Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsh23a1625b&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SH23A-1625
Other
7524 Magnetic Fields, 7529 Photosphere, 7537 Solar And Stellar Variability (1650), 7538 Solar Irradiance
Scientific paper
On September 13 2007, the Solar Bolometric Imager (SBI) observed the Sun in wide band spectrally integrated for 16 hours while suspended from a balloon at ~120,000 feet altitude above New Mexico. SBI represents a totally new approach in finding the sources of the solar irradiance variation. Its detector is an array of 320x240 thermal IR elements whose spectral sensitivity has been extended and flattened by a layer of gold-black deposited on its IR sensitive surface. The combination of bolometric array and telescope, a 30- cm Dall-Kirkham with uncoated primary and secondary Pyrex mirrors, provide an image of the Sun with constant spectral response between ~ 280 and 2600 nm, over a field of view of 960 x 720 arcsec with a pixel size of 3 arcsec. The September 13, 2007 flight provided bolometric (integrated light) maps of the photosphere when the Sun was near a minimum of activity. At the time of the flight no active regions were present giving us the opportunity to measure with high accuracy the bolometric contrast of the weak solar magnetic network from Sun center to the limb. The network was easily detectable by SBI near the limb. We measured an average bolometric contrast of ~ 0.8 to 1.0 %, which is slightly above the 5-minute oscillation brightness signal (the most prominent solar induced noise source for us). We were also able to detect the bolometric brightness signature of network near Sun center by averaging 720 bolometric images taken close to Sun center over a period of 1 hour. The resulting RMS noise was < 0.02% and most of the 5-minute oscillation brightness was removed in the average. This enabled us to measure an average network bolometric contrast at Sun center of 0.25% with a spread of about ± 0.05%. Ours is the first bolometric measurement (constant spectral sensitivity from 280 to 2600 nm) of the center-to-limb contrast of magnetic network. Our observations demonstrate that SBI can accurately measure the bolometric contrast of even quiet network across the solar disk. These measurements will enable a more precise estimate of the TSI contribution from changes of the enhanced magnetic network, which consist of larger elements than the quiet network. This will enable us to determine whether other low level brightness sources besides faculae and spots contribute to TSI and evaluate their possible long term influence in TSI change and climate.
Bernasconi Pietro N.
Foukal Peter V.
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