The helium chromosphere.

Abstract

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The helium chromosphere. Athay, R. G. Abstract The flash-spectrograms obtained by the High Altitude Observatory expedition to the 1952 eclipse in Khartoum contain several helium lines that are suitable for photometric reduction. Some of these lines have not been studied quantitatively at previous eclipses, and our spectrograms were taken with much greater height resolution than had been previously attempted. These features make a study of the helium data from the 1952 eclipse of special interest. The Hei data include the first four lines of the 23P - n3D series, the first three lines of the 2~P - n3S series, the first line of the 2'P - n'D series and the first three and fifth lines of the 2'? - n'S series. The Heii data are for X4686, 32D - 42F0. The data show maxima in the surface brightness of the chromosphere at the limb at heights of I 100 km for HeI emission and 1500 km for Heii emission. Both the hydrogen and helium data show that if the chromosphere is assumed to be uniform the radial optical depth at X5oo is 200 at the height where the maximum Hei emission occurs. These large optical depths exclude the possibility of excitation of chromospheric helium by ultraviolet coronal radiation if the chromosphere is uniform. Kinetic temperatures obtained from an analysis of the continuum data under the assumption of a uniform chromosphere are too low to produce the observed excitation of helium. Thus the assumption of a uniform chromosphere appears to be invalid for the helium data, and we assume a two-region model with all of the helium emission coming from one region. The triplet series of Hei have excitation temperatures of 50000, and the singlet series have excitation temperatures of 15,0000. If we assume approximate thermodynamic equilibrium for HeiT and the singlet series of Hei and use a relative abundance of hydrogen to helium of 7: 1, the observed line intensities for both Hei and Heii require kinetic temperatures of 19,000 in the regions above 1500 km that emit the helium radiation. If we vary b1 within the limits < b1 < i~~ the kinetic temperature in the helium region varies between 14,5000 < Te < 26,0000. The maximum in the Heii emission at 1500 km leads to Te 20,0000, in agreement with the values obtained by other methods. Continuum data show that the electron temperatures and densities below 4000 km in the regions that do not emit helium radiation are essentially unchanged from the results obtained on the assumption of a uniform chromosphere (Athay et al. 1955). Above 4000 km the nonuniform model leads to higher temperatures in these regions. The eclipse spectrograms show conclusively that above 5000 km the helium radiation is in the spicules. This work was supported by the Office of Naval Research and carried out in cooperation with the Naval Research Laboratory. Athay, R. G., Meozel, D. H. Pecker, J-C., and Thomas, R. N. 1955, Ap. J. Supplement 12, 505. H~gh Altitude Observatory, Boulder, Colo. Publication: The Astronomical Journal Pub Date: June 1955 DOI: 10.1086/107180 Bibcode: 1955AJ.....60..150A full text sources ADS |