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Title: A STUDY OF SOLAR PHOTOSPHERIC TEMPERATURE GRADIENT VARIATION USING LIMB DARKENING MEASUREMENTS

Abstract

The variation in area of quiet magnetic network measured over the sunspot cycle should modulate the spatially averaged photospheric temperature gradient, since temperature declines with optical depth more gradually in magnetic flux tube atmospheres. Yet, limb darkening measurements show no dependence upon activity level, even at an rms precision of 0.04%. We study the sensitivity of limb darkening to changes in area filling factor using a 3D MHD model of the magnetized photosphere. The limb darkening change expected from the measured 11-year area variation lies below the level of measured limb darkening variations, for a reasonable range of magnetic flux in quiet network and internetwork regions. So the remarkably constant limb darkening observed over the solar activity cycle is not inconsistent with the measured 11-year change in area of quiet magnetic network. Our findings offer an independent constraint on photospheric temperature gradient changes reported from measurements of the solar spectral irradiance from the Spectral Irradiance Monitor, and recently, from wavelength-differential spectrophotometry using the Solar Optical Telescope aboard the HINODE spacecraft.

Authors:
 [1];  [2]
  1. National Solar Observatory, Boulder, CO 80303 (United States)
  2. 192 Willow Road, Nahant, MA 01908 (United States)
Publication Date:
OSTI Identifier:
22663824
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 835; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCURACY; ACTIVITY LEVELS; COMPUTERIZED SIMULATION; LIMBS; LIMITING VALUES; MAGNETIC FLUX; MAGNETOHYDRODYNAMICS; PHOTOSPHERE; RADIANT FLUX DENSITY; SENSITIVITY; SOLAR CYCLE; SPACE VEHICLES; SPECTROPHOTOMETRY; SUN; SUNSPOTS; TELESCOPES; TEMPERATURE GRADIENTS; WAVELENGTHS

Citation Formats

Criscuoli, Serena, and Foukal, Peter. A STUDY OF SOLAR PHOTOSPHERIC TEMPERATURE GRADIENT VARIATION USING LIMB DARKENING MEASUREMENTS. United States: N. p., 2017. Web. doi:10.3847/1538-4357/835/1/99.
Criscuoli, Serena, & Foukal, Peter. A STUDY OF SOLAR PHOTOSPHERIC TEMPERATURE GRADIENT VARIATION USING LIMB DARKENING MEASUREMENTS. United States. doi:10.3847/1538-4357/835/1/99.
Criscuoli, Serena, and Foukal, Peter. Fri . "A STUDY OF SOLAR PHOTOSPHERIC TEMPERATURE GRADIENT VARIATION USING LIMB DARKENING MEASUREMENTS". United States. doi:10.3847/1538-4357/835/1/99.
@article{osti_22663824,
title = {A STUDY OF SOLAR PHOTOSPHERIC TEMPERATURE GRADIENT VARIATION USING LIMB DARKENING MEASUREMENTS},
author = {Criscuoli, Serena and Foukal, Peter},
abstractNote = {The variation in area of quiet magnetic network measured over the sunspot cycle should modulate the spatially averaged photospheric temperature gradient, since temperature declines with optical depth more gradually in magnetic flux tube atmospheres. Yet, limb darkening measurements show no dependence upon activity level, even at an rms precision of 0.04%. We study the sensitivity of limb darkening to changes in area filling factor using a 3D MHD model of the magnetized photosphere. The limb darkening change expected from the measured 11-year area variation lies below the level of measured limb darkening variations, for a reasonable range of magnetic flux in quiet network and internetwork regions. So the remarkably constant limb darkening observed over the solar activity cycle is not inconsistent with the measured 11-year change in area of quiet magnetic network. Our findings offer an independent constraint on photospheric temperature gradient changes reported from measurements of the solar spectral irradiance from the Spectral Irradiance Monitor, and recently, from wavelength-differential spectrophotometry using the Solar Optical Telescope aboard the HINODE spacecraft.},
doi = {10.3847/1538-4357/835/1/99},
journal = {Astrophysical Journal},
number = 1,
volume = 835,
place = {United States},
year = {Fri Jan 20 00:00:00 EST 2017},
month = {Fri Jan 20 00:00:00 EST 2017}
}
  • We report an photospheric limb-darkening measurements obtained at the McMath Solar Telescope in 1980 July, September, and October as part of a continuing program to study possible long-term variations in the photospheric emergent flux. A total of 243 usable full-diameter scans was recorded over 7 days in the clean continuum window at 4451.25 A. Individual scans were fitted to a fifth order polynomial in xi = ln..mu.., and a mean limb-darkening curve was derived for each day. The standard deviation of the daily mean of intensities ranged between 0.08% and 0.12% for all ..mu..>0.15. We note that the limb darkeningmore » decreased significantly between September 25 and 26. This change of the limb-darkening curve is not readily understandable in terms of variations in scattered light, seeing, or nonlinear photometric gain. We suggest that it is caused by a decrease of the temperature gradient in the upper photosphere, in the region above approximately tau/sub 5000/ = 0.5. The small increase in effective temperature that might accompany this limb-darkening variation is estimated using a standard radiative equilibrium photospheric model. We note that the increase of effective temperature corresponds in sign with an increase in the solar irradiance reported between September 25 and 26 from both the Nimbus 7 ERB and the SMM ACRIM radiometers.« less
  • Measurements of the contrast at 525 nm of solar faculae are presented. These measurements were obtained with an extreme limb photometer (ELP) in 19775 and 1979 at the San Fernando Observatory. The mean contrast of active regions was determined from the limb inward to 54'', averaged over an annulus determined by the slits of the ELP. We find that the contrast of faculae increases with heliocentric angle theta, from ..mu..roughly-equal0.25 to ..mu..roughly-equal0.065, where ..mu.. = cos theta. The average value of the mean contrast of facular regions is 2.1% with a possible upper limit of about 5% at ..mu..roughly-equal0.1, determinedmore » from regions with the greatest filling factor. This mean contrast, when corrected for a filling factor of 10-15% implies a contrast for individual faculae of 30-50%. The results from the two observing seasons, when combined, are not inconsistent with the ..mu..-dependence of the excess solar oblateness signal measured at Princeton in 1966. The 1979 facular data would have produced an apparent excess oblateness of 66% of the Dicke and Goldenberg value. Because the contrast of faculae appears to increase to within a few arcsec of the limb, the excess facular intensity is probably due not only to ''hot wall'' effects but also to local heating in the upper levels of facular flux tubes.« less