ALMA – The key to the Sun’s coronal heating problem (SolarALMA)

The SolarALMA project is funded with a Consolidator Grant by the European Research Council (ERC) and runs from September 2016 to August 2021. The aim is to utilize the first observations of the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA), which promises significant steps towards understanding the outer layers of the solar atmosphere with possible contributions to solving the coronal heating problem.

The Sun (top, credits: SDO/NASA) and some of the 66 ALMA antennas (bottom, credits:ALMA/ESO). (Hi-res version.)

Introduction - The chromospheric/coronal heating problem

Illustration of how the gas temperature changes throughout the Sun. After reaching a minimum just above the visible surface, the temperature (blue line) rises again to millions of degrees in the outermost layers.

The layers above the visible "surface" of our Sun (also referred to as the solar atmosphere) consists of - from the solar “surface” outwards -  the photosphere (from where most of the visible light originates), the chromosphere, the narrow transition region, and the corona, which is prominently visible during the solar eclipses. The gas temperature drops slightly with height from 5500o C at the surface before, counter-intuitively, rising in the chromosphere above and eventually jumping to values of a few millions of degrees C in the outermost layers (see the image to the right). Obviously, the outer layers are heated by physical processes, which provide and dissipate energy - a fact known since the late 1930s. Despite many attempts to solve this so-called coronal heating problem during the last decades, it remains one of the large open questions in contemporary solar physics. The same is true for the chromosphere, which is denser than the corona, and thus requires a larger heating rate, beside the fact that both layers are coupled in many ways. The source of high temperatures in the outer layers and corresponding emission of radiation is a general question for stars.

About the SolarALMA project

The key question of the SolarALMA project is how are the outer layers of the Sun heated to such high temperatures. A large number of heating mechanisms have been proposed as explanation but it remains open by which mechanisms are most important. The key to solving the chromospheric/coronal heating problem lies in accurate observations at high spatial, temporal and spectral resolution, facilitating the identification of the mechanisms responsible for the transport and dissipation of energy. This has so far been impeded by the small number of accessible diagnostics and the challenges with their interpretation. The interferometric Atacama Large Millimeter/submillimeter Array (ALMA), located at an altitude of 5000m in the Chilean Andes, now offers impressive capabilities. Due to the properties of the solar radiation at millimeter wavelengths, ALMA serves as a linear thermometer, mapping narrow layers at different heights. It can measure the thermal structure and dynamics of the solar chromosphere and thus sources and sinks of atmospheric heating. Radio recombination and molecular lines (e.g., CO) potentially provide complementary kinetic and thermal diagnostics, while the polarisation of the continuum intensity and the Zeeman effect can be exploited for valuable chromospheric magnetic field measurements. The first regular observations of the Sun with ALMA will begin in late 2016 (Cycle 4). Among them are solar observations in connection with this project (approved in August 2016).

The aim of the SolarALMA project is to develop the necessary diagnostic tools and use them for solar observations with ALMA. The preparation, optimisation and interpretation of these observations will be supported by state-of-the-art numerical simulations. A key objective is the identification of the dominant physical processes and their contributions to the transport and dissipation of energy. The results promise to be a significant step towards solving the coronal heating problem with general implications for stellar activity.

(Click here for the official abstract of the ERC project.)

The research team and job openings

Next to the PI (Sven Wedemeyer), the core team of the SolarALMA project will consist of 3 (post-doctoral) researchers and 2 PhD students. The team will collaborate with the local solar physics group and the international Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON), which are all hosted at the Institute of Theoretical Astrophysics, University of Oslo. Among our many international collaboration partners are the solar ALMA development teams organised by NRAO and ESO.

Active team members

Sven Wedemeyer

(Project leader)

Mikolaj Szydlarski


Andrew Rodger

(PhD student, University of Glasgow, at ITA: 1-5/2016)


New team members starting in 2017

(Post-doctoral) researcher 

(Call for applications closed)

PhD fellowship

(Call for applications closed)


Previous team members

Daniel Wass

(Post-graduate trainee, IAESTE, 11-12/2016)


Future job vacancies

(Post-doctoral) researcher 3

vacant, 2 years (1.9.2018 - 31.8.2020)

PhD fellowship 2

vacant, 4 years (1.9.2018 - 31.8.2022)


Master thesis projects

Master thesis projects can be offered as part of the SolarALMA project. Please contact Sven Wedemeyer for more information.

Tags: Atacama Large Millimeter/submillimeter Array (ALMA), SSALMON, Solar Chromosphere, Observations, Simulations, Japan, USA, Nordic
Published Jan. 26, 2016 11:29 AM - Last modified Feb. 3, 2017 10:08 PM