MHD wave refraction and the acoustic halo effect around solar active regions - a 3D study

Abstract

An enhancement in high-frequency acoustic power is commonly observed in the solar photosphere and chromosphere surrounding magnetic active regions. We perform 3D linear forward wave modelling with a simple wavelet pulse acoustic source to ascertain whether the formation of the acoustic halo is caused by MHD mode conversion through regions of moderate and inclined magnetic fields. This conversion type is most efficient when high frequency waves from below intersect magnetic field lines at a large angle. We find a strong relationship between halo formation and the equipartition surface at which the Alfvén speed $a$ matches the sound speed $c$, lending support to the theory that photospheric and chromospheric halo enhancement is due to the creation and subsequent reflection of magnetically dominated fast waves from essentially acoustic waves as they cross $a=c$. In simulations where we have capped $a$ such that waves are not permitted to refract after reaching the $a=c$ height, halos are non-existent, which suggests that the power enhancement is wholly dependent on returning fast waves. We also reproduce some of the observed halo properties, such as a dual 6 and 8 mHz enhancement structure and a spatial spreading of the halo with height.

References in corpus (13)

• Radiative MHD simulation of sunspot structure
• Local Helioseismology: Three Dimensional Imaging of the Solar Interior
• Modeling the Subsurface Structure of Sunspots
• MHS sunspot model from deep sub-photospheric to chromospheric layers
• Acoustic wave propagation in the solar sub-photosphere with localised magnetic field concentration: effect of magnetic tension
• Three dimensional numerical simulations of acoustic wave field in the upper convection zone of the Sun
• Sunspot seismic halos generated by fast MHD wave refraction
• Properties of high-frequency wave power halos around active regions: an analysis of multi-height data from HMI and AIA onboard SDO
• Acoustic oscillations in a field-free cavity under solar small-scale bipolar magnetic canopy
• Constructing and Characterising Solar Structure Models for Computational Helioseismology
• A Wave Scattering Theory of Solar Seismic Power Haloes
• Transmission and conversion of magnetoacoustic waves on the magnetic canopy in a quiet Sun region
• Sensitivity of helioseismic travel-times to the imposition of a Lorentz force limiter in computational helioseismology

Cited by in corpus (12)

• Three-dimensional simulations of solar magneto-convection including effects of partial ionization
• Magnetoacoustic Waves in a Stratified Atmosphere with a Magnetic Null Point
• 3D simulations of realistic power halos in magneto-hydrostatic sunspot atmospheres: linking theory and observation
• Observed local dispersion relations for magneto-acoustic-gravity waves in the Sun's atmosphere: Mapping the acoustic cut-off frequency
• On Mode Conversion, Reflection and Transmission of Magneto-Acoustic Waves from Above in an Isothermal Stratified Atmosphere
• On Strengthening of the Solar f-mode Prior to Active Region Emergence Using the Fourier-Hankel Analysis
• Statistical analysis of acoustic wave parameters near active regions
• Enhanced Acoustic Emission in Relation to the Acoustic Halo Surrounding Active Region 11429
• Probing sunspots with two-skip time-distance helioseismology
• A Study of Acoustic Halos in Active Region NOAA 11330 using Multi-Height SDO Observations
• Spatial variation of periods of ion and neutral waves in a solar magnetic arcade
• Investigation of surface effects of simple flux tubes using numerical simulations