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Observing the Sun with the Atacama Large Millimeter-submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping  

Stephen White   Submitted: 2017-05-24 16:59

The Atacama Large Millimeter-submillimeter Array (ALMA) radio telescope has commenced science observations of the Sun starting in late 2016. Since the Sun is much larger than the field of view of individual ALMA dishes, the ALMA interferometer is unable to measure the background level of solar emission when observing the solar disk. The absolute temperature scale is a critical measurement for much of ALMA solar science, including the understanding of energy transfer through the solar atmosphere, the properties of prominences, and the study of shock heating in the chromosphere. In order to provide an absolute temperature scale, ALMA solar observing will take advantage of the remarkable fast-scanning capabilities of the ALMA 12 m dishes to make single-dish maps of the full Sun. This article reports on the results of an extensive commissioning effort to optimize the mapping procedure, and it describes the nature of the resulting data. Amplitude calibration is discussed in detail: a path that utilizes the two loads in the ALMA calibration system as well as sky measurements is described and applied to commissioning data. Inspection of a large number of single-dish datasets shows significant variation in the resulting temperatures, and based on the temperature distributions we derive quiet-Sun values at disk center of 7300 K at 3 mm and 5900 K at 1.3 mm wavelengths. These values have statistical uncertainties of order 100 K, but systematic uncertainties in the temperature scale that may be significantly larger. Example images are presented from two periods with very different levels of solar activity. At a resolution of order 25 arcsec, the 1.3 mm wavelength images show temperatures on the disk that vary over about a 2000 K range. Active regions and plage are amongst the hotter features while a large sunspot umbra shows up as a depression and filament channels are relatively cool. Prominences above the solar limb are a common feature of the single-dish images.

Authors: S.M. White, K. Iwai, N.M. Phillips, R.E. Hills, A. Hirota, P. Yagoubov, G. Siringo, M. Shimojo, T.S. Bastian, A.S. Hales, T. Sawada, S. Asayama, M. Sugimoto, R.G. Marson, W. Kawasaki, E. Muller, T. Nakazato, K. Sugimoto, R. Brajsa, I. Skokic, M. Barta, S. Kim, A.J. Remijan, I. de Gregorio, S.A. Corder, H.S. Hudson, M. Loukitcheva, B. Chen, B. De Pontieu, G.D. Fleishmann, D.E. Gary, A. Kobelski, S. Wedemeyer, Y. Yan
Projects: ALMA

Publication Status: Solar Physics, in press
Last Modified: 2017-05-31 13:50
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Updated Expressions for Determining Temperatures and Emission Measures from GOES Soft X-ray Measurements  

Stephen White   Submitted: 2005-03-04 11:49

We investigate the conversion of the 0.5-4 and 1-8 Angstrom soft X-ray flux measurements made by detectors on the Geostationary Operational Environmental Satellites (GOES) into temperature and emission measures of coronal plasma using modern spectral models and modern understanding of coronal abundances. In particular, the original analysis by Thomas, Starr & Crannell (1985) is updated to take into account the realization that coronal abundances may be quite different from photospheric abundances. An important result of this analysis is that the derived temperatures and emission measures depend strongly on the assumed abundances even at high temperatures where continuum rather than spectral lines dominates the Sun's X-ray spectrum. This occurs because the higher coronal abundances mean that most of the continuum is due to free-bound emission processes, not free-free emission, and thus is abundance-dependent. We find significant differences between modern calculations of the temperature response of the flux measurements and the versions currently in use: for a typical flare, emission measures may be up to a factor of 4 smaller than the current software suggests. Derived temperatures are similar for both photospheric and coronal abundances for cool flares (e.g., 15 MK), but for hot flares (e.g., 35 MK) coronal abundances can lead to significantly (of order 25%) lower temperatures being derived.

Authors: S. M. White, R. J. Thomas and R. A. Schwartz
Projects:

Publication Status: Solar Physics, in press
Last Modified: 2005-03-04 11:50
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Observing the Sun with the Atacama Large Millimeter-submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping
Updated Expressions for Determining Temperatures and Emission Measures from GOES Soft X-ray Measurements

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