Speaker
Description
First Name: Benxu
Last Name: Niu
Affiliation: Beihang University
All Authors: Jie Jiang、Ruihui Wang
Abstract: Sunspot tilt angle is a crucial ingredient for Babcock–Leighton-type solar dynamos, yet whether this tilt is established within the convection zone or formed at the solar surface is actively debated. To address the issue, we analyze the sunspot tilt angles from the Debrecen Photoheliographic Database (DPD) spanning 1974 – 2018 and systematically examine their evolution by categorizing sunspot groups into four evolutionary stages. Our results show that during the initial emergence phase, tilt angles are highly scattered, with a mean value near zero and lacking any latitudinal dependence. As sunspots enter the stabilization phase, the tilt angles gradually develop, reaching their peak values and exhibiting a pronounced latitudinal dependence consistent with Joy’s law. During the subsequent decay phase, the tilt angles decrease. These evolutionary properties do not show area dependence. Moreover, sunspots that initially display an anti-Joy tilt commonly reverse their orientation during evolution and ultimately converge toward Joy’s law, with smaller sunspots showing a higher likelihood of such reversal. These results indicate that the systematic properties of sunspot tilt angles are established after emergence onto the photosphere and surface or near-surface processes play a dominant role in forming the observed tilt.