Bifurcation of drift shells near the dayside magnetopause
Bifurcation of drift shells near the dayside magnetopause
复制标题
日侧磁层顶附近漂移壳的分叉
DOI：
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发表时间：
2004
期刊：
影响因子：
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通讯作者：
R. Wolf
中科院分区：
文献类型：
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作者：
M. K. Öztürk;R. Wolf
[1]在向阳面磁层顶附近，有一个空间区域，其中每条磁力线有两个磁场极小值，一个在每个尖点附近。该区域位于当地中午附近，从磁层顶延伸约1-2 Re。以接近90°的赤道俯仰角进入该区域的粒子将不沿沿着赤道路径穿过昼侧，而是沿赤道平面两侧的两个分支之沿着穿过昼侧。两个分支在当地中午过后再次连接。即使在静态条件下，漂移壳层分叉（DSB）的过程也是非绝热的。两种物理机制可以导致这种非绝热性：一种是几乎所有的磁层磁场配置，另一种是依赖于磁场中南北和东西不对称的特定组合。本文只讨论第一种机制。对于南北和东西对称的构型，DSB改变了运动的第二个不变量I，改变的量很小，大约是回转半径的数量级（第一个不变量是完整的）。对于近赤道的粒子（I = 0），变化可以明显更大。假设南北对称和晨昏对称，我们给出了第二不变跳跃ΔI的一般理论表达式，它可以应用于各种磁场模型。结果表明，ΔI敏感地依赖于粒子在分岔线处的反弹相位。在反弹相系综上，ΔI的均方根值随镜场的减小和动能的增大而增大。我们验证这些结果与测试粒子模拟模型磁场。
[1] Close to the dayside magnetopause, there is a region of space where each field line has two magnetic field minima, one near each cusp. That region is located around local noon, and extends about 1–2 Re from the magnetopause. Particles that enter this region with equatorial pitch angles sufficiently close to 90° will cross the dayside not along an equatorial path, but along one of the two branches on either side of the equatorial plane. The two branches are joined again past local noon. This process of drift-shell bifurcation (DSB) is nonadiabatic even under static conditions. Two physical mechanisms can cause this nonadiabaticity: one that is operative for nearly all magnetospheric magnetic field configurations and another that depends on a particular combination of north-south and east-west asymmetry in the magnetic field. This paper deals only with the first mechanism. For configurations with north-south and east-west symmetry, DSB changes the second invariant I of the motion by a small amount that is of the order of the gyroradius (the first invariant is intact). For near-equatorial particles (I ≈ 0) the change can be significantly larger. Assuming north-south and dawn-dusk symmetry, we present general theoretical expressions for the second-invariant jump ΔI, which can be applied to a variety of magnetic field models. The results show that ΔI is sensitively dependent on the bounce phase of the particle at the bifurcation line. The RMS value of ΔI over a bounce-phase ensemble increases with decreasing mirror field and with increasing kinetic energy. We verify these results with test-particle simulations using model magnetic fields.