Study On The Rotation Of Solar Photospheric Magnetic Fields

The discovery of solar rotation is closely related to the observation of sunspots. Early in seventeenth, the solar rotation was observed. The rotation of the pattern of magnetic ?elds is studied after the continuous record of magnetograms.Solar magnetic ?elds play an important role in the existence and variability of solar activity. Much solar activity seems to be connected with the magnetic?elds. In the dynamo theories, the di?erential rotation provides a mechanism for the conversion of a poloidal magnetic ?eld into a toroidal ?eld. Doing research about the rotation of solar magnetic ?elds is helpful in understanding the physical process of solar activity.The thesis, which is presented in ?ve chapters, makes a study of the rotation of the photospheric magnetic ?elds about the rotation pro?le, the temporal variation and the di?erence in the rotation of di?erent polarities.In chapter one, a brief introduction about the photospheric magnetic ?elds,the surface magnetic features and the solar rotation relevant to my study is given?rst. The autocorrelation method and the cross-correlation method, which have been applied to analyze the rotation of the observed pattern of solar magnetic?elds, are described and contrasted in detail. Then a review of the present status of the rotation of the photospheric magnetic ?elds is present, including its latitude-variation, its temporal variation, its north-south asymmetry, as well as its relationship with the strength of magnetic ?elds. Two di?erent rotation laws,the normal di?erential rotation pro?le and the quasi-rigid rotation pro?le, as well as the possible reasons for the di?erence between them, are chie?y introduced. At the end of this chapter, the previous study of the rotation of di?erent polarities is reviewed.In chapter two, through a cross-correlation analysis of the Carrington synoptic maps, the sidereal rotation rates of solar magnetic ?elds between ±60?latitudes are investigated. The rotation pro?le of magnetic ?elds is obtained:the sidereal rotation rates decrease from the equator to mid-latitude and reachtheir minimum values of about 13.16 deg day-1(13.17 deg day-1, sidereal) at 53?(54?) latitude in the northern(southern) hemisphere, then increase toward higher latitudes. Some possible interpretations are discussed for the resulting rotation pro?le.In chapter three, the di?erences in sidereal rotation rates between the positive and negative magnetic ?elds in the latitude range of ±60?are obtained. And the time-latitude distribution of the rotation rate di?erences is shown,which looks like butter?y diagram at low latitudes. For comparison, the time-latitude distribution of longitudinally-averaged photospheric magnetic ?eld strength is shown. And it is concluded that the magnetic ?elds with leading polarity rotate faster than those with the following polarity at low and middle latitudes in both northern and southern hemispheres. However, at higher latitudes, the magnetic?elds with leading polarity do not always rotate faster than those with following polarity. Further more, the relationship between the rotation rate di?erences and solar magnetic ?eld strength is studied through the correlation analysis. Some possible interpretations are discussed for the resulting correlation coe?cients.In chapter four, the rotation of the magnetic ?elds for the current solar cycle 24 is investigated still through a cross-correlation analysis of the Carrington synoptic maps. The temporal variation of sidereal rotation rates of positive and negative magnetic ?elds at some latitudes are shown, and it can be found that,at low latitudes, the negative ?elds generally rotate faster than the positive ?elds in the northern hemisphere, the positive ?elds generally rotate faster than the negative ?elds in the southern hemisphere. The mean rotation pro?les of the total, positive and negative magnetic ?elds in the time interval are also obtained.The mean rotation rates of the positive polarity reach their maximum values at about 9?latitude in the southern hemisphere. The mean rotation rates of the negative polarity reach their maximum values at about 6?latitude in the northern hemisphere. The mean rotation pro?le of the total magnetic ?elds displays an obvious north-south asymmetry that the rotation seems to be more di?erential in the northern hemisphere. The latitude-variation of the rotation rate di?erence between positive and negative magnetic ?elds is further studied, and it is found that the magnetic ?elds with leading polarity rotate faster than those with thefollowing polarity in each hemisphere, except for the zones around 52?latitude of the southern hemisphere and around 35?latitude of the northern hemisphere.In the last chapter, a brief summary of this thesis is provided and some unsolved questions about the di?erential rotation of the photospheric magnetic?elds are mentioned. And a forecast for the future related research is put forward.