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Earlier observational studies have used the time evolution of emergingmagnetic flux regions at the photosphere to infer their subsurface structures,assuming that the flux structure does not change significantly over thenear-surface layer.In this study, we test the validity of this assumption bycomparing the horizontal and vertical motions of an emerging active region. Thetwo motions would be correlated if the emerging structure is rigid. Theselected active region (AR) NOAA 11645 is not embedded in detectablepreexisting magnetic field. The observed horizontal motion is quantified by theseparation of the two AR polarities and the extension of the region. Thevertical motion is derived from the magnetic buoyancy theory. Our results showthat the separation of the polarities is fastest at the beginning with avelocity 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-tubebuoyant velocity is between 1 and 3~Mm hr$^{-1}$ while the thin flux-tubeapproximation results in an unreasonably high buoyant velocity, consistent withthe expectation that the approximation is inappropriate at the surface layer.The observed horizontal motion is not found to directly correlate with eitherthe magnetic field strength or the derived buoyant velocities. However, thepercentage of the horizontally oriented fields and the temporal derivatives ofthe field strength and the buoyant velocity show some positive correlationswith the separation velocity. The results of this study imply that theassumption that the emerging active region is the cross section of a risingflux tube whose structure can be considered rigid as it rises through thenear-surface layer should be taken with caution.

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