Solar winds along curved magnetic field lines
Solar winds along curved magnetic field lines
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DOI：
10.1051/0004-6361/201116668
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发表时间：
2011-03
影响因子：
6.5
通讯作者：
Bo Li;L. Xia;Yao Chen
中科院分区：
文献类型：
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作者：
Bo Li;L. Xia;Yao Chen
语境。使用行星际闪烁（IPS）技术的遥感测量和尤利西斯航天器的现场测量都显示出太阳极小条件下太阳风的双峰结构。目前还有待解决为什么快风快、慢风慢的问题。虽然流管的冠状膨胀率 f(c) 与现场测量的速度 v 之间存在强大的经验相关性，但更详细的数据分析表明 v 不仅仅取决于 f(c)。目标。我们研究自然伴随任何非径向膨胀的磁力线的非径向形状是否可能是一个额外的几何因素。方法。我们求解了输运方程，其中包含了电子-质子太阳风的湍流阿尔文波的加热，沿着解析磁场模型给出的弯曲场线，该模型代表了太阳极小日冕。结果。研究发现，场线形状对太阳风参数有很大的影响，与忽略场线曲率的情况相比，渐进速度降低了约 130 km s(-1) 或相对而言约 28%。这种效应在太阳风能量添加的总体框架中得到解释：与直线情况相比，场线曲率增强了亚音速流的有效能量沉积，从而导致更高的质子通量和更低的终端质子速度。结论。我们的计算表明，场线曲率可能是一个几何因素，除了管膨胀之外，它还显着影响太阳风速。此外，虽然场线曲率不太可能影响太阳极小期的极地快速太阳风，但它确实有助于使低纬度地区的风变慢，这反过来又有助于更好地再现尤利西斯测量结果。
Context. Both remote-sensing measurements using the interplanetary scintillation (IPS) technique and in-situ measurements by the Ulysses spacecraft show a bimodal structure for the solar wind at solar minimum conditions. At present it still remains to address why the fast wind is fast and the slow wind is slow. While a robust empirical correlation exists between the coronal expansion rate f(c) of the flow tubes and the speeds v measured in situ, a more detailed data analysis suggests that v depends on more than just f(c). Aims. We examine whether the non-radial shape of field lines, which naturally accompanies any non-radial expansion, could be an additional geometrical factor. Methods. We solved the transport equations incorporating the heating from turbulent Alfven waves for an electron-proton solar wind along curved field lines given by an analytical magnetic field model, which is representative of a solar minimum corona. Results. The field line shape is found to influence the solar wind parameters substantially, reducing the asymptotic speed by up to similar to 130 km s(-1) or by similar to 28% in relative terms, compared with the case where the field line curvature is neglected. This effect was interpreted in the general framework of energy addition in the solar wind: compared to the straight case, the field line curvature enhances the effective energy deposition to the subsonic flow, which results in a higher proton flux and a lower terminal proton speed. Conclusions. Our computations suggest that the field line curvature could be a geometrical factor which, in addition to the tube expansion, substantially influences the solar wind speed. Furthermore, although the field line curvature is unlikely to affect the polar fast solar wind at solar minima, it does help make the wind at low latitudes slow, which in turn helps better reproduce the Ulysses measurements.