Observation of Differential Rotation Within a Sunspot Umbra During an X-Class Flare

Richard Grimes,Huw Morgan,Balázs Pintér

doi:10.1007/s11207-020-01657-5

Abstract

Sunspots and their dynamics dominate the magnetic topology and evolution of both the photosphere and the overlying coronal active regions. Thus a comprehensive understanding of their behaviour is essential to understanding the solar magnetic field. A new technique is presented for applying multiple ellipse fits as a method for rotation tracking of sunspot umbrae. The method is applied to a sunspot in NOAA active region AR 12158 during an X-class flare event and the resulting rotation rate correlates well with other measurements from literature. The method also reveals an apparent difference in rotation rate between the edge and the innermost region of the sunspot umbra of up to 2 degrees per hour. Such differential rotation must lead to the large-scale twisting of sunspot magnetic flux tubes with implications for models of coronal loops and the build-up of instabilities that may lead to eruptions.

Highlights

Sunspots are a key feature of study on the solar photosphere, as tracers of magnetic flux tubes and a major driver of solar flares and coronal mass ejections
A method is presented for determining the rotation rate of a sunspot umbra at multiple threshold levels and applied to a sunspot which rotates during an X1.9 class flare event
The Multi-Layer Thresholding (MLT) method provides a range of rotation rates consistent with other measurements of rotation and reveals evidence of differential rotation within the umbra

Summary

Introduction

Sunspots are a key feature of study on the solar photosphere, as tracers of magnetic flux tubes and a major driver of solar flares and coronal mass ejections. It has been known since at least the start of the previous century (Evershed, 1910; St. John, 1913; Hale and Luckey, 1915) that sunspots can rotate both about their centre and relative to companion spots. It has been long established that the motions of sunspots are related to flaring activity (Gopasyuk, 1965; Stenflo, 1969), in sunspot groups containing opposite-polarity umbrae ( known as δ spots Künzel, 1959; McCloskey, Gallagher, and Bloomfield, 2016).

Methods

Results

Conclusion