2D He + pickup ion velocity distribution functions: STEREO PLASTIC observations
2D He + pickup ion velocity distribution functions: STEREO PLASTIC observations
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DOI：
10.1051/0004-6361/201425271
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发表时间：
2014-12
影响因子：
6.5
通讯作者：
C. Drews;L. Berger;A. Taut;T. Peleikis;R. Wimmer–Schweingruber
中科院分区：
文献类型：
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作者：
C. Drews;L. Berger;A. Taut;T. Peleikis;R. Wimmer–Schweingruber
上下文。He+拾取离子要么来自我们日光层内的星际中性氦的电离，即所谓的星际拾取离子，要么来自太阳风离子与微小尘埃粒子的相互作用，即所谓的拾取离子的内源。到目前为止，对拾取离子的观测大多局限于一维简并速度谱，这不利于研究He+速度分布函数的某些特性。目标。通常认为，新产生的拾取离子的快速俯仰角散射会迅速导致完全各向同性的He+VDF。根据最近的观察，这一假设已被发现过于简单化，需要重新调查。方法：研究方法。利用STEREO A航天器上塑料仪器上的He+拾取离子数据，我们在二维上重建了He+VDF的简化形式。这使得我们能够研究2DHe+Vdf随日球层磁场fi构型的相对变化。结果。我们的观测表明，He+Vdf是高度各向异性的，甚至表明，至少对B的某些fi方程而言，它不是完全自旋的。我们的结果进一步表明，He+的观测速度和俯仰角强烈地依赖于局地太阳磁场fi场矢量B、黄道经度λ、太阳风速vSw和B的全球分布。结论。我们发现了两个不同的信号，它们随着B的排列而系统地改变：(1)在wSw>；0处最明显的环形波束分布。5，很可能归因于星际He+；(2)平行于B的光束信号，在wSw和lt；0时最明显。5，归因于内源He+。He+VDF的强各向异性和前述依赖关系也意味着对He+拾取离子一维速度谱的观测具有潜在的欺骗性。
Context. He + pickup ions are either born from the ionization of interstellar neutral helium inside our heliosphere, the so-called interstellar pickup ions, or through the interaction of solar wind ions with small dust particles, the so-called inner source of pickup ions. Until now, most observations of pickup ions were limited to reduced 1D velocity spectra, which are insu ffi cient to study certain characteristics of the He + velocity distribution function (VDF). Aims. It is generally assumed that rapid pitch-angle scattering of freshly created pickup ions quickly leads to a fully isotropic He + VDF. In light of recent observations, this assumption has found to be oversimplified and needs to be reinvestigated. Methods. Using He + pickup ion data from the PLASTIC instrument on board the STEREO A spacecraft, we reconstruct a reduced form of the He + VDF in two dimensions. This allows us to study relative changes of the 2D He + VDF as a function of the configuration of the heliospheric magnetic field. Results. Our observations show that the He + VDF is highly anisotropic and even indicates that, at least for certain configurations of B , it is not fully gyrotropic. Our results further suggest, that the observed velocity and pitch angle of He + depends strongly on the local solar magnetic field vector, B , the ecliptic longitude, λ , the solar wind speed, v sw , and the global distribution of B . Conclusions. We found two distinct signatures that systematically change as a function of the alignment of B : (1) a ring beam distribution that is most pronounced at w sw > 0 . 5 and likely attributed to interstellar He + ; (2) a beam signature aligned parallel to B that is most pronounced at w sw < 0 . 5 and attributed to inner-source He + . The strong anisotropy and the aforementioned dependencies of the He + VDF also imply that observations of 1D velocity spectra of He + pickup ions are potentially deceiving.