Astronomy and Astrophysics – Astronomy
Scientific paper
May 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21621901f&link_type=abstract
American Astronomical Society, AAS Meeting #216, #219.01; Bulletin of the American Astronomical Society, Vol. 41, p.885
Astronomy and Astrophysics
Astronomy
Scientific paper
Despite their enormous thermal inertia, many late - type stars exhibit luminosity fluctuations caused by changing photospheric magnetic structures. These fluctuations exist only because of the high heat diffusivity of stellar convection. Were it lower, the dark spots would be surrounded by intense bright rings, as Gene Parker pointed out in 1974. These rings would cancel the spot - induced luminosity dips. Conversely, dark rings around the bright faculae would cancel their positive luminosity contribution.
Photometric measurements of this heat diffusivity place independent constraints on solar magnetic diffusivities - a key parameter in dynamo models. Irradiance studies also suggest that the structure of emerging magnetic fields shifts toward lower spatial frequencies with increasing activity. This finding could provide new information on the field source function in dynamo models.
Differential and near - IR imaging photometry reveal the decreased temperature gradient of facular magnetic flux tubes and the sunspot- like darkness of their deepest observable layers. Both of these features support current mhd flux tube models. Bolometric imaging measures the wide- band contribution to total irradiance variation, of spot and facular magnetic flux tubes. The remarkably constant solar limb - darkening measured over the past 33 years constrains fluctuations in quiet photospheric temperature gradient and thus, in global convective efficiency over the past three solar cycles.
Reconstruction of irradiance variation over past millennia relies on radio- isotope studies. These provide many interesting insights, but they assume that C14 and Be10 are formed only by solar modulation of the galactic cosmic ray flux. This assumption would break down if solar activity and particle fluxes much exceeded levels experienced in cycle 19. Such a "hyperactive” Sun would vary more in its radiative outputs, be dimmer in total irradiance, although brighter in the EUV and X rays.
Work is supported by NASA grant NNX09AP96G
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