Center-limb dependence of photospheric velocities in regions of emerging magnetic fields on the Sun

TL;DR

This study analyzes the photospheric plasma velocities during the early emergence of active magnetic regions on the Sun, revealing that horizontal velocities surpass vertical ones and increase with heliocentric angle, independent of magnetic flux magnitude.

Contribution

It provides new insights into the velocity ratios during magnetic flux emergence, highlighting the dominance of horizontal plasma flows in early active region development.

Findings

Horizontal velocities exceed vertical velocities during emergence.

Maximum negative Doppler velocities increase with heliocentric angle.

No correlation between velocities and magnetic flux magnitude.

Abstract

We investigate the ratio of the vertical and horizontal velocities of the photospheric plasma flows in the region of emerging magnetic fields on the Sun. We carried out a study of photospheric velocities during the first hours of the appearance of 83 active regions with magnetic flux more than 10^21 Mx with data acquired by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO). The emerging magnetic fluxes under investigation were isolated from extended concentrations of existing magnetic fields; they have different spatial scales and are located at different distances from the solar disk center. We found that the values of maximum negative Doppler velocities that accompany the emergence of active region magnetic fields during the first 12 hours increase nonlinearly with the heliocentric angle. This result shows that the horizontal photospheric velocities of plasma outflows are higher than the vertical ones of the plasma upflows during the first hours of the emergence of active regions. The horizontal velocity component at the site of emerging active regions exceeds that of convective flows in the quiet Sun. A comparison between the velocities and the maximum value of the total magnetic flux has not revealed any relation.