SOLAR ACTIVITY — SPENCER JONES 235 



of the Ha line of hydrogen, either visually with the spectrohelioscope 

 or photographically with the aid of a monochromatic filter or with the 

 spectroheliograph. 



A characteristic magnetic disturbance which is termed a crochet is 

 found to be associated with each intense flare. The amplitude of this 

 magnetic disturbance is comparable with the amplitude of the normal 

 diurnal magnetic variation. These crochets are better observed at 

 })laces near the magnetic equator than at places in higher magnetic 

 latitudes ; this is understandable, as the effect on the ionospheric cur- 

 rent system tends to be greatest near the subsolar point. The emission 

 of ultraviolet light from a very bright flare is thus comparable with 

 the normal emission from the whole of the sun's disk. There is conse- 

 quently a great enhancement of ultraviolet light radiation from the 

 sun during a flare, which gives rise to greater ionization in the iono- 

 sphere. The crochet is the magnetic effect associated with this in- 

 creased ionization, arising from a great enhancement in the strength of 

 the electric-current system in the ionosphere. 



It is found that when a bright flare occurs on the sun there is a 

 practically simultaneous f adeout in short-wave radio transmissions on 

 channels passing over the sunlit hemisphere of the earth. The trans- 

 missions over the dark hemisphere remain unaffected. The enhanced 

 emission of ultraviolet light from the sun, which occurs while the flare 

 is in progress, causes the photoionization to extend to a much lower 

 level, where the atmospheric density is much greater. The radio waves 

 are then absorbed instead of being reflected and the phenomenon of a 

 radio f adeout occurs. The suddenness with which a fadeout develops 

 is very striking: in the course of a few seconds the radio waves can 

 fade from normal intensity to complete inaudibility. This is a con- 

 sequence of the extreme rapidity with which the outburst develops. 



A secondary effect associated with the flare outburst is an enhance- 

 ment in the reflection of the very long waves reflected by the D-layer, 

 which is a consequence of the increased ionization in this layer pro- 

 duced by the flare. The occurrence of a flare is thus accompanied by 

 a sudden enhancement of the atmospherics which come to us mainly 

 from the tropical regions where thmiderstorms are most numerous. 

 Thunderstorms are occurring throughout the 24 hours somewhere or 

 other on the earth ; a receiver which records the integrated intensity of 

 the atmospherics will show a sharp increase in intensity when a flare 

 occurs. Such receivers have been built and provide a simple means of 

 recording and timing the occurrence of flares which may not be ob- 

 served because of cloudy weather and, if the weather is not cloudy, of 

 enabling observations of a flare to be secured during its progress, if 

 the sun was not already under observation at the time. 



