Abstract
OBSERVATIONS during the solar cycle just completed have shown that a close correlation exists between solar flares—transitory patches of enhanced emission visible in hydrogen and calcium light near spots on the solar disk—and terrestrial magnetic storms. Great magnetic storms and auroral displays tend to occur about a day after the appearance of a brilliant flare, and it has been suggested that they are due to charged particles emitted from the sun simultaneously with the visible radiation. If the geometrical conditions are suitable, it is suggested, these particles begin to reach the earth 20–26 hours later, and cause disturbances of the magnetic traces which may last for a day or more. Nearly twenty years ago, Milne gave theoretical reasons for believing that atomic particles might be ejected from the sun at a speed of about 1,600 km./sec., which corresponds to a time of travel of 26 hours. Soon afterwards, Chapman suggested that a comparison should be made between solar spectra at times of magnetic quiet and of magnetic disturbance. The annual survey of the year's work at Mt. Wilson (Adams, Pub. Ast. Soc. Pacif., 56, 213; 1944) now states that solar spectrograms taken in the ultra-violet region during magnetic storms give some indication of two very shallow absorption bands beginning near the centre of the lines H and K and extending about 12 A. towards shorter wave-lengths. Their maximum depth is only 1 per cent of the background continuum. Maximum velocities of the order of 1,000 km./sec. and mean velocities of about 600 km./sec. are indicated. Control spectrograms taken during magnetic calm show no such absorption. This is the first direct observational evidence of the presence in interplanetary space of calcium ions approaching the earth from the sun at speeds comparable with those predicted both theoretically and by inference from geophysical observations.
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Atomic Particles from the Sun. Nature 155, 692 (1945). https://doi.org/10.1038/155692c0
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DOI: https://doi.org/10.1038/155692c0