银河旋臂、地核环流与地球大冰期

地球在其约４６亿年的生命史中，多次出现大冰期，关于其形成原因是地球科学家研究的热门课题。促使地球系统演化的力源主要来自哪一圈层？气体具有最大的激活能，但大气圈仅占地球总质量的１０－６，它不可能是主要圈层。固态的激活能最低，下地幔和地内核亦不大可能在地球系统演化中扮演主要角色。地球外核是液态，具有较高的激活能，它约占现代地球系统总质量的３０％，故可认为它是地球系统演化的主要活动圈层。作为旋转地球上的流体，外核环流存在着两种极端流型：一是“地转流型”，其速度场是二维场，垂直运动很弱（以下简称为Ｇ型）；二是“强对流型”，当Ｅｌｓｓａｓｅｒ数≥１时，流场的二维几乎完全被Ｌｏｒｅｎｚ力所破坏，对流充满整个地核（以下简称Ｃ型）。文章在事实分析的基础上提出了地球大冰期形成的如下假说：当地球背景磁场与银河旋臂磁场极性符号相同时，外磁场将激发地球外核环流转为Ｃ型，引起地壳和地幔强烈的垂直运动（强造山运动），致使大气热机效率亦大为提高，高纬地区强降温，这是大冰期形成的根本原因。这一假说的逆表述，即当地球背景磁场与银河旋臂磁场极性相反时，地核环流将转向Ｇ型，地壳表面将主要是“夷平作用”，致使大气热机效率亦降低，行星风系减弱，高纬?

GALACTIC ARMS, CIRCULATION IN CORE AND GREAT ICE AGES

There were several Great Ice Ages during the evolution of the earth. The cause of Great Ice Ages is a hot point that geoscientists are focusing on. Which sphere of the earth system acts as the key role to drive the evolution? Atmosphere(gas) has maximum activation energy, but it is only 10 -6 of the mass of earth system, so it is impossible for atmosphere to acts as the key role. Solid state has the minimum activation energy, so the lower mantle and inner core are also impossible to act as key role. Outer core is liquid, it is of higher activation energy, it is about 30% of the total mass of modern earth system, so, we may think that the main active sphere is the outer core in the earth evolution. As the fluid in rotating earth, the circulation in outer core has two extreme patterns: one is “geostrophic pattern”. Its velocity field is almost kept in two dimensions (horizontal), the vertical movement is weak (we call this pattern as “Pattern G”). The other is “strong convection pattern”. When Elssaser Number equals to or is bigger than 1, the two dimension (horizontal) movement may be destroyed by Lorenz force. It is full of convection in outer core (we call this pattern as “Pattern C”). On the basis of facts analysis, this paper puts forward a hypothesis on the Great Ice Ages formation of the earth as follows: when the magnetic field of the earth and magnetic field of the galactic arm have the same direction, the magnetic field of the galactic arm will excite the circulation in outer core into Pattern C, and cause the strong vertical movement of mantle and crust (strong orogeny). As a result, the heat engine efficiency of atmosphere improved and enormous cooling in higher latitude region happened. This is the fundamental cause of Great Ice Ages. When the magnetic field of the earth and the magnetic field of the galactic arm have opposite directions, the circulation in outer core will turn into Pattern G. During this pattern, the planation takes the leading place on the crust surface. Consequently, the heat engine efficiency of atmosphere and planetary wind is weakened, the temperature in higher latitude region rises. This is the key reason of Great Interglacial Periods of the earth.