The Configuration of Jupiter ’ s Magnetosphere
The Configuration of Jupiter ’ s Magnetosphere
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发表时间：
2003
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通讯作者：
K. Khurana;M. Kivelson;V. Vasyliūnas;N. Krupp;J. Woch;A. Lagg;B. Mauk;W. Kurth
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作者：
K. Khurana;M. Kivelson;V. Vasyliūnas;N. Krupp;J. Woch;A. Lagg;B. Mauk;W. Kurth
磁层是一个围绕行星产生影响的“球体”，在这个球体中，与行星磁场相关的力凌驾于所有其他力之上。这颗行星的磁场将太阳风从行星上转移开，形成一个空腔，其中包含来自太阳风或行星的低密度热等离子体。太阳风和磁层的等离子体被一个叫做磁层顶的薄边界层隔开，在这个边界层中，强大的表面电流循环。要形成和维持一个磁层，需要三个关键因素。它们是，一个足够强的行星磁场来阻止太阳风，一个磁层内部或外部的等离子体源来填充它，以及一个能量源来为它提供动力。太阳风驱动的磁层(地球和水星是其中的主要例子)的等离子体和能量主要来自太阳风。在旋转驱动的磁层中，大部分能量来自行星的自转，而等离子体来自行星或行星的卫星(木星和克罗尼亚磁层是这一类的主要例子)。木星内部是一台强大的发电机的所在地，它在赤道地区产生强度为∼4高斯的表面磁场。这种强磁场和木星的快速自转(自转周期∼9小时55分钟)在太阳系中创造了一个独特的磁层，太阳系以其巨大的尺寸(平均亚太阳磁层顶距离45-100RJ，其中1RJ=71492公里是木星的半径)和快速自转(见图24.1的木星磁层示意图)而闻名。木星的磁层与大多数其他磁层的不同之处在于，它的大部分等离子体来自木星的卫星木卫一。重等离子体主要由S和O的各种电荷态组成，在离心力和热压的共同作用下使磁层膨胀。很容易证明，在没有内部重等离子体的情况下，偶极场将平衡太阳次区域距离∼42RJ处的太阳风(0.08nPa.)的平均动态压力，而不是观测到的∼75RJ的平均磁层顶位置(见图24.1)。重等离子体也是在木星磁层赤道区域产生超过160MA的方位向电流的原因，在那里它被限制在一个薄的电流片(黎明部分的半厚度∼2RJ)。
A magnetosphere is a “sphere” of influence around a planet in which the forces associated with the magnetic field of the planet prevail over all other forces. The magnetic field of the planet diverts the solar wind away from the planet, carving a cavity which contains a low-density hot plasma derived from the solar wind, or the planet. The plasmas of the solar wind and the magnetosphere are kept apart by a thin boundary layer called magnetopause in which strong surface currents circulate. To form and maintain a magnetosphere, three key ingredients are required. These are, a strong enough planetary magnetic field that halts the solar wind, a source of plasma internal or external to the magnetosphere to populate it and a source of energy to power it. The solar wind driven magnetospheres (of which the Earth and Mercury are prime examples) derive their plasma and energy mainly from the solar wind. In rotationally driven magnetospheres, the bulk of the energy is derived from planet’s rotation whereas the plasma is derived from the planet or a satellite of the planet (jovian and kronian magnetospheres are the prime examples of this category). The interior of Jupiter is the seat of a strong dynamo that produces a surface magnetic field in the equatorial region with an intensity of ∼ 4 Gauss. This strong magnetic field and Jupiter’s fast rotation (rotation period ∼ 9 h 55 min) create a unique magnetosphere in the solar system which is known for its immense size (average subsolar magnetopause distance 45-100 RJ where 1 RJ = 71492 km is the radius of Jupiter) and fast rotation (see Figure 24.1 for a schematic of Jupiter’s magnetosphere). Jupiter’s magnetosphere differs from most other magnetospheres in the fact that it derives much of its plasma internally from Jupiter’s moon Io. The heavy plasma, consisting principally of various charge states of S and O, inflates the magnetosphere from the combined actions of centrifugal force and thermal pressure. It is readily shown that in the absence of an internal heavy plasma, the dipole field would balance the average dynamic pressure of the solar wind (0.08 nPa) at a distance of ∼ 42 RJ in the subsolar region as contrasted to the observed average magnetopause location of ∼75 RJ (see Figure 24.1). The heavy plasma is also responsible for generating an azimuthal current exceeding 160 MA in the equatorial region of Jupiter’s magnetosphere where it is confined to a thin current sheet (half thickness ∼ 2 RJ in the dawn sector).