Variation of the Sun's ultra‐violet radiation as revealed by ionospheric and geomagnetic observations

Summary—A study has been made of the short‐ and long‐period variations of critical frequencies of the E ‐, F 1‐, and F 2‐ionospheric layers at Washington, Huancayo, Watheroo, and Mount Stromlo during 1937–44. Terrestrial factors have been eliminated in order that the frequencies should represent the changes in the ionising ultra‐violet light. A similar study has also been made of the variations in the amplitude of the S q magnetic field at Apia, Watheroo, and Cape Town during 1937–43. Yearly, monthly , and daily variations have been compared with solar data by means of graphs and correlation‐methods. The following are the main conclusions: (a) The variable part of the Sun's ultra‐violet illumination comes mainly from active regions characterised by the appearance of sunspots, flocculi, and faculae. (b) The sources of ultra‐violet have a longer life than sunspots, and possibly a longer life than faculae. (c) The three ionospheric layers E , F 1, and F 2, and the S q ‐field are influenced by the same sources of ultra‐violet light. (d) The sources emit considerable ultra‐violet radiation when at the center of the Sun and probably emit some radiation when near the limb. (e) F 2 electrons take periods of one or two days to reach equilibrium concentration —at the level of S q ‐currents recombination is much more rapid. (f) The variable part of the ultra‐violet flux is proportional to sunspot‐number, and is about equal to the steady part at sunspot‐maximum. This applies to radiation exciting the E ‐, F 1‐, and F 2‐regions. (g) There is a linear relation between f 0 F 2 and sunspot‐number. (h) Faculae could produce continuous ultra‐violet radiation that might account for ionospheric and geomagnetic variations.