ions of elements with atomic number Z 18, which have been observed in laboratory measurements, the Sun, and in astrophysical sources (Gabriel 1972;Bitter et al 1979Bitter et al , 2003Gu et al 2012); in addition, lower-Z ions also display dielectronic satellite lines (Faucher et al 1983;Wargelin et al 2001). The reason for their importance is that they can be used as an accurate diagnostic of electron temperature (Dubau & Volonté 1980). Dielectronic satellite lines also play a role in L-shell emission spectra.…”

Autoionizing states of atomic boron

“…In contrast to the main series, the 2p 3 intruder state has a strong coupling to the 2s2p( 3 P )ε d channel, which, with a prevalence of almost 90%, completely dominates its branching ratio, and which imparts to the resonance a width more than one order of magnitude larger than that of any other autoionizing state in this interval. The first terms of the two main series, the 2s2p( 1 P )3s and 2s2p( 1 P )3d states, partly mix with the 2p 3 state, thus acquiring a noticeably larger decay component to the ε d channel. It is interesting to notice that the staggering prevalence of the decay of the 2p 3 state to the 2s2p( 3 P )ε d channel is not replicated by the higher terms of the 2p 2 ( 3 P )np autoionizing series in the next energy region.…”

“…Accurate records for photoexcitation and photoionization atomic spectra, obtained either from measurements or from state-of-the-art calculations, are instrumental in monitoring the properties of matter in extreme conditions, such as hightemperature plasma in laboratory [1], in stellar coronas [2,3], and in black-hole accretion disks [4,5] or the rarefied gases in galactic halos [6] and in the intergalactic medium [7], which are traversed by ionizing radiation [8,9]. Such atomic records are also essential to test new models for correlated electron dynamics in finite many-body systems.…”

RECOMBINATION RATE COEFFICIENTS OF BORON-LIKE Ne

“…In particular, recombination rate coefficients of highly charged ions are important for the study and modeling of astrophysical and laboratory plasmas as well as for the interpretation of astrophysical observations. Dielectronic recombination (DR) is considered to be an important source of emission lines from laboratory and astrophysical plasmas (Dubau & Volonté 1980;Hahn & LaGattuta 1988). This mechanism was first recognized by Burgess (1964) as being the most important and dominant recombination channel over radiative recombination (RR) in the solar corona at high temperatures.…”