HELIOSPHERIC STRUCTURE: THE BOW WAVE AND THE HYDROGEN WALL
HELIOSPHERIC STRUCTURE: THE BOW WAVE AND THE HYDROGEN WALL
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日球层结构：弓波和氢壁
DOI：
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发表时间：
2013
期刊：
影响因子：
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通讯作者：
D. McComas
中科院分区：
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作者：
G. Zank;J. Heerikhuisen;Brian E. Wood;N. Pogorelov;E. Zirnstein;D. McComas
最近的IBEX观测表明，本地星际介质（LISM）的流速比以前认为的要小（23.2 km s−1而不是26 km s−1）。合理的LISM等离子体参数表明，LISM流可能是边缘超快磁声或亚快磁声。这就提出了两个具有挑战性的问题：（1）一个仅仅是超快或亚快磁声的LISM模型能否解释严重依赖于氢壁（H壁）提供的额外吸收的Lyα观测结果？（2）如果LISM流是弱超快磁声流，跃迁是呈现传统激波的形式，还是中性氢（H）通过电荷交换介导激波耗散和结构？这两个问题都解决了使用三个三维自洽耦合磁流体动力学等离子体动力学H模型与不同的LISM磁场强度（2，3，和4 μG）以及等离子体和中性H数密度。2 μ G和3 μG模型是太阳风顶远上风处的快磁声模型，而4 μG模型是完全亚音速模型。在2 μG模型中，存在一个宽的（1050 -75 Au）类弓激波结构。3 μG模型有一个平滑的超快-亚快磁声跃迁，类似于一个非常宽的，1000 Au厚的弓形波。理论分析表明，从超快到亚快的磁声下游态的转变是由于分别在超音速太阳风和热的内日鞘中产生的快中性H和热中性H的电荷交换。对于2 μG和3 μG模型，超快磁声LISM流通过一个临界点，该临界点位于快磁声马赫数M = 1和Qe = γ/（γ − 1）UQm处，其中Qe和Qm是由于电荷交换引起的等离子体能量和动量源项，U是LISM流速，γ是等离子体绝热指数。由于马赫数在3 μG的情况下仅为超快磁音速，因此热和快速的中性H可以完全调解过渡并在结构上施加电荷交换长度尺度，使得太阳-风-LISM相互作用有效地无弓激波。因此，快速和热的日光层中性H的电荷交换在弱日光层弓形激波处提供了主要的耗散机制，在某些情况下有效地创建了一个激波日光层（即，仅日光层终端激波）。这两种超快磁声模型都产生了相当大的H壁。我们发现：（1）亚快磁声LISM流不能模拟沿沿着所考虑的四条视线（α Cen，36 Oph，DK UMa，和χ1 Ori-upwind，sidewind，and downwind）观测到的Lyα吸收剖面;（2）两种超快磁声模型都能解释Lyα观测，可能无弓激波3 μG模型稍有优势。根据进一步的建模和与更远的视线的比较，我们得出结论，IBEX可能已经发现了一类由中性H介导的星际激波。
Recent IBEX observations indicate that the local interstellar medium (LISM) flow speed is less than previously thought (23.2 km s−1 rather than 26 km s−1). Reasonable LISM plasma parameters indicate that the LISM flow may be either marginally super-fast magnetosonic or sub-fast magnetosonic. This raises two challenging questions: (1) Can a LISM model that is barely super-fast or sub-fast magnetosonic account for Lyα observations that rely critically on the additional absorption provided by the hydrogen wall (H-wall)? and (2) If the LISM flow is weakly super-fast magnetosonic, does the transition assume the form of a traditional shock or does neutral hydrogen (H) mediate shock dissipation and hence structure through charge exchange? Both questions are addressed using three three-dimensional self-consistently coupled magnetohydrodynamic plasma—kinetic H models with different LISM magnetic field strengths (2, 3, and 4 μG) as well as plasma and neutral H number densities. The 2 and 3 μG models are fast magnetosonic far upwind of the heliopause whereas the 4 μG model is fully subsonic. The 2 μG model admits a broad (∼50–75 AU) bow-shock-like structure. The 3 μG model has a smooth super-fast–sub-fast magnetosonic transition that resembles a very broad, ∼200 AU thick, bow wave. A theoretical analysis shows that the transition from a super-fast to a sub-fast magnetosonic downstream state is due to the charge exchange of fast neutral H and hot neutral H created in the supersonic solar wind and hot inner heliosheath, respectively. For both the 2 μG and the 3 μG models, the super-fast magnetosonic LISM flow passes through a critical point located where the fast magnetosonic Mach number M = 1 and Qe = γ/(γ − 1)UQm, where Qe and Qm are the plasma energy and momentum source terms due to charge exchange, U is the LISM flow speed, and γ is the plasma adiabatic index. Because the Mach number is only barely super-fast magnetosonic in the 3 μG case, the hot and fast neutral H can completely mediate the transition and impose a charge exchange length scale on the structure, making the solar-wind–LISM interaction effectively bow-shock-free. The charge exchange of fast and hot heliospheric neutral H therefore provides a primary dissipation mechanism at the weak heliospheric bow shock, in some cases effectively creating a one-shock heliosphere (i.e., a heliospheric termination shock only). Both super-fast magnetosonic models produce a sizeable H-wall. We find that (1) a sub-fast magnetosonic LISM flow cannot model the observed Lyα absorption profiles along the four sightlines considered (α Cen, 36 Oph, DK UMa, and χ1 Ori—upwind, sidewind, and downwind respectively); (2) both the super-fast magnetosonic models can account for the Lyα observations, with possibly the bow-shock-free 3 μG model being slightly favored. Subject to further modeling and comparison against further lines of sight, we conclude with the tantalizing possibility that IBEX may have discovered a class of interstellar shocks mediated by neutral H.