Spectroheliograph

The spectroheliograph, originally designed for photographing only the solar prominences, disclosed, in its first application by Hale at the Kenwood observatory (Chicago, 1891, 1892), a new and unexplored region of the sun's atmosphere. Photographs of the solar disk, taken with the H or K line, showed extensive luminous clouds (flocculi) of calcium vapour, vastly greater in area than the sunspots. About this time Deslandres introduced at the Paris observatory, the velocity spectrograph, which permits the motion in the line of sight of the calcium vapour at various levels to be measured on photographs of the H or K line in suc cessive sections of the sun. By setting the camera slit of a spectro heliograph so as to admit to the photographic plate the light of the denser calcium vapour, which lies at low levels, or that of the rarer vapour at high levels, the forms of the flocculi corresponding to various superposed regions of the atmosphere can be recorded. The lower and denser vapour appears as bright clouds, but the cooler vapour, at higher levels, absorbs the light from below and thus gives rise to dark clouds.

The first photographs of the sun in hydrogen light were made with the spectroheliograph at the Yerkes observatory in 1903.

These reveal dark hydrogen flocculi, which, like the dark calcium flocculi, lie at a level above that of the bright calcium flocculi.

They also show less extensive bright flocculi, usually in the im mediate neighbourhood of sunspots, and frequently eruptive in character. These rise from a low level, and sometimes reach con siderable elevations in the form of eruptive prominences.

In such an exploration of the sun's atmosphere it might be anticipated that definite currents, or some evidences of atmos pheric circulation analogous to those familiar in terrestrial meteorology, would be discovered. In the early work neither the forms nor the motions of the calcium flocculi revealed the existence of such currents, but in the higher region shown by the hydrogen photographs the distribution of the dark flocculi sug gested the operation of definite forces, though their full effect was not recognized until the spring of 1908. At that time mono chromatic photographs of the sun were first made by Hale and Ellerman on Mt. Wilson with the red (Ha) line of hydrogen, previous hydrogen photographs having been taken with H13, Hy or HS in the blue or violet. On account of the relatively great strength of Ha at a considerable distance from the photosphere, the new photographs recorded flocculi at high levels previously un explored. The forms and motions of these flocculi show that great vortices exist in the solar atmosphere above sunspots, which resemble terrestrial cyclones or tornadoes. About 75% of these high-level vortices indicate clockwise rotation in the southern hemisphere and counter-clockwise rotation in the northern, as in the case of terrestrial cyclones. The detection of these vortices

led, in 1908, to the discovery of magnetic fields in sunspots, ap parently due to electric convection in the primary vortices at lower levels which are the cause of the spots themselves. The long dark flocculi also shown on hydrogen spectroheliograms, identified as prominences projected against the surface of the sun, were subsequently called "filaments" by Deslandres, who has studied them extensively at Meudon. With a spectrohelio graph of high dispersion the centre of the hydrogen line H a shows the "alignments" discovered by Deslandres, which con stitute a network of wide mesh associated with the filaments.

Spectrohelioscope.

The spectrohelioscope renders visible to the eye many of the phenomena of the solar atmosphere photo graphed with the spectroheliograph and also permits their veloc ities in the line of sight to be measured. It thus combines some of the functions of the spectroheliograph and the velocity spectro graph. Its principle was suggested by Young in 1870, and tried provisionally by him for the observation of prominences at the sun's limb. He discarded it, however, when the use of a wide slit was proposed for this purpose by Zollner and Huggins. Hale, who revived and developed the method (1924), succeeded with its aid in observing the bright and dark hydrogen flocculi on the sun's disk, and in analyzing and measuring the motions of the gas in eruptions and vortices.

Imagine a narrow slit, rapidly oscillating, between the eye and a telescopic image of the sun. Through persistence of vision, the image can be seen in white light. Introduce a spectroscope, which excludes from the slit before the eye all light except that of the red hydrogen line. We then have a spectrohelioscope. For satis factory observations of the hydrogen atmosphere against the disk, fairly high dispersion is necessary. Hale uses a spectroscope of 13ft. focal length, having a plane grating ruled with about 15,00o lines to the inch, giving a first order spectrum which is very bright in the red. He has employed various means of moving the slits, one of which is a spinning disk pierced with 5o radial slits 0.004in. wide. A tin. solar image is formed on one side of the disk and the observer sees the hydrogen image of a portion of the sun through a low-power eyepiece focussed on the dia metrically opposite slits. J. A. Anderson and Sinclair Smith have suggested devices for producing an exactly similar monochromatic image with a spectroscope having fixed slits. These consist of a rotating prism of square section (Anderson) or an oscillating plane mirror (Smith) which cause a portion of the solar image to move rapidly across the fixed first slit of the spectroscope. The second slit is then viewed through another prism or mirror, mov ing at precisely the same rate and giving a fixed monochromatic image in the eyepiece.