Multi-Wavelength Observations of Sunspot Rotation and Analysis of Influencing Factors

Sunspots represent regions of highly concentrated magnetic fields on the solar surface. Multi-wavelength observations enable the acquisition of magnetic and dynamical information across different atmospheric layers, thereby facilitating investigations into the formation of non-potential magnetic field configurations. Based on observations from the Chinese H$\alpha$ Solar Explorer (CHASE), which offers the advantage of multi wavelengths observations within the H$\alpha$ spectral window (featuring spatial co-alignment and temporal simultaneity), four sunspots exhibiting light bridge structures were analyzed to investigate their rotational characteristics in both the photosphere and chromosphere. While employing the light bridges within sunspots as tracers, particular attention was paid to the influence of light bridge evolution on the determination of rotational velocities. Key findings include: (1) analysis of all four samples revealed that the angular velocity of sunspot rotation in the line wing ($\Delta\lambda=-0.8$ Å, $\Delta\lambda$ is the wavelength interval between the H$\alpha$ line core and other wavelengths) and continuum ($\Delta\lambda=-2.8$ Å) is consistent with angular velocities ranging approximately from $ 0.5\ (^{\circ})/\rm{h} $ to $ 2\ (^{\circ})/\rm{h} $; (2) three of the samples exhibited faster rotational velocities in the line core (chromosphere) compared to the line wing, with a velocity difference of $ 0.1\ (^{\circ})/\rm{h} $, however, dynamic evolution (e.g., morphology and brightness variations of light bridges) significantly influenced the measurements; (3) three sunspots featured radially penetrating light bridges traversing the umbra. Analysis demonstrated that one side of the light bridges displayed a rotational velocity profile characterized by “slow-fast-slow” variations with increasing radial distance across all wavelengths, while the opposite side exhibited a gradually increasing velocity trend, highlighting the non-rigid rotational nature of sunspots. It is critical to consider the effects of sunspot evolution across different spectral wavelengths for multi-band researches of sunspot rotation.