地球的差异旋转
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摘要
对于具有流体对流层的旋转星球 ,由于星球自转对对流的影响 ,必然会在星球对流层内部不同部分间以及星球的不同圈层间产生差异旋转 (differentialrotation) .所谓差异旋转是指旋转角速度随着深度 (星球不同圈层的旋转角速度不同 )以及纬度 (同一圈层内部不同部分间的角速度不同 )具有差异的现象 .地球是一个多圈层的旋转系统 ,主要由大气圈、水圈、岩石圈、地幔、外核以及内核组成 .大气圈和水圈具有明显的流体性质 ,并且在漫长的地质年代中 ,地幔、岩石圈和地幔之间的软流层以及外核均具有流体性质 ,而且在大气压力、热、重力和电磁力等的作用下发生了对流 .这些对流运动一旦受到地球自转的影响 ,就必然会致使地球各圈层间以及对流层内部不同部分间产生差异旋转 .几个重要现象 :基本地磁场的长期西漂、岩石圈的长期西漂、地球自转速率变化 (周日长度波动 )和固体内核各向异性对称轴的移动表明在固体地球内部各圈层—岩石圈、地幔、外核和内核间存在差异旋转 .来自地震学上的数据证明了固体内核与地幔之间存在较明显的差异旋转 ,速率可达 1.1°～ 3.0°/a.这跟其它数据如自由振荡数据以及地磁场长期西漂数据获得的结果具有很大的差异 .导致固体地球内核相对于地幔差异旋转的主要宏观机制为电磁力矩、引?
Differential rotation must take place among different layers or different sections of a single layer in a rotating star with convection zone(s) due to the effect of its rotation on convective motion . Differential rotation refers to the variation of angular velocity with depth (Different layers in a rotating star have different angular velocity) and latitude (Different sections of a single layer have different angular velocity). The earth is a multi-layered rotating system, which is mainly made up of aerosphere, hydrosphere, lithosphere, mantle, outer core and inner core. Obviously, aerosphere and hydrosphere have strong rheology and, in geologic time, mantle, asthenosphere and outer core also have rheology, as well as convection has occurred in their interiors due to atmospheric pressure, heat, gravity and electromagnetic force. Differential rotation must take place among different layers and different sections of a single layer as soon as the earth's rotation exerts effect on convective motion in these layers. A few phenomena, the westward drift of the earth's magnetic field and lithosphere, the variation of the earth's rotation (the change of the length of a day) and the motion of the anisotropic symmetry axis of inner core, have indicated the occurrence of differential rotation among different layers-lithosphere, mantle, outer core and inner core. Seismological evidences have suggested differential rotation between inner core and mantle is distinct, whose magnitude can be up to 1.1°～3.0°/a, which is greatly differ from the results from other observational data such as free oscillation and the westward drift of the earth's magnetic field. The main macro and micromechanisms, which lead to differential rotation between inner core and mantle, are electromagnetic, gravitational, topographic and viscous torque and convective motion, magnetic wind and thermal wind, respectively.
Differential rotation must take place among different layers or different sections of a single layer in a rotating star with convection zone(s) due to the effect of its rotation on convective motion . Differential rotation refers to the variation of angular velocity with depth (Different layers in a rotating star have different angular velocity) and latitude (Different sections of a single layer have different angular velocity). The earth is a multi-layered rotating system, which is mainly made up of aerosphere, hydrosphere, lithosphere, mantle, outer core and inner core. Obviously, aerosphere and hydrosphere have strong rheology and, in geologic time, mantle, asthenosphere and outer core also have rheology, as well as convection has occurred in their interiors due to atmospheric pressure, heat, gravity and electromagnetic force. Differential rotation must take place among different layers and different sections of a single layer as soon as the earth's rotation exerts effect on convective motion in these layers. A few phenomena, the westward drift of the earth's magnetic field and lithosphere, the variation of the earth's rotation (the change of the length of a day) and the motion of the anisotropic symmetry axis of inner core, have indicated the occurrence of differential rotation among different layers-lithosphere, mantle, outer core and inner core. Seismological evidences have suggested differential rotation between inner core and mantle is distinct, whose magnitude can be up to 1.1°～3.0°/a, which is greatly differ from the results from other observational data such as free oscillation and the westward drift of the earth's magnetic field. The main macro and micromechanisms, which lead to differential rotation between inner core and mantle, are electromagnetic, gravitational, topographic and viscous torque and convective motion, magnetic wind and thermal wind, respectively.
引文
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[32] HideR .Theearth’sdifferentialrotation[J].Q .J.R .Astron.Soc.,1986,278:3～14.
[33] KuangWJ,BloxhamJ .Onthedynamicsofthetopographicalcore mantlecoupling[J].Phys.EarthPlanet.Inter.,1997a,99:289～294.
[34] JaultD ,LemouelJ L .Core mantleboundaryshape:constrainsin ferredfromthepressuretorqueactingbetweencoreandmantle[J].Geophys.J.Int.,1990,101:233～241.
[35] AurnouJ,OlsonPControlofinnercorerotationbyelectromagnetic,gravitationalandmechanicaltorques[J].Phys.EarthPlanet.Inter.,2000,117:111～121
[2] R櫣digerGP.Atheoryofdifferentialrotationbasedonthediscussionofturbulenttransportofangularmomentum[J].Astron.Nachr.,1977,298(5):245～252.
[3] GilmanPA .NonlineardynamicsofBoussinesqconvectioninadeeprotatingsphericalshellI[J].Geophys.Astrophys.FluidDyn.,1977,8:93～135.
[4] 马宗晋.供现今地球动力学研究的8组全球级构造现象与问题[A].见:现今地球动力学问题讨论会论文集[C].张崇立,高祥林和商宏宽编.北京:地质出版社,1994.
[5] GubbinsD .Rotationoftheinnercore[J].J.Geophys.Res.,1981,86(B12):11695～11699.
[6] VoorhiesCV .Time varingfluidflowatthetopofearth’scorefromdefinitiveGeomagneticReferenceFieldModels[J].J.Geophys.Res.,1995,100:10029～10039.
[7] KautzlebenH .Onthetemporalvariationsofthegeomagneticfields[J].GerlandsBeitr.Geophys.,1972,81:233～239.
[8] YukutakeT .Astratiedcoremotioninferredfromgeomagneticsecularvariations[J].Phys.EarthPlanet.Inter.,1981,24:253～258.
[9] JaultD ,GireC ,LemouelJ L .Westwarddrift,coremotionsandex changesofangularmomentumbetweencoreandmantle[J].Nature,1988,333:353～356.
[10] BullardEC ,FreedmanC ,GellmanH ,etal.Thewestwarddriftoftheearth’smagneticfield[J].Philos.Trans.R .Soc.London,Ser.A ,1950,243:61～92.
[11] AurnouJ ,BritoD ,OlsonP .Anomalousrotationoftheinnercoreandthetoroidalmagneticfield[J].J .Geophys.Res.,1998,103(B5):9721～9738.
[12] BostromRC .Westwarddisplacementofthelithosphere[J].Nature,1971,234:536～538.
[13] KnopoffL ,LeedsA .Lithosphericmomentaandthedecelerationoftheearth[J].Nature,1972,237:93～95.
[14] MooreGW .Westwardtidallagasthedrivingforceofplatetectonics[J].Geology,1973,1:99～101.
[15] DoglioniD .Theglobaltectonicpatterns[J].J.Geodyn.,1990,12:21～38.
[16] DemetsC ,GordonRG ,ArgusDF ,etal.Currentplatemotions[J].Geophys.J.Int.,1990,101:425～478.
[17] RichardY ,DoglioniD ,SabadiniR .Differentialrotationbetweenlithosphereandmantle:aconsequenceoflateralmantleviscosityvariation[J].JGeophys.Res.,1991,96:8407～8415.
[18] GordonRG .Presentplatemotionsandplateboundaries[A].inGlobalEarthPhysics,ReferenceShelf1[C].ArthensT .J.(eds),Am.Geophys.Un.,Washington,1995.
[19] 马宗晋,高祥林,任金卫.现今全球构造特征及其动力学解释[J].第四纪研究,1992,12(4):325～332.
[20] 李四光.天文地质古生物资料摘要[M].北京:科学出版社,1972.
[21] RanalliG .Westwarddriftofthelithosphere:notaresultofrotationaldrag[J].Geophys.J.Int.,2000,141:535～537.
[22] HideR ,DickeyJO .Earth’svariablerotation[J].Science,1991,253:629～637.
[23] WahrJM .Theearth’srotation[J].Ann.Rev.EarthPlanet.Sci.,1988,16:231～249.
[24] LambeckK ,HopgoodP .Theearth’srotationandatmosphericcircu lationfrom1963—1973[J].Geophys.J.R .Astron.Soc.,1981,64:67～89.
[25] WahrJM ,OortAH .Friction andmountain torqueestimatesfromglobalatmosphericdata[J].J .Atmos.Sci.,1984,41:190～204.
[26] LandleyRB ,KingRW ,ShapiroII ,etal.Atmosphericangularmomentumandthelengthoftheday:acommonfluctuationwithape riodnear50days[J].Nature,1981,294:730～733.
[27] HolmeR .Electromagneticcore mantlecoupling,1:explainingdecadalchangesinthelengthofday[J].Geophys.J.Int.,1998,132:167～180.
[28] JaultD ,LemouelJ L .Exchangeofangularmomentumbetweenthecoreandmantle[J].J.Geomagn.Geoelectr.,1991,43:111～129.
[29] BuffettBA .Gravitationaloscillationsinthelengthofday[J].Geo phys.Res.Lett.,1996a,23(17):2279～2282.
[30] BuffettBA .Amechanismfordecadefluctuationsinthelengthofday[J].Geophys.Res.Lett.,1996b,23(25):3803～3806.
[31] KuangWJ,BloxhamJ.OntheeffectofboundarytopographyonflowintheEarth’score[J].Geophys.Astrophys.FluidDyn.,1993,72:161～195.
[32] HideR .Theearth’sdifferentialrotation[J].Q .J.R .Astron.Soc.,1986,278:3～14.
[33] KuangWJ,BloxhamJ .Onthedynamicsofthetopographicalcore mantlecoupling[J].Phys.EarthPlanet.Inter.,1997a,99:289～294.
[34] JaultD ,LemouelJ L .Core mantleboundaryshape:constrainsin ferredfromthepressuretorqueactingbetweencoreandmantle[J].Geophys.J.Int.,1990,101:233～241.
[35] AurnouJ,OlsonPControlofinnercorerotationbyelectromagnetic,gravitationalandmechanicaltorques[J].Phys.EarthPlanet.Inter.,2000,117:111～121
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