Abstract
This article reviews the use of magnetic bremsstrahlung at short radio wavelengths to measure solar magnetic fields. The vertical component of the chromospheric magnetic field can be deduced from the observed polarization and brightness temperature spectrum at millimeter wavelengths. State-of-the-art 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.
Highlights
Magnetic field can affect radio emission in two ways: via the Lorentz force, which causes the emitting electrons to spiral in the magnetic field providing a direct source of opacity for gyroemission, and via modification of the plasma response to electromagnetic fields, leading to different refractive index and polarization for the ordinary (o) and extraordinary (x) magnetoionic wave modes
While gyroresonance emission from non-relativistic plasma at low harmonics of the gyrofrequency, which is an example of the direct effect of magnetic field on opacity, is responsible for the coronal emission of non-flaring solar active regions observed at centimeter wavelengths, the thermal bremsstrahlung is dominant in the corona of the quiet Sun (QS), as well as in more dense chromospheric layers
This chapter presented a particular case of measuring the magnetic field at the chromospheric heights from the thermal free–free observed at the mm wavelengths in the light of the future circular polarization measurements with Atacama Large Millimeter/Submillimeter Array (ALMA)
Summary
Magnetic field can affect radio emission in two ways: via the Lorentz force, which causes the emitting electrons to spiral in the magnetic field providing a direct source of opacity for gyroemission, and via modification of the plasma response to electromagnetic fields, leading to different refractive index and polarization for the ordinary (o) and extraordinary (x) magnetoionic wave modes. While gyroresonance emission from non-relativistic plasma at low harmonics of the gyrofrequency, which is an example of the direct effect of magnetic field on opacity, is responsible for the coronal emission of non-flaring solar active regions observed at centimeter (cm) wavelengths, the thermal bremsstrahlung is dominant in the corona of the quiet Sun (QS), as well as in more dense chromospheric layers. Submillimeter (submm) emission gets optically thick already at the heights near the temperature minimum and in the lower chromosphere, while the emission at longest millimeter (mm) wavelengths originates from the upper chromosphere and the transition region This makes observations of free–free emission at mm and submm wavelengths a vital source of information about the enigmatic layers of the solar atmosphere, which play a significant role in defining the dynamics and energy budget of the solar corona, and of the solar wind. Polarization measurements contain information about the magnetic field strength, but indirectly, via the temperature gradient
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