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

Abstract

Aims: 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.
Methods: We carried out a study of photospheric velocities during the first hours of the appearance of 83 active regions with magnetic flux more than 10²¹ 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.
Results: We found that the values of maximum negative Doppler velocities that accompany the emergence of active region magnetic fields during the first 12 h 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.