Prominent Photospheric Downflows on Magnetic Neutral Line in a Delta-Type Sunspot

Abstract

It is known that δ-type sunspot groups have a high flare productivity and produce strong flares. In particular, βγδ type sunspots are the most active type of all. A βγδ active region NOAA 9957 with frequent flux cancellations but without any marked flare activity during its decay phase was studied in this work. Using SOHO/MDI Dopplergrams and magnetograms, we detected continuous prominent downflow motions of 1500 - 1700 m s⁻¹ for several hours on the magnetic neutral line in this region. In the downflow region, penumbral structures were observed to decay. We will interpret and discuss the phenomenon as a case of submergence of the magnetic flux.