what are faculae?

Solar-B 6th Science Meeting – Kyoto, Japan Tuesday 8-November-2005

What are Faculae?

Tom Berger, Alan Title, Ted TarbellLockheed Martin Solar and Astrophysics Lab

Luc Rouppe van der VoortInstitute for Theoretical Astrophysics, Oslo Norway

Mats Löfdahl, Göran ScharmerInstitute for Solar Physics, Royal Swedish Academy of Sciences


Introduction• Why study faculae?

90% (or 100%?) of the variation of total solar irradiance is accounted for by the “competition” between dark sunspots and bright facular and network areas.

Sunspot contrast is independent of disk position but facular contrast depends sensitively on disk position and wavelength: faculae are brightest near the limb and (sometimes) dark at disk center.

Accurate knowledge of the center-to-limb variation (CLV) of facular contrast is a key input to solar irradiance models.

• What do we know about faculae to date?They are associated with magnetic fields and have been theorized (and modeled) to be the hot walls of thin flux tubes in the photosphere.

Measurements of facular contrast CLV as a function of magnetic flux density have not been conclusive – spatial resolution has been a challenge.


Swedish 1-meter Solar Telescope: 2002


Tickmarks = 1 Mm

SST G-bandAR 1037706-June-2003 = 0.6 = 53°


SST 630.25nmAR 1037706-June-2003 = 0.6 = 53°

Tickmarks = 1 Mm


0.80Rs

0.79Rs0.78Rs

0.81Rs

SST G-bandAR 1037706-June-2003 = 0.6 =53°


1

2

3

4

5


Cut 1 Cut 2

Cut 3 Cut 4


Cut 5


Facular brightening ~ 400 km


Topka et al., ApJ 1997

Previous Analysis: Image Contrast vs. Magnetic Flux Density


430.5 nm G-band Facular Contrast vs. Magnetic Flux Density


436.4 nm continuum Facular Contrast vs. Magnetic Flux Density


Facular Contrast vs. Line-of-sight Angle


16-June-2003 G-band faculae: = 0.65 = 49º


3D compressible MHD models Carlsson, Stein, et al., ApJL 2004 July Keller, Schussler, et al., ApJL 2004 May


From Keller, Schussler, et al., ApJL 2004 May


Conclusions• Facular contrast as a function of magnetic flux density is

constant down to 200 G levels. Previous measurements had insufficient spatial resolution and binned on magnetogram signal thus blending micropores with the bright faculae.

• Magnetic element contrast is a strong function of flux density: magnetic elements are dark below ~900 G.Beck, Schmidt, et al., A&A 2005.

• Faculae are not magnetic element bright-points seen near the limb – they are not the hot walls of “flux tubes” Faculae are granules seen through the magnetic field.

– This explains why faculae appear so much “deeper” than flux tube hot walls and are much wider than any magnetic element bright points seen near disk center.

– Richard Muller was right after all to call them “facular granules” way back in 1982.


Solar-B Contributions

• Continual observations of uniform quality allows – Statistical studies of center-to-limb facular irradiance function.– “Irradiance” history of active regions from emergence to decay.– Joint observations of facular dynamics with bolometric

instruments in space (e.g. EVE on SDO)

• Duration of mission from solar minimum to solar maximum allows– Accurate study of the phase of irradiance cycle relative to

sunspot cycle.– Settlement of the irradiance vs. luminosity argument – are there

other contributions to TSI besides sunspots and faculae/network fields?

what are faculae?

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