Polarisation of dielectronic recombination satellite lines

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Abstract
The polarisation of line radiation emitted during dielectronic recombination is investigated theoretically. By taking into account interference effects due to overlapping levels, formulae have been derived for the linear polarisation of these lines (usually called dielectronic satellites). Special attention is given to recombination of He-like systems which play an important role in hot plasmas. As an illustration, for n=4, results are reported for Li-like 1s2nl-1s2pnl satellites which contribute to the observed intensity of the He-like 1s2-1s2p parent line. The interference effects increase the polarisation up to a factor of two as n to infinity . Expressions are also given for the polarisation of well isolated Li-like satellite lines excited by an arbitrary axially symmetric electron angular distribution. Detectable (>or=10%) polarisation of j and k satellites may be expected in a low-density plasma provided the dominant thermal electrons contribute by less than about 70% of the total emission of these satellites. Finally, the combined effects of anisotropy and power-law energy distribution of a suprathermal electron tail on the intensity ratio j/w are examined. It is concluded that to neglect the presence of a small percentage of these suprathermal electrons in a thermal plasma may lead to strong overestimates of the plasma temperature.
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