Browsing by Department "Professorship Korpi-Lagg Maarit"

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    Helicity proxies from linear polarisation of solar active regions
    (EDP SCIENCES, 2020-09-08) Prabhu, A.; Brandenburg, A.; Kaepylae, M. J.; Lagg, A.; Max Planck Institute for Solar System Research; Carnegie Mellon University; Professorship Korpi-Lagg Maarit; Department of Computer Science
    Context. The α effect is believed to play a key role in the generation of the solar magnetic field. A fundamental test for its significance in the solar dynamo is to look for magnetic helicity of opposite signs both between the two hemispheres as well as between small and large scales. However, measuring magnetic helicity is compromised by the inability to fully infer the magnetic field vector from observations of solar spectra, caused by what is known as the π ambiguity of spectropolarimetric observations. Aims. We decompose linear polarisation into parity-even and parity-odd E and B polarisations, which are not affected by the π ambiguity. Furthermore, we study whether the correlations of spatial Fourier spectra of B and parity-even quantities such as E or temperature T are a robust proxy for magnetic helicity of solar magnetic fields. Methods. We analysed polarisation measurements of active regions observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics observatory. Theory predicts themagnetic helicity of active regions to have, statistically, opposite signs in the two hemispheres. We then computed the parity-odd EB and TB correlations and tested for a systematic preference of their sign based on the hemisphere of the active regions. Results. We find that: (i) EB and TB correlations are a reliable proxy for magnetic helicity, when computed from linear polarisation measurements away from spectral line cores; and (ii) E polarisation reverses its sign close to the line core. Our analysis reveals that Faraday rotation does not have a significant influence on the computed parity-odd correlations. Conclusions. The EB decomposition of linear polarisation appears to be a good proxy for magnetic helicity independent of the π ambiguity. This allows us to routinely infer magnetic helicity directly from polarisation measurements.
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    Physically motivated heat conduction treatment in simulations of solar-like stars: effects on dynamo transitions
    (EDP SCIENCES, 2021-01-26) Viviani, M.; Käpylä, M. J.; Università della Calabria; Professorship Korpi-Lagg Maarit; Department of Computer Science
    Context. Results from global magnetoconvection simulations of solar-like stars are at odds with observations in many respects: Simulations show a surplus of energy in the kinetic power spectrum at large scales; anti-solar differential rotation profiles with accelerated poles, and a slow equator for the solar rotation rate; and a transition from axi-to nonaxisymmetric dynamos at a much lower rotation rate than what is observed. Even though the simulations reproduce the observed active longitudes in fast rotators, their motion in the rotational frame (the so-called azimuthal dynamo wave, ADW) is retrograde, in contrast to the prevalent prograde motion in observations. Aims. We study the effect of a more realistic treatment of heat conductivity in alleviating the discrepancies between observations and simulations. Methods. We use physically motivated heat conduction by applying Kramers opacity law to a semi-global spherical setup that describes the convective envelopes of solar-like stars, instead of a prescribed heat conduction profile from mixing-length arguments. Results. We find that some aspects of the results now better correspond to observations: The axi-to nonaxisymmetric transition point is shifted towards higher rotation rates. We also find a change in the propagation direction of ADWs that means that prograde waves are also now found. However, the transition from an anti-solar to solar-like rotation profile is also shifted towards higher rotation rates, leaving the models in an even more unrealistic regime. Conclusions. Although Kramers-based heat conduction does not help in reproducing the solar rotation profile, it does help in the faster rotation regime, where the dynamo solutions now better match the observations.