Solar and stellar science at millimetre and radio wavelengths
Millimetre and radio emission from the Sun and stars provides powerful diagnostics of the thermal and magnetic structure of their outer atmospheres, as well as of particle-acceleration processes. Historically, however, the long wavelengths limited the achievable spatial resolution of telescopes and therefore access to the small scales on which many key physical processes operate. Advances in interferometric aperture synthesis, receiver technology and detector systems are now rapidly transforming the field. At RoCS, we use the Atacama Large Millimeter/submillimeter Array (ALMA) and contribute to the development and scientific preparation of the next generation of facilities, including the Square Kilometre Array Observatory (SKAO), the Atacama Large Aperture Submillimeter Telescope (AtLAST), and the Solaris telescope network.
ALMA science at RoCS
The primary goal of our ALMA activities is to advance our understanding of the solar atmosphere and place the Sun in the broader context of other stars by exploiting ALMA's unique observational capabilities. ALMA observations are combined with complementary data from ground- and space-based observatories and with advanced numerical simulations. The simulations support the physical interpretation of the observations and are also used to develop improved data-processing methods and observing strategies.
RoCS(/ITA) activities related to ALMA began even before regular solar observations started in 2016, with staff contributing to the development of ALMA's dedicated solar observing modes.
From ALMA measurement sets to science-ready data
Solar ALMA observations require substantial processing before they can be transformed into science-ready time series of images. The complexity and novelty of solar interferometric observations make this a particularly challenging task. A major focus of the ERC-funded SolarALMA project at RoCS (2016-2021) was therefore the development and optimisation of data-processing methods, resulting in the Solar ALMA Pipeline (SoAP).
The processing includes calibration and interferometric imaging as well as the accurate co-alignment of ALMA observations with complementary space-based data from missions such as IRIS and SDO. Continued development has significantly improved the quality of reconstructed solar image series. Science-ready data produced with SoAP are made openly available through the Solar ALMA Science Archive (SALSA), hosted at RoCS/ITA.
First scientific results
ALMA observes the Sun in several receiver bands, sampling different characteristic heights in the solar atmosphere and thereby providing complementary views of the chromosphere. RoCS studies have investigated, among other topics, small-scale transient brightenings, signatures of propagating shock waves, and chromospheric oscillations.
Systematic comparisons of ALMA Band 3 and Band 6 observations across different solar environments have revealed notable differences between quiet regions and regions with stronger magnetic fields. The observations also provide new constraints on magnetohydrodynamic wave modes and their role in transporting energy through the solar atmosphere. Together, these studies demonstrate the diagnostic potential of millimetre observations for understanding the structure, dynamics and energy balance of the chromosphere.
Developing new scientific tools
The ALMA team at RoCS continues to contribute to the international development of solar observing modes, gradually expanding the capabilities available to the solar community. RoCS also led the ESO-funded development study “High-cadence imaging of the Sun”, which used synthetic observations to quantitatively assess and optimise solar ALMA observing and processing strategies.
The framework combines realistic Bifrost simulations produced at RoCS, the Advanced Radiative Transfer (ART) code developed in collaboration with J. de la Cruz Rodríguez at Stockholm University, and the Solar ALMA Simulator (SASim). SASim converts data from numerical models into realistic synthetic ALMA observations, which can then be processed with SoAP in the same way as real observations. Comparing the reconstructed images with the original simulation - the known “ground truth” - makes it possible to quantitatively evaluate and improve the observing and imaging procedures.
This approach has also been used to explore future capabilities, including observations with cadences as short as 0.1 s and the use of sub-arrays for observations at multiple frequency bands.
Reaching for the stars
With the start of the EMISSA project in 2019, the ALMA activities at RoCS were extended from the Sun to other stars. Stellar observations provide a much wider range of physical parameters against which the Sun can be compared as our uniquely well-resolved reference star.
EMISSA has compiled a catalogue of stellar ALMA observations complemented by measurements across a broad wavelength range, from the ultraviolet to radio wavelengths. The aim is to characterise the thermal properties of stellar outer atmospheres and their relation to magnetic activity. Comparing these observations with the Sun provides a new route towards understanding how atmospheric heating and magnetic activity vary across different types of stars.
AtLAST: the next step in solar millimetre observations
The proposed, UiO-led Atacama Large Aperture Submillimeter Telescope (AtLAST) is a 50-m single-dish telescope designed for a very large field of view and highly multiplexed instrumentation. For solar observations, AtLAST would complement ALMA in a crucial way: while ALMA provides high spatial resolution over relatively small fields, AtLAST could rapidly map much larger regions of the Sun, potentially including the entire solar disk.
Its broad (sub-)millimetre coverage would allow different layers of the solar atmosphere to be probed, while high-cadence large-area observations would enable systematic studies of chromospheric dynamics, flares and other transient events. Repeated synoptic observations could additionally provide a new view of the solar activity cycle at these wavelengths. RoCS researchers are actively contributing to defining the solar and stellar science cases and the corresponding technical requirements for AtLAST.
The simulation, observing and data-analysis expertise developed at RoCS for ALMA therefore provides a natural foundation for preparing the solar observing capabilities of AtLAST.
SKA: opening the radio corona
The SKA Observatory (SKAO) is constructing two major radio interferometers: SKA-Low in Australia, covering 50-350 MHz, and SKA-Mid in South Africa, covering approximately 0.35-15.4 GHz. Together, they will provide unprecedented capabilities for imaging the dynamic radio Sun across a wide frequency range.
For solar physics, the SKAO will open new possibilities for studying the structure and evolution of the corona, magnetic reconnection, solar flares, coronal mass ejections, and the acceleration and transport of energetic particles. In particular, recent work involving RoCS has investigated how SKA-Mid can resolve fine-scale coronal structures and constrain the plasma processes associated with flares and particle acceleration. (arXiv)
Combined with ALMA, SKA will allow RoCS to follow solar activity from the chromosphere into the corona across an exceptionally broad range of radio wavelengths.
Solaris: continuous monitoring of the millimetre Sun
While ALMA and future facilities such as AtLAST provide detailed observations, many studies of solar activity also require continuous monitoring. The Solaris project is developing compact single-dish radio telescopes for repeated full-disk imaging of the Sun around 100 GHz. Its Antarctic station can exploit the exceptionally long periods of solar visibility during the austral summer to observe the Sun for nearly 20 hours per day.
Solaris therefore provides a complementary capability: moderate spatial resolution but exceptionally high temporal coverage. Continuous observations can follow the long-term evolution of active regions and detect changes associated with flares and other transient activity. The longer-term concept is to combine stations in both hemispheres into a network capable of approaching continuous year-round monitoring of the millimetre Sun. RoCS contributes to this development, including efforts towards extending the network to northern latitudes
Together with targeted high-resolution observations from ALMA and, in the future, AtLAST, Solaris would provide the continuous context needed to connect short-lived events with the longer-term evolution of solar activity.