Emergence of magnetic flux at the solar surface and the origin of active regions

Abstract

The evolution of photospheric velocities from the first minutes after the emergence of fresh magnetic flux and the formation of the first pores in active region NOAA 10488 is studied with a time resolution of 1 min and spatial resolution of 4″. The emerging magnetic flux of a major active region is initially a bundle of magnetic-flux loops. Some of these loops erupt through the system of supergranular cells with speeds of up to 1 km/s within 15–25 min and form pores and small spots. It is suggested that the development of a pore represents the emergence of a horizontal magnetic field, which is converted into elements with a strong vertical magnetic field. The region of ascending plasma initially coincides with the zero line of a bipolar magnetic pair. Downflow and upflow regions are related to and appear with the development of pores. During the first hours of their evolution, the trailing-polarity pores exhibit downflows with mean speeds of ∼500 m/s, while upflows with speeds of ∼250 m/s dominate near the leading-polarity pores. It is concluded that a matter flow from the leading to the trailing end is present in the rising loop of a magnetic flux tube, in agreement with well-known numerical-simulation results. The flow that develops in the magnetic-flux tube erupting through the convection zone persists when pores and small spots emerge in the photosphere, at least during the first hours of their evolution.