Cosmic rays in the immediate local interstellar medium

直接局部星际介质中的宇宙射线

• 批准号: 490751943
• 负责人: Professor Dr. Dieter Breitschwerdt
• 金额: --
• 依托单位: Zentrum für Astronomie und Astrophysik
• 依托单位国家: 德国
• 项目类别: Research Grants
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/490751943?language=en
• 关键词: Cosmic; rays; immediate; local; interstellar

Precise observations of cosmic rays (CRs) are only available from our position in the Galaxy and are affected by the structure of our local environment: The Earth is situated in the heliosphere, which suppresses fluxes at energies smaller than a few GeV. The heliosphere itself is embedded in a low-density superbubble, called the Local Bubble (LB). The exact history and spatial distribution of nearby sources are also unknown, leading to sizeable uncertainties of the spectra at high and very low energies. Recently, the first direct observations outside the heliosphere have become available at energies below a GeV thanks to the two Voyager probes which have crossed into interstellar space. We note that compared to the dimensions of the LB, these measurements are still only probing the immediate local interstellar medium (ISM).Diffuse emission, most prominently from CR nuclei producing gamma-rays, and ionisation rates in diffuse molecular clouds (MCs) remotely probe CRs elsewhere in the Galaxy. This approach gives complementary constraints, but is affected by uncertainties in the astrophysics and interstellar chemistry involved. While the inferences from the gamma-ray measurements have displayed some interesting anomalies, the ionisation rates show an even stronger deviation, with the ones inferred from MCs elsewhere in the Galaxy one to two orders of magnitude large than what is inferred from the locally measured fluxes. The discrepancy is even stronger for measurements of ionisation rates close to the galactic centre which are almost four orders of magnitude larger than the local rate!For the resolution of this oftentimes ignored anomaly, we consider mainly two scenarios: First, the fluxes of CRs at energies below a GeV fluctuate spatially within the Galaxy. This is due to the combined effect of the discrete nature of sources and the limited range due to ionisation and Coulomb losses at low energies. If the locally observed flux were at the low end of the distribution of fluxes or the observed ionisation rates were biased towards the high end of the distribution, this could explain the discrepancy. We will predict the distribution of fluxes and statistically evaluate the compatibility between local spectra and ionisation measurements.Second, the flux of local CRs could be suppressed due to the presence of the LB. On small scales, CR transport is anisotropic due to the presence of the regular magnetic field. With little observational constraints on the regular component of the LB, we will instead consider a variety of analytical and numerical models, and study the diffusion of primary and secondary CRs. Eventually, we will combine both scenarios in a consistent global picture.The synergy between the Paris partners, experts on diffuse emission and ionisation rates, the Aachen ones, experts on CR transport, and the Berlin ones, experts on hydrodynamical simulations of the ISM, will guarantee resolving one of the most puzzling anomalies in CRs.

对宇宙射线（CR）的精确观测只能从我们在银河系的位置进行，并且受到当地环境结构的影响：地球位于日光层，它抑制能量小于几吉伏的通量。日光层本身嵌入一个低密度超级气泡中，称为局部气泡（LB）。附近源的确切历史和空间分布也是未知的，导致高能量和极低能量的光谱存在相当大的不确定性。最近，由于两个穿越星际空间的航行者探测器，首次在日光层外以低于 GeV 的能量进行直接观测。我们注意到，与LB的尺寸相比，这些测量仍然只是探测直接的本地星际介质（ISM）。漫射发射，最显着的是来自产生伽马射线的CR核，以及漫射分子云（MC）中的电离率远程探测银河系其他地方的CR。这种方法给出了补充约束，但受到所涉及的天体物理学和星际化学的不确定性的影响。虽然伽马射线测量的推论显示出一些有趣的异常现象，但电离率显示出更大的偏差，从银河系其他地方的MC推断出的电离率比从当地测量的通量推断出的电离率大一到两个数量级。对于接近银河系中心的电离率的测量，这种差异甚至更大，该电离率几乎比本地电离率大四个数量级！为了解决这种经常被忽视的异常现象，我们主要考虑两种情况：首先，能量低于GeV的CR通量在银河系内空间波动。这是由于源的离散性质和低能量下的电离和库仑损失导致的有限范围的综合影响。如果局部观察到的通量位于通量分布的低端，或者观察到的电离率偏向分布的高端，这可以解释这种差异。我们将预测通量的分布，并统计评估局部光谱和电离测量之间的兼容性。其次，由于 LB 的存在，局部 CR 的通量可能会受到抑制。在小尺度上，由于规则磁场的存在，CR 传输是各向异性的。由于对 LB 常规成分的观测限制很少，我们将考虑各种分析和数值模型，并研究初级和次级 CR 的扩散。最终，我们将把这两种情况结合在一个一致的全球图景中。巴黎合作伙伴（扩散发射和电离率专家）、亚琛合作伙伴（CR 传输专家）和柏林合作伙伴（ISM 流体动力学模拟专家）之间的协同作用，将保证解决 CR 中最令人费解的异常现象之一。