Physical Properties of Wave Motion in Inclined Magnetic Fields within Sunspot Penumbrae

Abstract

At the surface of the Sun, acoustic waves appear to be affected by the presence of strong magnetic fields in active regions. We explore the possibility that the inclined magnetic field in sunspot penumbrae may convert primarily vertically-propagating acoustic waves into elliptical motion. We use helioseismic holography to measure the modulus and phase of the correlation between incoming acoustic waves and the local surface motion within two sunspots. These correlations are modeled by assuming the surface motion to be elliptical, and we explore the properties of the elliptical motion on the magnetic-field inclination. We also demonstrate that the phase shift of the outward-propagating waves is opposite to the phase shift of the inward-propagating waves in stronger, more vertical fields, but similar to the inward phase shifts in weaker, more-inclined fields.

ANATOMY OF A SOLAR FLARE: MEASUREMENTS OF THE 2006 DECEMBER 14 X-CLASS FLARE WITH GONG,HINODE, ANDRHESSI

“…The field inclination is also mixed within the vicinity of seismic sources, with the inclined field seen throughout the active region, but with more significant changes seen in regions 1-4 during the scan covering the time of the flare. This can be important for the generation of helio- since mode conversion will occur more effectively when the sound and Alfvén speeds coincide (Cally 2000;Schunker et al 2008).…”

Analysis of the Helioseismic Power-Spectrum Diagram of a Sunspot

“…Schunker et al (2008) explored the wave motions with the presence of inclined magnetic field inside sunspot penumbra, and Felipe, Khomenko, and Collados (2010) discussed the wave mode conversions in the magnetized plasma by using 3D MHD simulations. More recently, Kitiashvili et al (2011) showed through MHD simulations that vertical magnetic field would introduce some acoustic frequency shifts toward higher frequency, but an inclined magnetic field would shift the frequency more substantially.…”

The role of magnetic fields in the scattering of p-modes

“…For example, Cally et al (2003) investigated mode-conversion mechanism in inclined fields and reproduced the observed phase shifts calculated from Hankel analysis by Braun (1995). Recently, Lindsey & Braun (2005) and Schunker et al (2008) have shown that the phase shift of the outward propagating waves is opposite to the phase shift of the inward propagating waves in stronger, vertical fields. Couvidat & Rajaguru (2007) also report possible contamination of helioseismic inversions for sound speed beneath sunspots, possibly due to near surface magnetic field effects.…”

“…Although this ratio cannot be currently measured accurately from observations, one can constrain it for this model by surface observations of the vector magnetic field of sunspots (see e.g. Schunker et al 2008). Thus we expect typical value of D/R to be 0.4 < D/R < 2 for the inclination observed in the sunspots between umbrae and penumbrae.…”