A statistical analysis of solar flux variations over time scales of solar rotation: 1978–1982

Abstract

Differences in the temporal behavior of the ultraviolet irradiance at 205 nm and the 10.7‐cm radio flux, the ultraviolet irradiance at 121.6 nm, model calculations of the 205‐nm irradiance derived from Ca II K plage emission, and the sunspot‐blocking function are examined during a 5‐year period near the maximum of solar cycle 21. Because of solar rotation the dominant variance in each of these time series occurs at 27 days, but real temporal differences arise because the five solar time series are each formed at different heights within the solar atmosphere and are associated with a variety of solar active region phenomena having different spatial and temporal characteristics on the solar disc. These differences may be important if the ground‐based solar activity time series are used instead of the measured UV irradiances in correlation studies of solar variability with atmospheric parameters such as ozone densities and temperature. Recognizing the presence of autocorrelation in the UV irradiance time series is also important in solar terrestrial correlation studies, since it complicates the use of classical statistical techniques for estimating the significance of the results.

Correlated brightness variations in solar radiative output from the photosphere to the corona

“…Thus, we conclude that removing the contributions of the most pronounced magnetic active region variability sources in total and X-ray solar fluxes (i.e., the dark sunspots and brightest X-rays, respectively) reveals mutually correlated variations in coronal, chromospheric and photospheric brightnesses. This suggests a common brightness source for the variations, connected with the bright magnetic structures seen in Ca K images which are already known to be associated with Lyman a irradiance variability [Lean and Repoff, 1987]. Having identified correlated brightness variations in radiative output formed throughout the solar atmosphere, we expect that in the future a detailed analysis of Yohkoh and Ca K images and magnetograms will provide new insights, not only to coronal radiative output variations, but for the variability of the entire solar spectrum.…”

“…The periodograms in Figure 1 reflect the relative importances of these distinct active region variability sources. The strong 27-day peak in the periodogram of the Lyman a data is a direct result of rotational modulation of active region plage emission [Lean and Repoff, 1987]. Significant power at 27 days is absent in both the coronal soft X-ray and photospheric total irradiance data.…”

Thermospheric densities derived from spacecraft orbits: Application to the Starshine satellites

“…A particular difference is that chromospheric flux rotational modulation is smaller, relative to its solar cycle amplitude, than is the case for coronal flux [ Donnelly et al , 1986]. The phasing of the 27‐day rotational modulation also differs for chromospheric and coronal indices [ Lean and Repoff , 1987]. These differences motivate the determination of separate linear relationships for the 81‐day means, F 10.7 A and Mg A , of the two indices, and for the residuals, F 10.7 − F 10.7 A and Mg − Mg A , for the time period of each Starshine mission.…”

Time variations of solar UV irradiance as measured by the Solstice (UARS) instrument

“…The maximum to minimum ratios for these averaged distributions were 1.17, 1.03, and 1.01 for Ly-a, 200-205, a•d 250-255, respectively. The ratio for the averaged Ly-a distribution is only slightly smaller than that given ea•- Analyses of the SBUV and SME observations at the time of strong solar disturbances in 1982 (e.g., Lean and Repoff, 1987;London and Rottman, 1990) also indicated the presence of significant variations of both 27day and 13.5-day oscillations during periods of strong solar disturbance. Although a 13.5-day subharmonic could arise from the asymmetry of the principal 27-day oscillation, as suggested from Figure 3, The magnitudes of irradiance variations over a solar rotation are strongly wavelength dependent.…”

“…The Ly-a data derived from the SME observations (e.g., Rottman, 1988) indicated similar differences in 27-day Ly-a maximum to minimum ratios. It is hoped that documentation of the erage irradiance decrease to solar minimum values during solar cycle 22 will be provided by SOLSTICE observations made from 1992 to 1997.Variations of F10.7 solar irradiance are associated with episodes of strong solar disturbances that are apparent in the chromosphere and lower corona (e.g.,Lean and Repoff, 1987). Time variations of the F10.7 radio irradiance, covering the same period of observations as in Pigure 2amaximum to minimum ratio (~2) at the time of the highly active period than at the moderate period (•1.5).…”