Abstract
Context. The study of solar oscillations (helioseismology) has been a very successful method of researching the Sun. Helioseismology teaches us about the structure and mean properties of the Sun. Together with mid-resolution data, the local properties were uncovered in quiet-Sun regions. However, magnetic fields affect the oscillations and prevent us from studying the properties of magnetically active regions with helioseismology. Aims. We aim to create a new methodology to suppress the negative effects of magnetic fields on solar oscillations and measure plasma properties close to active regions. Methods. The methodology consists of new averaging geometries, a non-linear approach to travel-time measurements, and a consistent inversion method that combines plasma flows and sound-speed perturbations. Results. We constructed the one-sided arc averaging geometries and applied them to the non-linear approach of travel-time measurements. Using the one-sided arc travel times, we reconstructed the annulus travel times in a quiet-Sun region. We tested the methodology against the validated helioseismic inversion pipeline. We applied the new methodology for an inversion for surface horizontal flows in a region with a circular H-type sunspot. The inverted surface horizontal flows are comparable with the output of the coherent structure tracking, which is not strongly affected by the presence of the magnetic field. We show that the new methodology suppresses the negative effects of magnetic fields up to outer penumbra. We measure divergent flows with properties comparable to the moat flow. Conclusions. The new methodology can teach us about the depth structure of active regions and physical conditions that contribute to the evolution of the active regions.
Highlights
Helioseismology studies the solar interior via properties of the solar oscillations caused by the sound and the surface gravity waves that propagate through the Sun
We show that the new methodology suppresses the negative effects of magnetic fields up to outer penumbra
We introduce a new type of averaging geometry: one-sided arc geometries
Summary
Time–distance helioseismology estimates the properties of the plasma flows and the sound-speed perturbations via the travel times of the waves between two points on the solar surface. Zhao & Kosovichev (2003) inverted for plasma flows and sound-speed perturbations beneath a sunspot. They have compared results with and without a maskedout active region but did not find significant differences between the corresponding results. Hughes et al (2005) compared the inverted sound-speed perturbations using single-skip and double-skip geometries They found a good agreement at depths larger than about 5 Mm below a sunspot. This is in contrast with the fact that the large active regions are potentially dangerous for a technical civilisation via solar flares
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