SPECTRA OF THREE BRIGHT PROMINENCES

Abstract

The Sacramento Peak Observatory has made routine spectral observations at the sun's limb on every possible day for the past eight years. The purpose of this patrol is to record principally the red and green coronal lines. In the course of this series of observations, spectra of thousands of solar prominences have been recorded. Newkirk has reported on the spectroscopic composition of prominences in the first five years of operation of the patrol.1 The brightest prominence he found showed some continuous emission, and about a dozen atomic emission lines. In the past few months, just at maximum of the current solar cycle, this same routine program has produced spectra of several prominences incomparably brighter and richer in spectral features. This report concerns observations of three of these objects. Previous descriptions of prominence spectra observed in eclipse (Unsold2), and outside eclipse (Richardson3), agree essentially with those reported below. However, no other prominence spectrum discussed has equaled two of these objects in plenitude, nor the third in degree of excitation. It is important to understand the nature of coronal spectrographs in order to appreciate the limitations of such spectra in this connection. A coronagraph is a monochromatic instrument, since the occulting disk completely eliminates scattered disk light from the secondary optical system only for a unique wavelength range, in practice about 100 A wide. Moreover, since the first objective of a coronagraph is a simple lens, the diameter of the sun's first image is wavelength dependent. Without a hyperchromatic secondary optical system, the diameter of the final image will be so also. As a result, the spectrograph slit images differ-