Ada Ortiz-Carbonell - Solar Irradiance Variability
A whole spectrum of magnetic structures can be found on the solar surface forming a hierarchy with a wide range of sizes, field strengths a degrees of compactness. My research interests are focused on the solar irradiance variability induced by the emergence of such magnetic structures into the solar photosphere, as well as in the intrinsic relationship between magnetism and solar irradiance. Solar magnetism is responsible for the solar activity, which ultimately drives the observed irradiance variations. Specifically, my interests are centered on those magnetic structures that are brighter than the quiet photosphere - faculae and magnetic network elements - and thus produce a positive contribution to the observed irradiance variations.
I am studying the dependence of the facular and network contrast with relevent parameters such as position, magnetic field, size, wavelength and spatial resolution (see Figure 1, that shows the contrast of different magnetic features as a function of their position on the solar disk). The reason to study these dependencies is that a good knowledge of the contrast of those magnetic features is required in order to determine their contribution to solar variability, since the contrast is an important input for models of reconstruction of irradiance variations. In addition, the contrast variation with the position of the feature on the solar disk (CLV or center-to-limb variation) serves as a test for the existing theoretical flux tube models, as the nature of their CLV is driven by the tube properties. Often contrast studies have analyzed the dependence of the contrast with position over the solar disk, and less frequently with magnetic flux or wavelength; however the dependence with the resolution has been barely investigated although it plays an important role when comparing observations with MHD (magnetohydrodynamic) simulations. For this, I am comparing contrast measurements as taken by the MDI instrument on board the SOHO spacecraft (one wavelength, 4" resolution) with those measurements taken with the PSPT telescope (Precision Solar Photometric Telescope, Mauna Loa Solar Observatory, Hawaii) at three different wavelengths and a resolution of 1". This work is currently in progress and will be published in the following months. I expect to have a more complete view on the radiative properties of small magnetic elements present in the solar surface.
Together with Mark Rast (formerly at HAO/NCAR) I am studying the latitudinal variation of the photospheric intensity. In order to reproduce differential rotation profiles similar to those observed, solar differential rotation models must impose latitudinal variations of the entropy in the tachocline (the tachocline is the transition layer between two distinct rotational regimes in the interior of the Sun: the differentially-rotating solar envelope and the radiative interior where the rotation is uniform). This entropy variation is transported through the convection zone and translates into an expected temperature (or equivalently irradiance) difference of about 10 K between the equator and the poles. However, up to now photometers did not have enough photometric precision to detect such small intensity variations, and hence those variations had not been totally confirmed. The PSPT telescope has allowed us to accurately detect such latitudinal variations thanks to its 0.1% photometric precision per pixel, as well as observing an increase of the population of the smallest magnetic features towards the poles, likely due to their poleward accumulation produced by meridional circulation. This latitudinal variation of the photospheric intensity is entirely thermal and therefore not related to solar activity. The PSPT CaIIK images can be used as a proxy for the magnetic flux, and therefore we have used the best quality CaIIK images to mask out any activity found on the solar disk. These results will be published soon and will be an observational confirmation for a theoretical prediction.ASP Spotlight September 2006
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