Measuring the internal magnetic fields of the Sun would provide important constraints on our understanding of the mechanisms that underly solar activity. In this work, we have used a full solar cycle's worth of high quality helioseismic data from the Michelson Doppler Imager (MDI) instrument onboard the SOHO spacecraft to explore changes in the interior thermal structure, using the techniques of both global and local helioseismology. We have also used these data to attempt to directly measure the magnetic fields in the convection zone.
We have found that the interior of the Sun changed slightly but significantly over the course of the last solar cycle. Analyzing the global mode frequencies measured during solar cycle 23, we find a change, which we interpret as a change in the sound speed, at the base of the convection zone. At r = [special characters omitted], the change in sound speed is a decrease of δc²/ c² = (7.23 ± 2.08) × 10–5. Modeling the effects of magnetic fields on the helioseismic splitting coefficients, we find that a field of strength necessary to cause the thermal change we find is not detectable with our data. We find that the signal that is there can be explained by a shallow toroidal field with a weak poloidal component. This field is tightly correlated with surface activity. We find that the toroidal field peaks at r0 = 0.999[special characters omitted] and r = 0.996[special characters omitted], with peak field strengths of 380 ± 30 G and 1.4 ± 0.2 kG for the shallower and deeper fields, respectively. The peak strength of the poloidal field is 124 ± 17 G.
We employ ring diagram analysis to explore these layers of the Sun in more detail. We confirm earlier results that helioseismic frequencies increase in active regions, where strong surface magnetic fields are present, and that acoustic power is suppressed. We find that the changes in frequency depend somewhat on the surface geometry of the magnetic fields. Finally, we find in a large sample of active regions that the thermal structure beneath sunspots is a two layer structure with slower soundspeed in the shallower layers (above approximately r = 0.98[special characters omitted]) and a faster sound speed between approximately r = 0.975[special characters omitted] and r = 0.985[special characters omitted].
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