ALMA observations of the Sun
The Atacama Large Millimeter/submillimeter Array (ALMA) is a valuable tool for observing the chromosphere of our Sun at (sub-)millimeter wavelengths at high spatial, temporal, and spectral resolution with large potential to address long-standing scientific questions in solar physics. The interferometric array is located on the Chajnantor plateau in the Chilean Andes at an altitude of 5000 m and consists of 66 antennas (most with a diameter of 12 m). By combining the antennae, they act like a giant telescope with baselines of up to 16 km.
ALMA started to offer solar observing in 2016 and since then increased the offered capabilities. As the Sun differs from most other astronomical objects by being an extended source in the sky that is dynamic on very short timescales, customized observing and post-processing strategies are needed.
The role of numerical simulations
Numerical simulations of the solar chromosphere play an important role in the planning, optimization, and interpretation of solar ALMA observations. Synthetic brightness temperature maps, which are calculated from numerical models, can be used to simulate what ALMA would observe (see animation to the right). This way, different instrumental set-ups can be tested and adjusted to the scientific requirements. The activities of our network will focus on all related simulation and modelling aspects from calculating models of the solar atmosphere, producing synthetic brightness temperature maps, applying instrumental effects to comparison with real ALMA observations of the Sun. The general procedure is outlined in the illustration below (click for large version).
The SSALMONetwork
The network was initiated in 2014 in preparation for the first regular observations of the Sun with ALMA and grew to 87 members from 20 countries across the globe. This initiative was tightly coupled to international development studies that worked on the implementation of solar observing modes:
- "Advanced Solar Observing Techniques" - A project within the North American Study Plan for Development Upgrades of the ALMA
(PI: T. Bastian, National Radio Astronomy Observatory (NRAO), USA - "Solar Research with ALMA" - A project carried out at the Czech ARC node of European ALMA Regional Center (EU ARC at Ondrejov, Czech Republic) in the frame of the ESO program "Enhancement of ALMA Capabilities/EoC",
(PI: Roman Brajsa, Hvar Observatory, Croatia).
Key goal 1 - Raising awareness of science opportunities with ALMA.
Simulations should anticipate and stimulate the solar physics community in thinking about the science opportunities with ALMA. Such simulations demonstrate what could be possibly observed with ALMA and which scientific problems could therefore be addressed in the future. Many scientific topics, which are currently worked on with other instruments, could possibly also be investigated by using ALMA. Many scientists outside the solar radio community are certainly not fully aware of the opportunities and potential of ALMA. Motivating these colleagues to consider ALMA observations for their research could strengthen the visibility and potential throughput of the solar ALMA community.
Key goal 2 - Clear visibility of solar science within the ALMA community.
Solar observations differ technically from other ALMA observations. Consequently, solar observation runs - apart from initial tests - have been postponed to later observation cycles. We should use the time to demonstrate the wider ALMA community that solar observations have a large potential to produce fascinating and important scientific results and solar observations deserve higher priority than it is currently the case. Simulations have and can produce illustrative results, which not only demonstrate the feasibility of solar applications but which can also be used to make the scientific potential more visible to the wider ALMA community.
Key goal 3 - Constraining ALMA observing modes with numerical simulations.
"Observing simulations", i.e., artificial observations based on numerical models of the solar atmosphere are a valuable tool, which will help to constrain and optimize ALMA observing modes. Such simulations will show what kinds of observations should ALMA perform in order to usefully test science questions and which of these scientific applications are most promising in view of ALMA's wavelength range and angular resolution. Observing simulations should be used to examine relevant issues like observing bands, antenna configuration, pointing strategy (single pointing, mosaic, on-the-fly mapping), time resolution, etc. This step will involve simulating an observation with the instrument and assessing the results (i.e., determining whether a given simulation adequately addresses the science goals). As a result, recommendations regarding the appropriate observing strategy would be made for particular science goals. On this basis, the development study groups would then consider how best to implement corresponding supported observing modes. In summary, the observing simulations serve two purposes:
- Demonstration of the feasibility of using ALMA to address the science questions raised.
- Definition of the specific requirements that ALMA operations must satisfy in order to successfully support such observations.
Highlighted results
The major results have been summarised in several publications, most notably the following two:
Solar Science with the Atacama Large Millimeter/Submillimeter Array - A New View of Our Sun
Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu, B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin, P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H.-G.; Benz, A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.; Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary, D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.; Selhorst, C. L.; Barta, M.
Space Science Reviews, Vol. 200, 1-4, 1-73 (2016)
- ADS link: http://adsabs.harvard.edu/doi/10.1007/s11214-015-0229-9
- Published Online First (in 2015, see below)
- DOI 10.1007/s11214-015-0229-9
- Space Science Reviews: ISSN 0038-6308
SSALMON White Paper - ALMA OBSERVATIONS OF THE SUN IN CYCLE 4 AND BEYOND
Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.; Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.; Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson, M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.; Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov, V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões, P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig, A.; White, S.; Yagoubov, P.; Zaqarashvili, T.
- Latest version: 1.2 (March 29th, 2016)
- ADS link: http://adsabs.harvard.edu/abs/2016arXiv160100587W
- Astro-ph link: http://arxiv.org/abs/1601.0058
