Measuring Magnetic Field With Atacama Large Millimeter/Submillimeter Array

Abstract

The article discusses the use of magnetic bremsstrahlung at short radio wavelengths for measuring solar magnetic fields. The polarization and brightness spectra observed at millimeter wavelengths can be used to deduce the vertical component of the chromospheric magnetic field both in the quit Sun and in active regions. State-of-the-art three-dimensional (3D) radiative magnetohydrodynamic (R-MHD) simulations of the quiet solar atmosphere were used to synthesize observational deliverables at the wavelengths of the Atacama Large Millimeter/Submillimeter Array (ALMA) and to test the applicability of the method. The article provides selected observational examples of the successful application of the method and presents an overview of the recent developments and potential of the magnetic field measurements with ALMA.

Radio Measurements of the Magnetic Field in the Solar Chromosphere and the Corona

“…Among the new instruments, the Siberian Solar Radioheliograph (SSRH) is starting, a new, solardedicated system for the Owens Valley Radio Observatory-Long Wavelength Array (OVRO-LWA) is nearing completion, the Chinese Mingantu Ultrawide Spectral Radioheliograph (MUSER) is in operation, the next generation VLA (ngVLA) is under consideration, while the Square Kilometer Array (SKA) is not too far below the horizon. The Atacama Large mm and submm Array (ALMA) cannot observe solar circular polarization yet (see Loukitcheva, 2020 in this special research topic collection); when this option becomes available, a 2,000 G sunspot will give an easily measurable polarization of 10% at 3 mm (Equation 23). In addition, the continued operation of existing instruments, such as the Nançay radioheliograph and RATAN-600 must be assured; the shut down of the Nobeyama radioheliograph on March 31, 2020 was a severe loss to the community.…”

Observing the Sun with the Atacama Large Millimeter/submillimeter Array – from continuum to magnetic fields

“…These tasks will require significant efforts from the involved scientific community. The necessary development can and should be guided by forward modelling as presented here and already by, e.g., Loukitcheva et al (2017), Loukitcheva (2020) and Nita et al (2015) (see also references therein). The circular polarisation signals in Quiet Sun regions are expected to be typically smaller than 0.1 % for the currently available receiver bands, whereas a few percent are expected for regions above sunspots.…”

Measuring the magnetic origins of solar flares, CMEs and Space Weather

“…• Measurements at 30 GHz and above made with the ALMA interferometer facility can in principle yield line-of-sight magnetic field strengths using Gelfreikh's (1994) theory of continuum polarization (Loukitcheva 2014(Loukitcheva , 2020), but we have been unable to find reports of solar polarization measurements with ALMA. It can observe from 0.4 to 8.8 mm wavelengths (750 and 34 Ghz respectively), thereby sampling heights from 600 km to 2000 km in the chromosphere.…”

“…However, for weaker fields, the Hanle effect becomes the dominant source of polarized light variations in response to the solar magnetic fields. Following early theoretical work by Belluzzi & Trujillo Bueno (2012), the scattering polarization, Hanle and Zeeman effects across the h and k lines was investigated in semi-empirical 1D models (Alsina Ballester et al 2016;del Pino Alemán et al 2016, 2020. The Hanle field of the k line is 23 G (see Figure 5).…”