Horizontal Flows in Active Regions from Ring-diagram and Local Correlation Tracking Methods

TL;DR

This study compares surface and sub-surface horizontal flows in active regions using Dopplergrams, finding strong correlation but significant differences in flow magnitudes and directions between the two measurement methods.

Contribution

It demonstrates the consistency between local correlation tracking and helioseismic ring-diagram methods in measuring flows in active regions, despite differences in magnitude and direction.

Findings

Strong positive correlation between flow measurements from both methods.

Flows from helioseismology are roughly twice as large as surface flows.

Median directional difference of 49 degrees between methods.

Abstract

Continuous high-cadence and high-spatial resolution Dopplergrams allow us to study sub-surface dynamics that may be further extended to explore precursors of visible solar activity on the surface. Since the p-mode power is absorbed in the regions of high magnetic field, the inferences in these regions are often presumed to have large uncertainties. In this paper, using the Dopplergrams from space-borne Helioseismic Magnetic Imager (HMI), we compare horizontal flows in a shear layer below the surface and the photospheric layer in and around active regions. The photospheric flows are calculated using local correlation tracking (LCT) method while the ring-diagram (RD) technique of helioseismology is used to infer flows in the sub-photospheric shear layer. We find a strong positive correlation between flows from both methods near the surface. This implies that despite the absorption of acoustic power in the regions of strong magnetic field, the flows inferred from the helioseismology are comparable to those from the surface measurements. However, the magnitudes are significantly different; the flows from the LCT method are smaller by a factor of 2 than the helioseismic measurements. Also, the median difference between direction of corresponding vectors is 49 degree.