Abstract
A STRONOMERS are accustomed to divide the outer regions of the sun into four parts: (1) the photospheric layers, (2) the reversing layer, (3) the chromosphere, (4) the corona. In this address I wish to deal particularly with the chromosphere, but before doing so I should like to dwell a little on certain aspects of this fourfold division. Meteorologists make a similar subdivision of the earth's atmosphere. We have the troposphere, the stratosphere, the conducting layers, the auroral layers, and so on. But meteorologists have at least one advantage over solar physicists—meteorologists know where the earth's atmosphere begins. It may leave off very indefinitely, but it certainly begins quite definitely-it begins at the solid and liquid crust of the earth. It is sharply bounded below. But on the sun, and indeed on any star, no such sharp base exists. Whether the sun is wholly gaseous, or whether with Dr. Jeans we suppose it to be ultimately in a liquid state in the far interior, we are at least certain that owing to the high surface temperature and the positive temperature gradient implied by the outflow of heat, the entire outer layers, down to a depth much greater than the furthest depth we can see, are in the gaseous state. We therefore have no datum line for the base of the solar atmosphere.
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MILNE, E. The Sun's Outer Atmosphere1. Nature 121, 911–913 (1928). https://doi.org/10.1038/121911a0
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DOI: https://doi.org/10.1038/121911a0