Solar-cycle-related Variation of Differential Rotation of the Chromosphere

Abstract

Solar-cycle-related variation of the solar chromospheric rotation is studied by analyzing the chromospheric rotation rate of 938 synoptic maps generated from the Ca ii K line at the Mount Wilson Observatory during the period of 1915 August 10 to 1985 July 7. The results obtained are as follows: (1) The parameters A (the equatorial rotation rate) and B (the latitudinal gradient of rotation) in the standard form of differential rotation both show a decreasing trend in the considered time frame, although A has weak statistical significance. (2) There is a significant negative correlation between the level of solar activity and parameter B, indicating that there seems to be a correlation between field strength and chromospheric differential rotation. (3) During solar cycles 15, 16, 19, 20, and 21, the southern hemisphere rotates faster, whereas in cycles 17 and 18, the northern hemisphere rotates faster. (4) There exists a significant negative correlation between the N–S asymmetry of the chromospheric rotation rate and that of solar activity, indicating that differential rotation of the chromosphere seems to be strengthened by stronger magnetic activity in a certain hemisphere. Possible explanations for the above results are given.

Rotational Characteristics of the Solar Transition Region Using SDO/AIA 304 Å Images

“…In addition, we discussed the correlation between the rotation rate and the solar activity represented by the yearly mean total sunspot numbers, and found the rotation rate of the STR follows the overall trend as solar activity, which supports the results reported by Japaridze & Chargeishvili (2016), Sharma et al (2021), Wan & Gao (2022), and Wan & Li (2022). In the comparison of the rotational coefficients, we found the STR rotates more slowly and more rigidly than the solar chromosphere and the corona.…”

“…These behaviors suggest that the rotation rate of the STR follows the same trend as solar activity and is modulated by the solar cycle as well. Some authors have reported the Sun rotates faster during the solar cycle maxima by analyzing different data sets (Japaridze & Chargeishvili 2016;Sharma et al 2021;Wan & Gao 2022;Wan & Li 2022), and our finding is in agreement with their results. Especially for the result of Sharma et al (2021), who studied the rotation of the STR as well.…”

“…Wöhl et al (2010) presented that the southern solar hemisphere rotates more rapidly for the period 1998-2006. Wan & Gao (2022) observed the southern hemisphere rotates faster at most cycles during the years 1915-1985 in the study of chromospheric rotation. However, our result did not find any specific variations between the north-south asymmetry of rotation rate and solar activity.…”

“…It is a common practice to use sunspot numbers to represent solar activity (Jurdana-Šepić et al 2011;Sharma et al 2021;Wan & Gao 2022). In order to explore the dependence between the solar rotation rate and solar activity, we compared the average rotation rate of the STR to the annual sunspot numbers from 2011 to 2022.…”

“…Using the microwave low-brightness-temperature regions (LTRs) and Hα filaments as tracers, Brajša et al (1997) presented that the Sun revealed a higher rotation rate on average during the maxima of the solar activity cycles 21 and 22, and the rotation coefficients A and B determined by tracing LTRs are 14.28( ± 0.06) and −1.82( ± 0.28), respectively. On the chromospheric rotation, Wan & Gao (2022) proposed the solar equator rotates slightly faster over all solar cycle maxima by analyzing the synoptic maps generated from the Ca II K line at the Mount Wilson Observatory (1915Observatory ( -1985. Sharma et al (2021) presented the average solar rotation rate follows the trend of solar activity by investigating SFD images at 30.4 nm wavelength observed from STEREO (2008STEREO ( -2018.…”