Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): High-Resolution Interferometric Imaging

Abstract

Observations of the Sun at millimeter and submillimeter wavelengths offer a unique probe into the structure, dynamics, and heating of the chromosphere; the structure of sunspots; the formation and eruption of prominences and filaments; and energetic phenomena such as jets and flares. High-resolution observations of the Sun at millimeter and submillimeter wavelengths are challenging due to the intense, extended, low-contrast, and dynamic nature of emission from the quiet Sun, and the extremely intense and variable nature of emissions associated with energetic phenomena. The Atacama Large Millimeter/submillimeter Array (ALMA) was designed with solar observations in mind. The requirements for solar observations are significantly different from observations of sidereal sources and special measures are necessary to successfully carry out this type of observations. We describe the commissioning efforts that enable the use of two frequency bands, the 3-mm band (Band 3) and the 1.25-mm band (Band 6), for continuum interferometric-imaging observations of the Sun with ALMA. Examples of high-resolution synthesized images obtained using the newly commissioned modes during the solar-commissioning campaign held in December 2015 are presented. Although only 30 of the eventual 66 ALMA antennas were used for the campaign, the solar images synthesized from the ALMA commissioning data reveal new features of the solar atmosphere that demonstrate the potential power of ALMA solar observations. The ongoing expansion of ALMA and solar-commissioning efforts will continue to enable new and unique solar observing capabilities.

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Notes

  1. 1.

    Atacama Compact Array (ACA: also known as the Morita Array) is a short-spacing imaging system consisting of the TP array and 7-m array (Iguchi et al., 2009).

  2. 2.

    The URLs of the “Scientific Verification Data” in each ARC web site are as follows:

    EA-ARC: almascience.nao.ac.jp/alma-data/science-verification ;

    EU-ARC: almascience.eso.org/alma-data/science-verification ;

    NA-ARC: almascience.nrao.edu/alma-data/science-verification .

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Acknowledgments

The ALMA solar-commissioning effort was supported by ALMA Development grants from NAOJ (for the East Asia contribution), NRAO (for the North American contribution), and ESO (for the European contribution). The help and cooperation of engineers, telescope operators, astronomers-on-duty, Extension and Optimization of Capabilities (EOC; Formerly Commissioning and Science Verification) team, and staff at the ALMA Operations Support Facility was crucial for the success of solar-commissioning campaigns in 2014 and 2015. We are grateful to the ALMA project for making solar observing with ALMA possible. This article makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00020.SV, ADS/JAO.ALMA#2011.0.00001.CAL. ALMA is a partnership of ESO (representing its member states), NSF (USA), NINS (Japan), together with NRC (Canada), NSC, ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. SDO is the first mission to be launched for NASA’s Living With a Star (LWS) Program. IRIS is a NASA small explorer mission developed and operated by LMSAL with mission operations executed at NASA Ames Research center and major contributions to downlink communications funded by ESA and the Norwegian Space Center. This work was partly carried out on the solar data-analysis system and common-use data-analysis computer system operated by the Astronomy Data Center of NAOJ. M. Shimojo was supported by JSPS KAKENHI Grant Number JP17K05397. R. Brajša acknowledges partial support of this work by Croatian Science Foundation under the project 6212 Solar and Stellar Variability and by the European Commission FP7 project SOLARNET (312495, 2013 – 2017), which is an Integrated Infrastructure Initiative (I3) supported by the FP7 Capacities Program. G.D. Fleishman acknowledges support from NSF grants AGS-1250374 and AGS-1262772. The trip of Y. Yan to 2015 ALMA Solar Campaign was partially supported by NSFC grant 11433006. S. Wedemeyer acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No. 682462).

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This article is a companion of the article available at doi: 10.1007/s11207-017-1123-2 .

Appendix

Appendix

ALMA antennas measure the two orthogonal linear polarizations X and Y, and the 64-input baseline correlator measures the products of the linearly polarized antenna voltages. For a pair of antennas, \(m\) and \(n\), the correlation products are \(v_{\mathrm {x}_{\mathrm{m}} \mathrm{x}_{\mathrm{n}}}\), \(v_{\mathrm {y}_{\mathrm{m}}\mathrm{y}_{\mathrm{n}}}\), \(v_{\mathrm {x}_{\mathrm{m}} \mathrm{y}_{\mathrm{n}}}\), and \(v_{\mathrm {y}_{\mathrm{m}} \mathrm{x}_{\mathrm{n}}}\). For well-designed antenna feeds and weakly polarized emission (Cotton, 1999), the response of the interferometer can be expressed as

$$\begin{aligned} v'_{\mathrm {xx}} =& g_{\mathrm {mx}} g^{\ast}_{\mathrm {nx}} (I + Q \cos2\chi+ U \cos 2\chi) + \sigma'_{\mathrm {xx}} , \\ v'_{\mathrm {xy}} =& g_{\mathrm {mx}} g^{\ast}_{\mathrm {ny}} \bigl(\bigl(d_{\mathrm {mx}}-d^{\ast}_{\mathrm {ny}}\bigr)I - Q \cos2 \chi+ U \cos2\chi+ jV\bigr) + \sigma'_{\mathrm {xy}} , \\ v'_{\mathrm {yx}} =& g_{\mathrm {my}} g^{\ast}_{\mathrm {nx}} \bigl(\bigl(d^{\ast}_{\mathrm{nx}}-d_{\mathrm {my}}\bigr)I - Q \cos2 \chi+ U \cos2\chi- jV\bigr) + \sigma'_{\mathrm {yx}}, \\ v'_{\mathrm {yy}} =& g_{\mathrm {my}} g^{\ast}_{\mathrm {ny}} (I - Q \cos2\chi- U \cos 2\chi) + \sigma'_{\mathrm{yy}}, \end{aligned}$$

where \(I\) is the Stokes parameter describing the total intensity of the radiation, \(Q\) and \(U\) are the Stokes parameters characterizing linearly polarized radiation, and \(V\) is the Stokes parameter characterizing circularly polarized radiation. The parallactic angle [\(\chi\)] includes the effects of rotation of the alt–az ALMA antennas as viewed from the source. The \(g\)-factors are complex gain factors established by calibration and the \(d\)-terms represent polarization “leakage” which, by careful design, are small but measurable complex numbers, also determined by calibration. The noise in each correlation measurement is represented by \(\sigma'\). At present ALMA does not support full Stokes polarimetry and in particular, measurements of Stokes-\(V\), which requires calibration of the complex leakage terms. However, it is expected that support of full Stokes polarimetry will be implemented soon, thereby enabling a powerful new probe of chromospheric magnetic fields. For the present purpose, however, only the parallel correlations are of interest here. Rearranging \(v_{xx}\) and \(v_{yy}\) we have

$$\begin{aligned} (I + Q \cos2\chi+ U \cos2\chi) =& v'_{\mathrm {xx}}/ \bigl(g_{\mathrm {mx}} g^{\ast}_{\mathrm {nx}}\bigr) + \sigma_{\mathrm {xx}}'/\bigl(g_{\mathrm {mx}} g^{\ast}_{\mathrm {nx}} \bigr) = v_{\mathrm {xx}} + \sigma_{\mathrm {xx}}, \\ (I - Q \cos2\chi- U \cos2\chi) =& v'_{\mathrm {yy}}/ \bigl(g_{\mathrm {my}} g^{\ast}_{\mathrm {ny}}\bigr) + \sigma_{\mathrm {yy}}'/\bigl(g_{\mathrm {my}} g^{\ast}_{\mathrm {ny}} \bigr) = v_{\mathrm {yy}} + \sigma_{\mathrm {yy}}, \end{aligned}$$

where the unprimed quantities represent calibrated measurements. Summing and differencing these quantities and propagating the noise terms yields

$$\begin{aligned} I =& {1\over 2}(v_{\mathrm {xx}} + v_{\mathrm {yy}}) + \sigma_{\mathrm {I}}, \\ Q \cos2\chi +& U \cos2\chi= {1\over 2}(v_{\mathrm {xx}} - v_{\mathrm {yy}})+ \sigma_{\mathrm {I}}, \end{aligned}$$

where \(\sigma_{\mathrm {I}}=\sqrt{\sigma^{2}_{\mathrm {xx}}+\sigma ^{2}_{\mathrm {yy}}}/2\). It is seen that the sum of the calibrated correlation products \(v_{\mathrm {xx}}\) and \(v_{\mathrm {yy}}\) for a given antenna pair represents the interferometer’s response to Stokes-\(I\). While the Stokes-\(V\) parameter may be non-zero, the Stokes-\(Q\) and \(U\) parameters are expected to be zero for thermal solar emission and so \((v_{\mathrm {xx}} - v_{\mathrm {yy}})/2 = \sigma_{\mathrm {I}}\). Note further that, for emission that is not linearly polarized, the calibrated noise terms are such that \(\sigma_{\mathrm {xx}}=\sigma_{\mathrm {yy}}\) and so \(\sigma_{\mathrm{ I}}=\sigma _{\mathrm {xx}}/\sqrt{2} = \sigma_{\mathrm {yy}}/\sqrt{2}\). Since synthesis maps represent a linear superposition of interferometric measurements, the same relation holds true for synthesis images.

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Shimojo, M., Bastian, T.S., Hales, A.S. et al. Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): High-Resolution Interferometric Imaging. Sol Phys 292, 87 (2017). https://doi.org/10.1007/s11207-017-1095-2

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Keywords

  • Radio emission, millimeter wave
  • Interferometer, ALMA
  • Instrumentation and data management