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Variability of the Solar He I 10830 Å Triplet

Published online by Cambridge University Press:  03 August 2017

J. W. Harvey  and
W. C. Livingston
J. W. Harvey
Affiliation:
National Solar Observatory, National Optical Astronomy Observatories,* P. O. Box 26732, Tucson, AZ 85726, U.S.A.
W. C. Livingston
Affiliation:
National Solar Observatory, National Optical Astronomy Observatories,* P. O. Box 26732, Tucson, AZ 85726, U.S.A.

Abstract

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The He I 10830 Å triplet gives a unique view of the solar chromosphere. Digital spectroheliograms have been made regularly since early 1974 using this line and the NSO Vacuum Telescope on Kitt Peak. For many purposes (detection of coronal holes, giant two-ribbon flares, and dark point events) these images are sufficient. A Sun-as-a-star signal is also produced by averaging all the pixels in each daily image. To calibrate this ‘irradiance’ signal in terms of line equivalent width, a comparison is made with integrated sunlight spectrophotometric measurements obtained less frequently. After correction for the effects of water vapor blends, we find a linear relation between the two measurements. The daily averages have been assembled into a time series covering nearly two solar cycles. This time series shows cycle modulation of about ±30% and rotational modulation of about ±10%. The general variation is similar to that of other activity indices but with some interesting small differences. Since images are available, it has been possible to decompose the full disk index into components due to plages, filaments, coronal holes and background. At all times during the cycle, most of the signal comes from the background but most of the variability from plages.

Keywords

He I 10830 Åinfrared: starsSun: activitySun: chromosphere
Type
Part 1: Infrared Diagnostics of the Solar Atmosphere and Solar Activity
Information
Symposium - International Astronomical Union , Volume 154: Infrared Solar Physics , 1994 , pp. 59 - 64
Copyright
Copyright © Kluwer 1994 

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