Probing the Shallow Convection Zone: Rising Motion of Subsurface Magnetic Fields in the Solar Active Region

TL;DR

This study uses helioseismology to detect and analyze the rising motion of magnetic flux in the Sun's shallow convection zone, revealing deceleration patterns consistent with theoretical models of flux emergence.

Contribution

It provides the first seismological evidence of subsurface magnetic flux rising and decelerating before surface emergence, with quantitative velocity estimates at various depths.

Findings

Detection of magnetic flux rise several hours before surface appearance

Velocity of flux emergence decreases from several km/s to about 0.5 km/s near the surface

Results support models of flux slowing in the upper convection zone before surface emergence

Abstract

In this Letter we present a seismological detection of a rising motion of magnetic flux in the shallow convection zone of the Sun, and show estimates of the emerging speed and its decelerating nature. In order to evaluate the speed of subsurface flux that creates an active region, we apply six Fourier filters to the Doppler data of NOAA AR 10488, observed with SOHO/MDI, to detect the reduction of acoustic power at six different depths from -15 to -2 Mm. All the filtered acoustic powers show reductions, up to 2 hours before the magnetic flux first appears at the visible surface. The start times of these reductions show a rising trend with a gradual deceleration. The obtained velocity is first several km s^-1 in a depth range of 15--10 Mm, then ~1.5 km s^-1 at 10-5 Mm, finally ~0.5 km s^-1 at 5-2 Mm. If we assume that the power reduction is actually caused by the magnetic field, the velocity of order of 1 km s^-1 is well in accordance with previous observations and numerical studies. Moreover, the gradual deceleration strongly supports the theoretical model that the emerging flux slows down in the uppermost convection zone before it expands into the atmosphere to build an active region.