Abstract
Aims. We aim to gain insight into the effect of network and faculae on solar irradiance from their apparent intensity. Methods. Taking full-disc observations from the Solar Dynamics Observatory, we examined the intensity contrast of network and faculae in the continuum and core of the Fe I 6173 Å line and 1700 Å, including the variation with magnetic flux density, distance from disc centre, nearby magnetic fields, and time. Results. The brightness of network and faculae is believed to be suppressed by nearby magnetic fields from its effect on convection. We note that the degree of magnetically crowding of an area also affects the magnetic flux tube sizes and the depth at which magnetic concentrations are embedded in intergranular lanes, such that intensity contrast can be enhanced in magnetically crowded areas at certain flux densities and distances from disc centre. The difference in intensity contrast between the quiet-Sun network and active region faculae, noted by various studies, arises because active regions are more magnetically crowded and is not due to any fundamental physical differences between network and faculae. These results highlight that solar irradiance models need to include the effect of nearby magnetic fields on network and faculae brightness. We found evidence that suggests that departures from local thermal equilibrium (LTE) might have limited effect on intensity contrast. This could explain why solar irradiance models that are based on the intensity contrast of solar surface magnetic features calculated assuming LTE reproduce the observed spectral variability even where the LTE assumption breaks down. Certain models of solar irradiance employ chromospheric indices as direct indications of the effect of network and faculae on solar irradiance. Based on past studies of the Ca II K line and on the intensity contrast measurements derived here, we show that the fluctuations in chromospheric emission from network and faculae are a reasonable estimate of the emission fluctuations in the middle photosphere, but not of those in the lower photosphere. This is due to the different physical mechanisms that underlie the magnetic intensity enhancement in the various atmospheric regimes, and represents a fundamental limitation of these solar irradiance models. Any time variation in the radiant properties of network and faculae is, of course, relevant to their effect on solar irradiance. The data set, which extends from 2010 to 2018, indicates that their intensity contrast was stable to about 3% in this period. Conclusions. This study offers new insights into the radiant behaviour of network and faculae, with practical implications for solar irradiance modelling.
Highlights
The variation in the solar radiation that enters the Earth’s atmosphere is a key consideration in the climate change debate (Gray et al 2010)
This is due to the effect of how magnetically crowded a given region is on flux tube sizes and how deeply embedded magnetic concentrations are in intergranular lanes
We examined the variation in intensity contrast with distance from disc centre and magnetic flux density, B, the average magnetic flux density in the greater area about each point, called the local magnetic flux density, B loc, and time
Summary
The variation in the solar radiation that enters the Earth’s atmosphere is a key consideration in the climate change debate (Gray et al 2010). Making use of similar HMI data and simultaneous observations from the Atmospheric Imaging Assembly (AIA; Lemen et al 2012), on board SDO, we study the intensity contrast in the AIA 1700 Å channel along that in the Fe I 6173 Å line In another advance, we examine the CLV and magnetogram signal dependence, and the variation in intensity contrast with the average magnetic flux density over the neighbourhood of each network and facular image pixel, called here the local magnetic flux density, and time. We extend this earlier study by examining the variation in intensity contrast with local magnetic flux density across the entire solar disc This is to shed light on how the observation that network has a higher heating efficiency than faculae might be appropriately described in solar irradiance models. AIA 4500 Å images have severe artefacts that are caused by damage to a filter
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