What Irradiance Studies Tell Us about Solar/Stellar Convection and Magnetism
by
Peter Foukal(Heliophysics Inc, Nahant MA, USA)
→
Europe/Stockholm
122:026
122:026
Description
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 that would cancel the spot - induced luminosity dips. Photometric measurements of this heat diffusivity place independent constraints on solar magnetic diffusivities - a key parameter in dynamo models.
Saturation of solar irradiance increase observed at high activity levels suggests 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. Broad band imaging from the balloon – borne Solar Bolometric Imager directly measures the contribution to total irradiance variation of spots and faculae enabling higher sensitivity in the search for larger scale convective efficiency variations. 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.
