Theoretical oscillator strengths are reported for transitions between excited terms of the singlet and triplet S, P, D, F and G manifolds of atomic magnesium. The principal quantum numbers of the jumping electron range from n = 4 to 100. The calculations are based on the Coulomb approximation. Low-n results, for which other data are available previously, agree reasonably well with the sophisticated calculations of Moccia and Spizzo (1988). Data for 1,3 F–1,3 G transitions may deviate from hydrogenic values by as much as 50%.

The 11 July 1991 total solar eclipse over Mauna Kea was a unique opportunity to study the limb profile of the 12.32 μm MgI emission line. Our observations used the NASA 3-meter Infrared Telescope Facility,1 and a new Goddard large cryogenic grating spectrometer. Spectra of the line were taken in the slitless mode at second contact. The results show that the emission peaks within ~ 300 km of the 12-μm continuum limb. This agrees with recent theoretical predictions for this line as a NLTE upper photospheric emission feature. However, the increase in optical depth for this extreme limb-viewing situation means that most of the observed emission arises from above T min, and we find that this emission is extended to altitudes well in excess of the model predictions. The line emission can be traced to altitudes as high as 2000 km above the 12-μm continuum limb, whereas theory predicts it to remain observable no higher than 500 km above the continuum limb. The substantial limb-extension observed in this line is qualitatively consistent with limb-extensions seen by other observers in the far-IR continuum, and may be indicative of departures from gravitational hydrostatic equilibrium in the upper solar atmosphere, and/or may result from temperature and density inhomogeneities. The extended altitude of formation of this line enhances its value as a Zeeman probe of magnetic fields.

We have investigated the impact broadening of the 12 μm lines. Ion broadening is found to follow the adiabatic theory, whereas electron broadening needs a non-adiabatic treatment. This relaxes the fixed width/shift ratio, as found by Chang and Schoenfeld (1991), using a pure adiabatic analysis.