ALMA image of an enormous sunspot taken at a wavelength of 3 millimeters. This image is essentially a map of the temperature differences in a layer of the Sun’s atmosphere known as the chromosphere.

The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful instrument which can be used to observe the Sun at high spatial, temporal, and spectral resolution. ALMA consists of a giant array of 12-m antennas, with baselines up to 16 km, and an additional compact array of 7-m and 12-m antennas. It is located on the Chajnantor plateau, Chile, at an altitude of 5000m.

The solar radiation observed by ALMA originates mostly from the chromosphere. This is a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere.

The ALMA compact array (Image credit: ALMA).


Despite decades of intensive research, the solar chromosphere is still elusive owing to the complicated nature of the partially ionized chromospheric plasma and its intricate interaction with radiation, in addition to technical challenges for observing the emergent radiation. Advances in instrumentation during recent years have led to improved chromospheric diagnostics, sparking renewed attention for this important interface layer.

Radiation at millimeter wavelengths has great diagnostic potential but could not be fully exploited so far due to technical limitations, in particular telescope apertures were too small for the relatively long wavelengths. Consequently, single dish observations did not have sufficient temporal or spatial resolution, and small interferometric arrays could not measure the full range of spatial scales in the chromosphere. ALMA’s unprecedented capabilities will allow solar astronomers to address a wide range of topics and major questions in contemporary solar physics, including the dynamics, thermal structure and energy transport in the “quiet” solar chromosphere, active regions and sunspots, spicules, prominences and filaments, and flares. The solar group at the Armagh Observatory and Planetarium are involved in this project.


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Article by Gerry Doyle, Research Astronomer

Gerry Doyle, Research Astronomer