| The rotation of the sun has been known for the past four centuries.Galileo first spotted it with his first optical telescope.The study of the interior of the sun and the rotation of its atmosphere has aroused great interest over the past forty years.The study of the internal rotation of the sun is conducted through helioseismology(Howe 2009 and its references).The rotation of the solar atmosphere--the solar sphere,chromosphere and corona--has been studied from ground and satellite observations.The first studies of the sun’s rotation observed sunspots as tracers indicating the sun’s motion.Since then,many other quasi-permanent solar features have been discovered that can be used as tracers to measure the sun’s rotation.These include solar flocculus,s olar flare,coronal holes,supergranulation,etc.(Beck,Duvall and Scherrer 1998 and references therein).Using these tracers to measure the rotation rate,different numerical results and motion laws are obtained.Some of these measu rements do match each other,and many do not.In this paper,we use ensemble empirical mode decomposition(EEMD)and wavelet analysis to analyze the coronal index,and explore the evolution law of the coronal rotation,so as to further understand the rotation properties of the sun.First,an ensemble empirical mode decomposition of the daily coronal index between January 1,1939,and September 30,2022,breaking it down into signals with different periods,and finding the average period of all coronal rotations in this time range,according to the analysis of white noise in Wu and Huang(2004),the significance test of these periods was carried out.At the same time,the rotation period of the corona was determined in combination with previous studies and t he recognized solar rotation period of 27 days,and the rotation signal was extracted from the EEMD results.Then,the rotation signals extracted in the previous step are explored in every activity cycles.From the 18 th to the 24 th active cycles(1944 to2019),the rotation signals of each active cycle were decomposed by empirical mode,and the rotation period of each active cycle was calculated respectively to observe whether the coronal rotation of several active weeks is consistent and what the difference s are.Furthermore,by means of wavelet analysis,the frequency is used to calculate the daily period,and the evolution law of the cycle is analyzed from a long-term perspective.Finally,the daily coronal index is compared with the periodic variation of the sun’s other tracers(sunspots,for example,because they are easiest to see and have been studied the most)to see how the coronal rotation relates to other solar activity.Based on these analysis processes,the main findings are as follows:(1)The decomposition of the daily corona index found the following cycles:2.86 days,5.88 days,16.99 days,27.80 days,68.92 days,154.01 days,318.89 days(0.87 years),678.83 days(1.86 years),3735.02 days(10.23years),3975.45 days(10.89 years).(2)The rotation period from the 18 th to the 24 th active cycle was found,and it was found that the length of the rotation period first increased and then decreased during the period studied.(3)The rotation period shows a decreasing trend in the long term.(4)The trend of the daily coronal index and the number of sunspots in the same period is basically the same. |
