A Model for the Total Solar Irradiance Based on Active Region Decay
- 10 April 2008
- journal article
- research article
- Published by American Astronomical Society in The Astrophysical Journal
- Vol. 677 (1), 723-741
- https://doi.org/10.1086/527433
Abstract
We present a model for the total solar irradiance. The model takes the observed location, timing, and area of emerging active regions as input and produces a time-evolving size distribution of magnetic structures over the solar surface. We assume that the bright magnetic structures (faculae), which counteract the irradiance deficit caused by sunspots, consist of the products of active region decay. We simulate the decay process as a combination of fragmentation and boundary erosion of large-scale magnetic structures. The model has several adjustable parameters that control the decay processes and the irradiance contribution from the quiet Sun and the small-scale magnetic elements that are produced during the decay process. We use a genetic algorithm to estimate these parameters by fitting to the observed irradiance and daily sunspot area time series over the 1978-2007 time interval. Given the simplifications associated with the model, the resultant parameter values are well constrained within the imposed ranges. In addition, the irradiance and daily sunspot area time series produced by the best-fit models agree very well with the observations, although the sunspot area fits tend to perform better than the irradiance fits. However, it is evident that the model is neglecting a significant source of excess brightness, which manifests itself in two ways. First, the optimal values for the lifetime and intensity contrast of the bright, small-scale flux elements are both larger than expected. Second, the synthetic irradiance consistently underestimates the observations during the ascending phase of a cycle, despite the daily sunspot area fitting the observations quite well during these times. We also show that this genetic forward modeling approach can be used to detect a long-term trend of decadal timescale in the quiet-Sun irradiance. Assuming a constant quiet-Sun irradiance, we reconstruct the total solar irradiance over the 1874-1978 time interval, for which observational data of emerging active regions are available.Keywords
This publication has 91 references indexed in Scilit:
- The Rate of Emergence of Magnetic Dipoles in Coronal Holes and Adjacent Quiet-Sun RegionsThe Astrophysical Journal, 2006
- On the size distribution of sunspot groups in the Greenwich sunspot record 1874–1976Astronomy & Astrophysics, 2005
- Global‐Scale Turbulent Convection and Magnetic Dynamo Action in the Solar EnvelopeThe Astrophysical Journal, 2004
- Observational Evidence of Magnetic Flux Submergence in Flux Cancellation SitesThe Astrophysical Journal, 2004
- GONGp‐Mode Frequency Changes with Solar ActivityThe Astrophysical Journal, 1999
- On the Origin of Magnetic Fields in the Quiet PhotosphereThe Astrophysical Journal, 1999
- On the Dynamics of Small-Scale Solar Magnetic ElementsThe Astrophysical Journal, 1996
- Modelling solar irradiance variations with an area dependent photometric sunspot indexSolar Physics, 1994
- Distribution of sunspot umbral areas - 1917-1982The Astrophysical Journal, 1988
- The use of solar faculae in studies of the sunspot cycleSolar Physics, 1980