Observing The Corona Without An Eclipse

Once astronomers had understood the basics of the solar spectrum, through the forward leaps in knowledge in the 1860s, it became possible to observe the corona at times other than during a total solar eclipse, by using a suitable filter.

We have already seen how the photosphere produces a continuous spectrum, which is modified by absorption from cooler overlying gases. Consider the red Ha line at 6,563 A. At that specific wavelength the cool hydrogen at the top of the photosphere absorbs much of the light flux. But the hotter hydrogen in the chromosphere and corona above it is madly emitting at the same wavelength. If one observed the Sun using a filter that lets through only light within a narrow band about 6,563 A, then much of the photospheric spectrum would be cut out; what comes through would be the emission from these higher reaches of the solar atmosphere. The solar disk is therefore being blocked not by the opaque Moon, as in an eclipse, but by the clever use of a spectral filter. (It is called an "Ha" filter because it blocks all light except the wavelength corresponding to the Ha hydrogen spectral line.)

Astronomers soon seized upon this, and from the 1870s onwards it has been a fundamental technique allowing the changing form of the chromosphere to be followed. If you are ever in the presence of a group of astronomers, among the jargon bandied about the term "aitch-alpha filter" will often be heard.

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