Solar-stellar atmospheric tomography with mm-radio snapshot spectroscopic imaging

TL;DR

This paper advocates atmospheric tomography of solar and stellar atmospheres by combining millimeter and metrewave snapshot spectroscopic imaging to map heights from the chromosphere to the corona with sub-second cadence. It outlines the observational framework enabled by modern interferometers, introduces data-analysis tools SPREDS and VISAD for extracting fine-grained spectral–spatial information, and reviews key solar and stellar activity science cases, including flares, quasi-periodic pulsations, turbulence, CMEs, and quiet-Sun variability, with ALMA-based chromospheric tomography serving as a pivotal link. The work highlights robust activity indicators such as mm-band brightness-temperature spectra and discusses how these diagnostics extend to unresolved stars out to several hundred parsecs, facilitating cross-type comparisons and improved space-weather characterizations. Overall, the study sets out a path for leveraging large, high-fidelity mm–radio datasets to constrain atmospheric heating, magnetic activity, and particle acceleration across solar-type stars, with broad implications for exoplanet habitability and stellar physics.

Abstract

Millimter (mm) frequencies are primarily sensitive to thermal emission from layers across the stellar chromosphere up to the transition region, while metrewave (radio) frequencies probe the coronal heights. Together the mm and radio band spectroscopic snapshot imaging enables the tomographic exploration of the active atmospheric layers of the cool main-sequence stars (spectral type: FGKM), including our Sun. Sensitive modern mm and radio interferometers let us explore solar/stellar activity covering a range of energy scales at sub-second and sub-MHz resolution over wide operational bandwidths. The superior uv-coverage of these instruments facilitate high dynamic range imaging, letting us explore the morphological evolution of even energetically weak events on the Sun at fine spectro-temporal cadence. This article will introduce the current advancements, the data analysis challenges and available tools. The impact of these tools and novel data in field of solar/stellar research will be summarised with future prospects.

Figures (5)

• Figure 1: Standard flare model showing a flaring active region loop. The magnetic reconnection site is shown with the current sheet and bi-directional accelerated particle beams marked. The different emission regions across all atmospheric layers are also shown (see, text for details). Important spectral lines are mentioned in the box. Evaporation and downflows produce spectral Doppler shifts and assymmetric line profiles. Cliver22_extremeSolarEvents_Rev.
• Figure 2: Graphical summary of the idea of tomographic imaging. (a) Metrewave frequencies arising from varying atmospheric heights as the accelerated electron beams trigger instability across iso-density layers. (b) Millimeter emission contribution functions computed for the different ALMA bands using radiative transfer simulations applied on 3D atmospheric models. Plot for the sun is adapted from Sven16_ALMA_science.
• Figure 3: (a) Top: A sample snapshot spectroscopic image made at 0.5 s and 160 kHz resolution using an MWA dataset from 2014-11-03 Atul19_ARTB_microflare. Red ring marks the optical solar disk and the pink dotted ellipse marks the chosen 2$\times$psf region for deriving SPREDS shown in the Bottom panel. (b): SPREDS derived by fitting 2D Gaussian functions to a type-III bust source in a different dataset Atul21_dNN_vsht.
• Figure 4: (a): STOKES V VISAD for AD Leo (M4V) at 50 s and 5 MHz resolution. (b) Band averaged STOKES I lightcurve with circular polarisation % marked (Mohan et al., in prep).
• Figure 5: (a): mm-T$_\mathrm{B}(\nu)$ for $\alpha$ Cen A. Dotted line shows the photospheric emission model and the green curve shows the best fit powerlaw to T$_\mathrm{B}(\nu)$ with index $\alpha_{mm}$. Different markers show data from different observation cycles (denoted by 'C') and telescopes. (b): $\alpha_{mm}$ versus T$_\mathrm{eff}$ for the ALMA detected stellar sample Atul22_EMISSAII. Bigger markers denoted old ($>$1 Gyr) stars. Red line shows the power-law fit. A-type stars (T$_\mathrm{eff}$$>$ 8000 K) are shown for comparison. (c): $\alpha_{mm}$ versus pressure scale height for the same sample