The Origin of Deep Acoustic Sources Associated with Solar Magnetic Structures

It is generally accepted that solar acoustic (p) modes are excited by near-surface turbulent motions, in particular by downdrafts and interacting vortices in intergranular lanes. Recent analysis of Solar Dynamics Observatory data by Zhao et al. (2015) revealed fast-moving waves around sunspots, which are consistent with magnetoacoustic waves excited approximately 5 Mm beneath the sunspot. We analyzed 3D radiative MHD simulations of solar magnetoconvection with a self-organized pore-like magnetic structure, and identified more than 600 individual acoustic events both inside and outside this structure. By performing a case-by-case study, we found that acoustic sources surrounding the magnetic structure are associated with downdrafts. Their depth correlates with downdraft speed and magnetic field strength. The sources often can be transported into deeper layers by downdrafts. The wave front shape, in the case of a strong or inclined downdraft, can be stretched along the downdraft. Inside the magnetic structure, excitation of acoustic waves is driven by converging flows. Frequently, strong converging plasma streams hit the structure boundaries, causing compressions in its interior that excite acoustic waves. Analysis of the depth distribution of acoustic events shows the strongest concentration at 0.2–1 Mm beneath the surface for the outside sources and mostly below 1 Mm inside the magnetic region, that is, deeper than their counterparts outside the magnetic region.