  This study aims at setting observational constraints on the continuum and
line core intensity contrast of network and faculae, specifically, their
relationship with magnetic field and disc position. Full-disc magnetograms and
intensity images by the HMI instrument onboard SDO were employed. Bright
magnetic features, representing network and faculae, were identified and the
relationship between their intensity contrast at continuum and line core with
magnetogram signal and heliocentric angle examined. Care was taken to minimize
the inclusion of the magnetic canopy and straylight from sunspots and pores as
network and faculae. In line with earlier studies, network features, on a per
unit magnetic flux basis, appeared brighter than facular features. Intensity
contrasts in the continuum and line core differ considerably, most notably,
they exhibit opposite centre-to-limb variations. We found this difference in
behaviour to likely be due to the different mechanisms of the formation of the
two spectral components. From a simple model based on bivariate polynomial fits
to the measured contrasts we confirmed spectral line changes to be a
significant driver of facular contribution to variation in solar irradiance.
The discrepancy between the continuum contrast reported here and in the
literature was shown to arise mainly from differences in spatial resolution and
treatment of magnetic signals adjacent to sunspots and pores. HMI is a source
of accurate contrasts and low-noise magnetograms covering the full solar disc.
For irradiance studies it is important to consider not just the contribution
from the continuum but also from the spectral lines. In order not to
underestimate long-term variations in solar irradiance, irradiance models
should take the greater contrast per unit magnetic flux associated with
magnetic features with low magnetic flux into account.
