2.2. Helium 3
3He has been detected in a solar prominence from its isotope shift relative to the 10830 emission line with an estimated abundance consistent with that in the Solar wind (Hall 1975). 3He is also observed in the ISM, in HII regions, using large radio dishes, by virtue of the hyperfine transition of 3He+ at 3.46 cm (Bania, Rood & Wilson 1987). Abundance determinations relative to nearby H recombination lines involve correction for clumping and for differing degrees of ionisation of hydrogen and helium, but the most difficult part is subtraction of the baseline from the data, since the line is exceedingly weak. The resulting abundance ratios 3He / H range from 1.5 × 10-4 to under 10-5, without any clear pattern with galactocentric distance such as might have been expected from the trends of oxygen and nitrogen abundance. Some (though not necessarily all) of the source-to-source variations appear to be real.