docHigh Resolution Views of Solar Faculae
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High Resolution Views of Solar Faculae
High Resolution Views of Solar Faculae Tom Berger Lockheed Martin Solar and Astrophysics Lab [email protected] Outline What the heck are “solar faculae” anyway?? Why do we care? Solar irradiance! Measurements of center-to-limb variation High resolution images of solar faculae High resolution movies of solar faculae Theoretical models of faculae Introduction: what are “faculae”? Faculae = plural of facula - Latin for “small torch” History of observations: Originally discovered near the limb in full-disk images of the Sun, faculae are the small, bright, patterns around dark sunspots and in the “photospheric network”. First modern (photometric) study of faculae by Rogerson (1961). Older studies by Waldmeier and Ten Bruggencate (1939), Lockyer, Secchi, Galileo… First connection to magnetic field by Chapman & Sheeley (1968) and Frazier (1970). First sub-arcsecond (sub 1000 km) resolution imaging by Dunn & Zirker (1973), Mehltretter (1974), and Muller (1980s). Facular appearance and wavelength BBSO Ca II Kline MDI Continuum 28-October-2003 28-October-2003 Why do we care? Small-scale magnetic flux and irradiance The Total Solar Irradiance (TSI) increases by ~0.1% at sunspot maximum: the more dark sunspots there are on the Sun, the brighter it gets. Why? Because bright faculae “outnumber” and outlast dark sunspots and overcompensate the sunspot irradiance deficit. ACRIM Composite TSI Wolf Sunspot Number Courtesy J. Lean Small-scale magnetic flux and irradiance Sunspot irradiance deficit (darkness): easy to model ¾ ¾ ¾ Only weakly dependent on size and wavelength in the visible and UV - not dependent on disk position. Sunspots are relatively large and easy to observe and measure. Physical mechanisms of sunspot darkening are well understood (magnetic suppression of convective heat flow). Facular brightness: difficult to model ¾ ¾ ¾ Depends sensitively on wavelength, size, and disk position. Small-scale magnetic flux ranges in angular size from 1--2 arcsecond (micropores) down to less than 0.1 arcsecond (flux tubes”) – very hard to observe. Hardest thing to measure/model: the Center-to-Limb Variation (CLV) in brightness of faculae as a function of size and/or magnetic field strength. Measurements of CLV Definitions: θ = angle between observer (Earth) and solar surface normal. µ = cosθ, µ = 0 at the limb; µ = 1 at disk center. Contrast = (Ifac – Iqs)/ Iqs Libbrecht & Kuhn ApJ, 1984 “Hot Wall” Model 1.0 0.8 0.6 0.4 Chapman & Klabunde, 525.0 nm ApJ, 1982 0.2 0 ⃝ Wang & Zirin, 525.0 nm Sol Phys, 1987 Measurements of CLV SVST 630.2 nm continuum data Topka, Tarbell, et al. ApJ, 1997 Spatial resolution = 0.5” µ = 0.31 – 0.996 MDI 676.8 nm continuum data Ortiz, Solanki, et al. A&A, 2002 Spatial resolution = 2” High resolution images of faculae AR 10039 24-July-2002 TRACE white light image µ ~ 0.45 Tickmarks = 10 Mm High resolution images of faculae AR 10039 24-July-2002, G-band 430.5 nm, µ = 0.45 Tickmarks = 1 Mm High resolution images of faculae AR 10039 24-July-2002, G-band 430.5 nm, µ = 0.45 Tickmarks = 1 Mm High resolution images of faculae AR 10039 24-July-2002, G-band 430.5 nm, µ = 0.45 Tickmarks = 1 Mm High resolution images of faculae High resolution images of faculae SST G-band, disk center 25-May-03 Disk center 13-May-2003 High resolution images of faculae TRACE WL AR 10377 06-June-2003 µ = 0.6 High resolution images of faculae SST G-band AR 10377 06-June-2003 µ = 0.6 θ = 53° Tickmarks = 1 Mm High resolution images of faculae SST 630.25nm AR 10377 06-June-2003 µ = 0.6 θ = 53° Tickmarks = 1 Mm High resolution images of faculae SST G-band AR 10377 06-June-2003 µ = 0.6 θ =53° 0.81Rs 0.80Rs 0.79Rs 0.78Rs Tickmarks = 1 Mm High resolution images of faculae 2 1 5 4 3 High resolution images of faculae Cut 1 Cut 3 Cut 2 Cut 4 High resolution images of faculae Cut 5 High resolution images of faculae Tickmarks = 1 Mm High resolution images of faculae Tickmarks = 1 Mm High resolution images of faculae High resolution images of faculae High resolution MOVIES of faculae Swedish 1-meter Solar Telescope, La Palma 16-June-2003, G-band 430.5 nm µ = 0.65 Courtesy B. de Pontieu, LMSAL High resolution MOVIES of faculae Swedish 1-meter Solar Telescope, La Palma 16-June-2003, G-band 430.5 nm µ = 0.65 Courtesy B. de Pontieu, LMSAL Theoretical models of faculae “Hot Cloud” or “Hot Hill” models Rogerson (1961), Muller (1975, “Facular Granules”), Hirayama & Moriyama (1979), Schatten (1986, “Hillock & Cloud”) “Hot Wall” flux tube models Spruit (1976), Knölker & Schüssler (1988, 2D MHD models) Theoretical models of faculae 67 – 117 km Topka, et al. ApJ 1997 Zw = Wilson depression = Zw(B), function of magnetic field strength D = diameter of magnetic “flux tube” tau = optical depth. tau=1 is the “surface” from which light is emitted Theoretical models of faculae 3D compressible MHD models Keller, Schussler, et al., ApJL 2004 May Carlsson, Stein, et al., ApJL 2004 July Theoretical models of faculae arcsec µ = 0.5 Conclusions Observations and models are converging. CLV of 3D MHD models replicates observations, including bright faculae out to µ = 0.08 (θ = 88º). Models still lack complexity & number density of bright faculae for equivalent magnetic flux density. Models have yet to demonstrate the dynamic effects seen in the movie data: faculae are dynamic on time scales of minutes. Lateral radiation transfer (“hot wall”) mechanism responsible for facular brightening. But Wilson depression wall is not what we see as bright – it’s an extended sample of the granule behind the flux tube. TBD: Thorough CLV survey at 100 km resolution, including narrowband continuum and other line region wavelengths. Spectral line measurements of temperature, density, velocity, mag field for comparison to models Stokes polarimetry line diagnostics. ab initio facular irradiance calculations using new MHD models.
