Examining the Magnetic Field Strength and the Horizontal and Vertical Motions in an Emerging Active Region

TL;DR

This study investigates the motions and magnetic field evolution of an emerging active region to assess the validity of the rigid flux tube assumption in subsurface magnetic structure inference.

Contribution

It provides observational evidence that challenges the assumption of a rigid flux tube structure during magnetic flux emergence.

Findings

Horizontal separation velocity decreases after flux emergence peak.

Thin flux-tube approximation yields unrealistically high buoyant velocities.

Partial correlations found between field orientation, strength derivatives, and separation velocity.

Abstract

Earlier observational studies have used the time evolution of emerging magnetic flux regions at the photosphere to infer their subsurface structures, assuming that the flux structure does not change significantly over the near-surface layer.In this study, we test the validity of this assumption by comparing the horizontal and vertical motions of an emerging active region. The two motions would be correlated if the emerging structure is rigid. The selected active region (AR) NOAA 11645 is not embedded in detectable preexisting magnetic field. The observed horizontal motion is quantified by the separation of the two AR polarities and the extension of the region. The vertical motion is derived from the magnetic buoyancy theory. Our results show that the separation of the polarities is fastest at the beginning with a velocity of $\approx$~4~Mm hr$^{-1}$ and decreases to $\le$~1~Mm hr$^{-1}$ after the main growing phase of flux emergence. The derived thick flux-tube buoyant velocity is between 1 and 3~Mm hr$^{-1}$ while the thin flux-tube approximation results in an unreasonably high buoyant velocity, consistent with the expectation that the approximation is inappropriate at the surface layer. The observed horizontal motion is not found to directly correlate with either the magnetic field strength or the derived buoyant velocities. However, the percentage of the horizontally oriented fields and the temporal derivatives of the field strength and the buoyant velocity show some positive correlations with the separation velocity. The results of this study imply that the assumption that the emerging active region is the cross section of a rising flux tube whose structure can be considered rigid as it rises through the near-surface layer should be taken with caution.