Two recommendations are made that can eliminate persistent confusion in the study of diatomic spectroscopy by providing uniform and consistent definitions of the electronic transition moments and the rotational line intensity factors. First, it is recommended that the equation for the line strength of asingle rotational line be adopted to specify the relationship between the electronic transition moment and the rotational line intensity factor. Second, it is recommended that the electronic transition moment operator for perpendicular transitions be defined by (1/2]")( Jo, + iby). The adoption of these conventions results in a value of (2s + 1)(2J + 1) for the sum rule of the rotational line intensity factor for 2% * P+ transitions and a value of 2(2S + 1)(2J + 1) for the sum rule for all other spin-allowed transitions.
We continue previous research on the limb flare of 30 April, 1980, 20:20 UT, observed in X-rays by several instruments aboard the Solar Maximum Mission (SMM). It is shown quantitatively that the flare originated in an emerging magnetically confined kernel (diameter ~20") which existed for about ten to fifteen minutes, and from which energetic electrons streamed, in at least two injections, into a previously existing complicated magnetic loop system thus forming a less bright but extended and long-lived tongue. The tongue had a length of ~35 000 km and lasted ~90 min in X-rays (~ 10 keV); at lower energies ( ~ 0.7 keV) it was larger ( ~ 80 000 km) and lasted longer. The total number of energetic electrons ( ~ 1037) initially present in the kernel is of the same order as the number present in the tongue after the kernel's decline. This gives evidence that the energetic electrons in the tongue originated mainly in the kernel. The electron number densities in the kernel and tongue at maximum brightness were ~4.5 • 10 ~ and ~ 1 • 10 ~J cm -3, respectively. During the first eight minutes of its existence the tongue was hotter than the kernel, but it cooled offgradually. Its decline in intensity and temperature was exponential; energy was lost by radiation and by conduction through the footpoints of the loop system. These footpoints have a cross-section of only ~3 • 106 km 2. This small value, as well as photographs in a Clv UV emission line, suggests a highly filamentary structure of the system; this is further supported by the finding that the tongue had a 'filling factor' of ~ 10-2. Several faint X-ray brightenings (< 0.005 of the flare's maximum intensity) were observed at various locations along the solar limb for several hours before and after the flare. At ~ 30 min before the flare's onset a faint (< 0.02) flare precursor occurred, coinciding in place and shape with the flare. First the kernel precursor was brightest but the tongue precursor increased continuously in brightness and was the brightest part of the precursor some 10-15 min after the first visibility of the kernel precursor, until the start of the main flare. This suggests (weak) continuous electron acceleration in the tongue during a period of at least 30 min. The main flare was caused by strong emergence of magnetic field followed by two consecutive field line reconnections and accelerations in a small loop system, causing footpoint heating. Subsequently plasma streamed (convectively) into a pre-existing system of larger loops, forming the tongue.
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