Ionization in the Solar Chromosphere. 47«^ 



It appears that no satisfactory explanation of this fact, as 

 well as of the extraordinary height reached by these lines, 

 has yet been offered. It is intimately connected with the 

 physical mechanism of the arc and the spark. In this con- 

 nexion, it is well to recall Lockyer's original hypothesis, 

 which, however, does not seem to have been, at any time, 

 much in favour with the phj r sicists. According to Lockyer, 

 the passage from the arc to the spark means a great, though 

 localised, increase of temperature, to which mainly the en- 

 hancement of the lines was to be ascribed. But, apart from 

 its physical incompleteness, Lockyer's theory launches us 

 amidst great difficulties as far as the interpretation of solar 

 phenomena is concerned. It would lead us to the hypothesis 

 that the outer chromosphere is at a substantially higher 

 temperature than the photosphere, and the lower chromo- 

 sphere ; and that the temperature of the sun increases as we 

 pass radially outwards. This hypothesis is, however, quite 

 untenable and is in flagrant contradiction to all accepted 

 theories of physics. 



A much more plausible explanation is that the lines in 

 question are not due to radiations from the normal atom 

 of the element, but from " an ionized atom, i. e., one which 

 has lost an electron.*' The high-level chromosphere is, 

 according to this view, the seat of very intense ionization. 

 Let us see briefly how this hypothesis has grown up. 



Modern theories of atomic structure and radiation leave 

 little doubt that the ''enhanced lines" are due to the ionized 

 atom of the element. As a concrete example, let us take 

 the case of the calcium H, K, and g lines. The " H, K'" 

 lines are of the enhanced type, while "//" is of the normal 

 type. The " H, K " are the leading members of the 

 principal pair-series of the system of double lines of Calcium, 

 while the " g-" line is the first member of the system of 

 single lines of Calcium. Lorenser and Fowler * have shown 

 that the series formula of the double lines is of the type 



1 1 



/(m)} 2 {$(,<) 



^ 4N [}w 



while the series formula of the single lines is of the type 



v = S 



r - - 1 i 



where /(m), </>(/i) are functions of the form m + a, according 



* Fowler, Phil. Trans, vol. ccxiv. 

 Phil. Mag. S. 6. Vol. 40. No. 238. Oct. 1920. 2 I 



