Origin of Deep Acoustic Sources Associated with Solar Magnetic Structures

TL;DR

This study uses 3D radiative MHD simulations to investigate the origins of solar acoustic sources near magnetic structures, revealing different excitation mechanisms inside and outside sunspots and their relation to downdrafts and converging flows.

Contribution

It provides new insights into the physical processes generating acoustic waves around magnetic structures, especially the roles of downdrafts and converging flows in wave excitation.

Findings

Outside sources are associated with downdrafts and their depth correlates with downdraft speed.

Inside the magnetic structure, acoustic waves are driven by converging flows and boundary interactions.

Strongest acoustic source concentration is at 0.2-1 Mm outside and 2.5-3 Mm inside the structure.

Abstract

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 of this structure. By performing a case-by-case study, we found that surrounding the magnetic structure, acoustic sources are associated with downdrafts. Their depth correlates with downdraft speed and magnetic fields. 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 2.5 - 3 Mm for the excitation event inside the structure.