On the solar rotation and activity

Abstract

Thei nteractionb etween differentialr otationa nd magnetic fieldsi nthe solar convectionz one wasr ecently modelledb y Brun ( 2004). Onec onsequence of that model is that theM axwells tresses can oppose theR eynolds stresses, and thus contribute to thet ransport of thea ngular momentum towards thes olar poles, leading to ar educed differentialr otation. So,when magnetic fieldsare weaker,amore pronounced differentialrotationcan be expected, yielding ahigher rotation velocity at lowlatitudes takenonthe average. This hypothesisisconsistent with thebehaviour of thesolar rotationduring theM aunder minimum. In this work we search for similars ignatures of ther elationshipb etween thes olar activity and rotationd etermined tracing sunspot groups and coronal bright points. We use thee xtended Greenwich datas et and as eries of full-discs olar images takena t2 8.4n mw ith theE IT instrument on theS OHO spacecraft (1998)(1999)(2000). We investigatethe dependence of thesolar rotationonthe solar activity (describedbythe relative sunspot number) andthe interplanetary magnetic field (calculatedfromthe interdiurnal variability index).Possible rotationalsignatures of twoweak solar activity cycles at thebeginning of the20thcentury (Gleissbergminimum) are discussed. The interaction between differential rotation and magnetic fields in the solar convection zone was recently modelled by Brun (2004). One consequence of that model is that the Maxwell stresses can oppose the Reynolds stresses, and thus contribute to the transport of the angular momentum towards the solar poles, leading to a reduced differential rotation. So, when magnetic fields are weaker, a more pronounced differential rotation can be expected, yielding a higher rotation velocity at low latitudes taken on the average. This hypothesis is consistent with the behaviour of the solar rotation during the Maunder minimum. In this work we search for similar signatures of the relationship between the solar activity and rotation determined tracing sunspot groups and coronal bright points. We use the extended Greenwich data set and a series of full-disc solar images taken at 28.4 nm with the EIT instrument on the SOHO spacecraft (1998)(1999)(2000). We investigate the dependence of the solar rotation on the solar activity (described by the relative sunspot number) and the interplanetary magnetic field (calculated from the interdiurnal variability index). Possible rotational signatures of two weak solar activity cycles at the beginning of the 20th century (Gleissberg minimum) are discussed.

Large Scale Flows in the Solar Convection Zone

“…Some indication of the weakening of the solar differential rotation in the presence of strong magnetic field have been observed recently (Ambroz 2004), with a weaker zonal flow during the peak of magnetic activity (see also Brajsa et al 2007). Eddy et al (1976) have also shown that during the Maunder minimum the Sun had a greater angular velocity contrast.…”

A precise measurement of the solar differential rotation by tracing small bright coronal structures in SOHO-EIT images

“…), we will investigate the relationship between the parameters of the solar rotation and the solar activity using the monthly and yearly values of the solar rotation parameters presented in this work. In this way, we would like to complement some earlier studies of the coronal rotation variation (e.g., Nash et al 1988;Rybák 1994Rybák , 2000Altrock 2003;Brajša et al 2007;Gissot 2009, and references therein) and to compare those results for coronal structures with those obtained using other tracers and methods in different time periods (Table 16 in the present paper).…”

“…23, while the whole time period (1998−2006) embraces most of the activity maximum of the same solar cycle. The observed north-south rotational asymmetry can be interpreted in the framework of a model of interdependence of solar rotation and activity, taking into account the balance between Reynolds and Maxwell stresses (Brun 2004;Brun et al 2004;Brajša et al 2006Brajša et al , 2007. During most of the 23rd solar cycle, the southern solar hemisphere was more active (Temmer et al 2006, their Fig.…”

Consistent long-term variation in the hemispheric asymmetry of solar rotation

“…Discrepancies for the same tracers (for various authors) may be due to the time intervals and lengths of data included. Increasing evidence (Balthasar et al 1986;Bai 2003;Brajša et al 2006Brajša et al , 2007Balthasar 2007;Heristchi & Mouradian 2009) suggests that the solar differential rotation changes from cycle to cycle and even within one cycle. Bouwer (1992) claimed that precise rotation periods between 27 and 28 days persist only for a short time, sometimes only for a few solar rotations.…”