 
solar magnetism and irradiance
understanding the influence of magnetic field on solar irradiance
This
web page is the product of a team that was set under the auspices of
ISSI on 2004. The aim
of thr team is to find, from the experimental point of
view, a consensus on the present knowledge of irradiance versus
photospheric
magnetic field properties and from the theoretical point of view to
review where we stand on
solar atmosphere models, on magnetic field structures and their effect
on
irradiance, and the progress to be effected.
There
is a clear
direct correlation between photospheric magnetism and irradiance; among
the
earliest published work on the subject is Willson et al. (1981), in
which the
strong correlation between 0.2% decreases in solar irradiance and the
passage
of a large sunspot group across the disk was noted. There is now a
consensus
there is a direct correlation between photospheric magnetism and
irradiance and
it is "almost" proven that photospheric magnetic field is the sole
cause of solar irradiance variations on timescales shorter than one
solar cycle
(on time scales of days to several months). The explanation of
longer time scale variations is less
clear. We are also still far from having a
satisfactory
description of the physics that relate magnetism and solar irradiance,
either
theoretically or experimentally. A goal of the program proposed here is
to
establish an observational and theoretical framework that will allow
extrapolation of the available irradiance measurements to the longer
time
scales needed for application to paleoclimate records and forecasting.
During the last 20 years there has been significant progress both in the observation of magnetic fields in the photosphere and on the study of total solar irradiance, spectral irradiance, and localised sources of irradiance variation through photometrical imaging. A significant part of solar cycle 23 is being observed by several instruments that provide information with state-of-the-art techniques (by SOHO, ACRIMSAT, and SORCE in space, and by ground based observatories at San Fernando Observatory/CSU Northridge, PSPT / Hawaii and Rome, the Kitt Peak SOLIS project, and others). There are several studies that attempt to evaluate the influence of photospheric magnetic elements on the solar irradiance, yet there are significant apparent discrepancies in the results obtained. Recently several studies on the effect of faculae and network on solar irradiance have published results (Preminger et al., 2002, Krivova et al., 2003, Walton et al., 2003, Ermolli et al., 2003) with significant differences in their conclusions. It is interesting to note that the definition of faculae and network is not quite the same in the quoted papers. A question of fundamental importance is the cause of longer term, secular changes in solar irradiance, which are the most interesting for climate studies. The new sensitive magnetometers are now able to detect magnetic signals over a large fraction of the solar surface, i.e., outside the active regions and the network appearing in routine magnetograms (e.g., Kitt Peak mag. or MDI). (See, e.g., Dominguez Cerdeña et al., 2003) The existence of this complex but ubiquitous magnetism will become even more evident with the advent of modern space borne magnetographs (Solar-B or Sunrise), thus it will be mandatory that we understand the role of the ubiquitous magnetism for the solar irradiance variations. A second unanswered observational question is the amount of variability in each wavelength. Unruh, Solanki and Fligge (1999, 2000) and Fontenla et al. 1999 combine model spectra of solar features with information on the area coverage of sunspots and faculae to predict the integrated solar flux spectrum at various stages of the solar cycle. The spectral irradiance variations from space now being made by SORCE/SIM should help clarify these questions, which are important in understanding the effect on Earth. However, computations of radiative transfer through three dimensional magnetic structures is still in an early state, despite of the success of such models in reproducing sensitive observations of bright points in molecular bands (Schüssler et al 2003). It is fair to say that a quantitative understanding of the relation between magnetic field and brightness is still hampered by the fact that it depends critically on the size of the magnetic structures, of which the distribution is poorly known. On the theoretical side, significant advances are being made on several fronts. Three dimensional magnetohydrodynamic simulations (Emonet and Cattaneo 2001) and realistic 3-D compressible radiation-MHD simulations (Vögler and Schüssler 2003; Stein and Nordlund 2002) relevant to magnetic features have recently been carried out. A new generation of instruments is being assembled to operate during solar cycle 24, such as SDO, Solar-B, Picard, SUNRISE, Solar Orbiter (solar cycle 24 or 25), GREGOR telescope, and these will improve our observational data set, but will none of themselves provide the needed relation between irradiance and magnetic field. We aim to put together a baseline description for this relation now, which will be good preparation for the proper exploitation of the new generation of observations. Solar activity and magnetic fields
magnetic
structures
Observations and data processing
Intensity
images and magnetograms MDI calibration issues
(P. Scherrer)
PSTP,
feature identification
methods, modelling
irradiacnce variations (I.
Ermolli)
Total and
spectral solar irradiance
TheoryActive regions and irradianceactive regions enhanced cooling (H. Spruit)MHD simulationssimulations of magnetoconvection (S. Solanki, A. Vögler)Radiative models PSTP_2004, spectrum synthesis (P. Fox)
radiative models .(P. Fox) modelling spectral UV variability (M. Haberreiter)
Comparison of observations and models COSI, model to calculate
solar intensity spectra (M. Haberreiter)
PSTP, feature identification methods, modelling irradiacnce variations (I. Ermolli) The TeamV.
Domingo, University of Valencia, Spain (team leader,
vdomingo@uv.es)
I. Ermolli, INAF Osservatorio Astronomico di Roma, Italy P. Fox, High Altitude Observatory, Boulder, NCAR, Boulder, Colorado C. Fröhlich, Davos Physikalisch-Meteorologisches Observatorium / WRC, Davos, Switzerland G. Kopp, Univ. of Colorado / Laboratory for Atmospheric and Space Physics, Boulder, Colorado M. Haberreiter, Davos Physikalisch-Meteorologisches Observatorium / WRC, Davos, Switzerland J. Pap, Goddard Space Flight Center, Greenbelt, Maryland J. Sánchez Almeida, Instituto de Astrofísica de Canarias, La Laguna, Spain P. Scherrer, Stanford University, California W. Schmutz, Davos Physikalisch-Meteorologisches Observatorium / WRC, Davos, Switzerland M. Schüssler, Max-Planck-Institut für Sonnensystemforschung, Lindau, Germany S. Solanki, Max-Planck-Institut für Sonnensystemforschung, Lindau, Germany H. Spruit, Max Plank Institut für Astrophysik, Garching, Germany Y. Unruh, Imperial College, London, United Kingdom W. Vögler, Max-Planck-Institut für Sonnensystemforschung, Lindau, Germany Splinter groups1) TSI
& spectral
irradiance: Kopp, Fröhlich, Pap, Unruh,
Haberreiter. Responsible: Kopp & Fröhlich 2) Imaging intensity and
magnetic measurements, data analysis and feature identification: Fox,
Scherrer (tbc), Ermolli, Pap, Turmon (tbc), Walton (tbc). Responsible:
Ermolli. 3) Radiative models,
description and comparison with measurements, Fox, Haberreiter,
Unruh, Ermolli. Responsible: Fox |