Reexamination of the coronal index of solar activity

Abstract

The coronal index (CI) of solar activity is the irradiance of the Sun as a star in the coronal green line (Fe XIV, 530.3 nm or 5303 Å). It is derived from ground‐based observations of the green corona made by the network of coronal stations (currently Kislovodsk, Lomnický Štít, Norikura, and Sacramento Peak). The CI was introduced by Rybanský (1975) to facilitate comparison of ground‐based green line measurements with satellite‐based extreme ultraviolet and soft X‐ray observations. The CI since 1965 is based on the Lomnický Štít photometric scale; the CI was extended to earlier years by Rybanský et al. (1994) based on cross‐calibrations of Lomnický Štít data with measurements made at Pic du Midi and Arosa. The resultant 1939–1992 CI had the interesting property that its value at the peak of the 11‐year cycle increased more or less monotonically from cycle 18 through cycle 22 even though the peak sunspot number of cycle 20 exhibited a significant local minimum between that of cycles 19 and 21. Rušin and Rybanský (2002) recently showed that the green line intensity and photospheric magnetic field strength were highly correlated from 1976 to 1999. Since the photospheric magnetic field strength is highly correlated with sunspot number, the lack of close correspondence between the sunspot number and the CI from 1939 to 2002 is puzzling. Here we show that the CI and sunspot number are highly correlated only after 1965, calling the previously‐computed coronal index for earlier years (1939–1965) into question. We can use the correlation between the CI and sunspot number (also the 2800 MHz radio flux and the cosmic ray intensity) to recompute daily values of the CI for years before 1966. In fact, this method can be used to obtain CI values as far back as we have reliable sunspot observations (∼1850). The net result of this exercise is a CI that closely tracks the sunspot number at all times. We can use the sunspot‐CI relationship (for 1966–2002) to identify which coronal stations can be used as a basis for the homogeneous coronal data set (HDS) before 1966. Thus we adopt the photometric scale of the following observatories for the indicated times: Norikura (1951–1954; the Norikura photometric scale was also used from 1939 to 1954); Pic du Midi (1955–1959); Kislovodsk (1960–1965). Finally, we revised the post‐1965 HDS and made several small corrections and now include data from Kislovodsk, Norikura, and Sacramento Peak to fill gaps at Lomnický Štít.