Self‐consistent simulation of the photoelectron‐driven polar wind from 120 km to 9 R E altitude
Self‐consistent simulation of the photoelectron‐driven polar wind from 120 km to 9 R E altitude
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DOI：
10.1029/97ja03085
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发表时间：
1998-02
影响因子：
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通讯作者：
Yi‐Jiun Su;J. Horwitz;G. Wilson;P. Richards;D. G. Brown;C. W. Ho
中科院分区：
文献类型：
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作者：
Yi‐Jiun Su;J. Horwitz;G. Wilson;P. Richards;D. G. Brown;C. W. Ho
人们早就认识到，光电子可以增强影响极风外流的双极电场[例如，Axford, 1968；勒麦尔1972]。由于电离层离子和电子在很大程度上是由低海拔中性大气的光电离产生的，而最大的光电子产生速率发生在130-140公里的高度范围内，因此从E区到几个地球半径的高度对这种光电子驱动的极风进行自一致的建模是必要的。本文描述了一种新的稳态耦合流体-半动力学模型，以有效地将源区与高海拔区耦合起来。该模型结合了120-800 km高度范围内的流体处理，800 km至2 RE高度范围内的广义半动力学（GSK）处理和2 - 9 RE高度范围内的稳态无碰撞半动力学方法。我们应用该模型研究了电离层条件从太阳极小值（F10.7 = 90）到太阳极大值（F10.7 = 200）的光电子驱动极风。在3re高度以下，从太阳极小期到太阳极大期，O+和H+密度分别增加约5倍和2倍。在3 RE以下，平行体速度随F10.7的增加变化不大。在3 RE以上，当包含的向下磁鞘电子通量（如极雨）不足以平衡电离层光电子通量时，形成40 V左右的电势层。这样的势层将电离层离子在高海拔加速到超音速，高于3re，但在低海拔则不然。我们还发现，当磁层电子密度从0.05增加到2 cm−3时，在太阳最小条件下电位层从40减小到8.5 V，在太阳最大条件下电位层从46减小到12 V。
It has long been recognized that photoelectrons can enhance the ambipolar electric fields affecting polar wind outflows [e.g., Axford, 1968; Lemaire, 1972]. Since ionospheric ions and electrons are produced in large part by photoionization of the neutral atmosphere at lower altitudes, and the maximum photoelectron production rate occurs in the 130–140 km altitude range, it is essential to model this photoelectron-driven polar wind self-consistently from the E region to an altitude of several Earth radii. Here we describe a new steady state coupled fluid-semikinetic model to efficiently couple the source region to the high-altitude regions. This model couples a fluid treatment for the 120–800 km altitude range, a generalized semikinetic (GSK) treatment for the altitude range 800 km to 2 RE, and a steady state collisionless semikinetic method for the altitude range 2–9 RE. We apply this model to investigate the photoelectron-driven polar wind with ionospheric conditions ranging from solar minimum (F10.7 = 90) to solar maximum (F10.7 = 200). The O+ and H+ densities are found to increase by factors of approximately 5 and 2, respectively, from solar minimum to solar maximum below 3 RE altitude. However, the parallel bulk velocities display little variation with increased F10.7 for altitudes below 3 RE. An electric potential layer of the order of 40 V develops above 3 RE altitude, when the included downward magnetosheath electron fluxes (such as polar rain) are insufficient to balance the ionospheric photoelectron flux. Such potential layers accelerate the ionospheric ions to supersonic speeds at high altitudes, above 3 RE, but not at low altitudes. We also found that the potential layer decreases from 40 to 8.5 V for solar minimum conditions and from 46 to 12 V for solar maximum conditions when the magnetospheric electron density is increased from 0.05 to 2 cm−3.