A.— MATHEMATICAL AND PHYSICAL SCIENCES. 35 



the heavy positive ions would penetrate deeper and be diverted to the 

 polar regions by the magnetic field of the earth. 



Maris and Hulbert attribute the increase in ionisation to the action of 

 ultra-violet light. They conclude that at heights of 300 to 400 kilometres 

 temperatures of 1,000° K are reasonable, and at heights exceeding 400 kilo- 

 metres the free paths of the particles are very long, the motions due to 

 formal impact considerable, and the ionisation entirely due to the action 

 of ultra-violet light. When the activity of the sun increases it is assumed 

 that there is a tremendous increase of ultra-violet light ; thus. Maris and 

 Hulbert estimate that if one ten-thousandth part of the solar surface 

 (temperature 6,000°) were removed and there were exposed regions of 

 black body temperature 30,000°, the total ultra-violet energy would be 

 increased 10^ times, whereas the solar constant would be increased by 

 only 1 per cent. The ejected ions give rise to a magnetic storm, for under 

 the influence of gravity in a magnetic field the positive and negative ions 

 will move in opposite directions and at right angles to the gravitational 

 and magnetic forces, and so produce electric currents, the direction of 

 which is roughly in circles corresponding to magnetic lines of latitude. 

 Such currents will induce others in the earth, and owing to the rapidity 

 of the changes the currents will be nearer the surface of the earth than 

 in the case already considered, where the normal diurnal changes are 

 comparatively slow. But the general effect of the induced currents will 

 be the same, i.e. a greater variation in the horizontal force. Maris and 

 Hulbert calculate that a blast of ultra-violet light may thus give rise to 

 currents of the order 10® amperes, and a change of magnetic field of the 

 order of 0.001 C.G.S. 



Recently, Chapman and Ferraro have suggested that magnetic storms 

 are essentially connected with the approach of a neutral ionised stream 

 towards the earth, the more important changes in the stream taking place 

 in the direction of the sun at a distance equal to a few times the radius 

 of the earth. As the stream is a highly conducting body and cuts the 

 earth's magnetic field, electric currents are set up in the surface layers, 

 and the first stage of the magnetic storm is attributed to the magnetic 

 effect of these currents. Retardation of the stream results, and this 

 retardation is naturally greatest at that part of the front of the stream in 

 direct line with the centre of the earth. On either side the stream will 

 advance and partly enclose the earth, and along the sides of the enclosure 

 there will be charged layers due to the polarisation of the stream by the 

 magnetic field. Across the space on the dark side of the earth it is assumed 

 that a westerly current is set up due to charges passing over the space 

 between the charged layers. It will be observed that this theory, which 

 is being developed, has one distinct feature, inasmuch as the main electric 

 current flows at a distance a few radii from the earth. 



Whatever the ultimate action may be of a stream of charged particles 

 from the sun, which Maunder, who first suggested such a theory, 

 appropriately called a ' hose,' it appears appropriate to consider the 

 sunlit hemisphere to be the one first affected by the stream, and to 

 associate streams of great intensity with sunspots. It is known that 

 sunspots are rare in a zone 5° either side of the equator, and are chiefly 

 congregated in two zones, the mean latitude of which is about 20°, As we 



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