Cassini's final phase of exploration The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere. Science, this issue p. eaat5434, p. eaat1962, p. eaat2027, p. eaat3185, p. eaat2236, p. eaat2382 INTRODUCTION Starting on 26 April 2017, the Grand Finale phase of the Cassini mission took the spacecraft through the gap between Saturn’s atmosphere and the inner edge of its innermost ring (the D-ring) 22 times, ending with a final plunge into the atmosphere on 15 September 2017. This phase offered an opportunity to investigate Saturn’s internal magnetic field and the electromagnetic environment between the planet and its rings. The internal magnetic field is a diagnostic of interior structure, dynamics, and evolution of the host planet. Rotating convective motion in the highly electrically conducting layer of the planet is thought to maintain the magnetic field through the magnetohydrodynamic (MHD) dynamo process. Saturn’s internal magnetic field is puzzling because of its high symmetry relative to the spin axis, known since the Pioneer 11 flyby. This symmetry prevents an accurate determination of the rotation rate of Saturn’s deep interior and challenges our understanding of the MHD dynamo process because Cowling’s theorem precludes a perfectly axisymmetric magnetic field being maintained through an active dynamo. RATIONALE The Cassini fluxgate magnetometer was capable of measuring the magnetic field with a time resolution of 32 vectors per s and up to 44,000 nT, which is about twice the peak field strength encountered during the Grand Finale orbits. The combination of star cameras and gyroscopes onboard Cassini provided the attitude determination required to infer the vector components of the magnetic field. External fields from currents in the magnetosphere were modeled explicitly, orbit by orbit. RESULTS Saturn’s magnetic equator, where the magnetic field becomes parallel to the spin axis, is shifted northward from the planetary equator by 2808.5 ± 12 km, confirming the north-south asymmetric nature of Saturn’s magnetic field. After removing the systematic variation with distance from the spin axis, the peak-to-peak “longitudinal” variation in Saturn’s magnetic equator position is <18 km, indicating that the magnetic axis is aligned with the spin axis to within 0.01°. Although structureless in the longitudinal direction, Saturn’s internal magnetic field features variations in the latitudinal direction across many different characteristic length-scales. When expressed in spherical harmonic space, internal axisymmetric magnetic moments of at least degree 9 are needed to describe the latitudinal structures. Because there was incomplete latitudinal coverage during the Grand Finale orbits, which can lead to nonuniqueness in the solution, regularized inversion techniques were used to construct an internal Saturn magnetic field model up to spherical harmonic degree 11. This model matches Cassini measurements and retains minimal internal magnetic energy. An azimuthal field component two orders of magnitude smaller than the radial and meridional components is measured on all periapses (closest approaches to Saturn). The steep slope in this component and magnetic mapping to the inner edge of the D-ring suggests an external origin of this component. CONCLUSION Cassini Grand Finale observations confirm an extreme level of axisymmetry of Saturn’s internal magnetic field. This implies the presence of strong zonal flows (differential rotation) and stable stratification surrounding Saturn’s deep dynamo. The rapid latitudinal variations in the field suggest a second shallow dynamo maintained by the background field from the deep dynamo, small-scale helical motion, and deep zonal flows in the semiconducting region closer to the surface. Some of the high-degree magnetic moments could result from strong high-latitude concentrations of magnetic flux within the planet’s deep dynamo. The periapse azimuthal field originates from a strong interhemispherical electric current system flowing along magnetic field lines between Saturn and the inner edge of the D-ring, with strength comparable to that of the high-latitude field-aligned currents (FACs) associated with Saturn’s aurorae. A meridional view of the results of the Cassini magnetometer observations during the Grand Finale orbits. Overlain on the spacecraft trajectory is the measured azimuthal field from the first Grand Finale orbit, revealing high-latitude auroral FACs and a low-latitude interhemispherical FAC system. Consistent small-scale axisymmetric internal magnetic field structures originating in the shallow interior are shown as field lines within the planet. A tentative deep stable layer and a deeper dynamo layer, overlying a central core, are shown as dashed semicircles. The A-, B-, C-, and D-rings are labeled, and the magnetic field lines are shown as solid lines. RS is Saturn’s radius, Z is the distance from the planetary equator, ρ is the perpendicular distance from the spin axis, and Bϕ is the azimuthal component of the magnetic field. During 2017, the Cassini fluxgate magnetometer made in situ measurements of Saturn’s magnetic field at distances ~2550 ± 1290 kilometers above the 1-bar surface during 22 highly inclined Grand Finale orbits. These observations refine the extreme axisymmetry of Saturn’s internal magnetic field and show displacement of the magnetic equator northward from the planet’s physical equator. Persistent small-scale magnetic structures, corresponding to high-degree (>3) axisymmetric magnetic moments, were observed. This suggests secondary shallow dynamo action in the semiconducting region of Saturn’s interior. Some high-degree magnetic moments could arise from strong high-latitude concentrations of magnetic flux within the planet’s deep dynamo. A strong field-aligned current (FAC) system is located between Saturn and the inner edge of its D-ring, with strength comparable to the high-latitude auroral FACs.

中文翻译：

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卡西尼号总决赛揭示的土星磁场

卡西尼号探索的最后阶段 卡西尼号飞船绕土星运行了 13 年；由于燃料不足，轨迹被更改为尚未访问的样本区域。一系列靠近环的轨道之后是大结局轨道，该轨道使航天器穿过土星与其环之间的间隙，然后航天器在进入土星高层大气时被摧毁。本期报告中的六篇论文来自卡西尼号任务的这些最后阶段。多尔蒂等人。测量了靠近土星的磁场，这意味着行星内部存在复杂的多层发电机过程。鲁索斯等人。检测到一个额外的辐射带被困在环内，由自由中子的放射性衰变维持。拉米等人。当卡西尼号飞过发射千米辐射的区域时进行的当前等离子体测量，这些区域与行星的极光相连。许等人。确定了从环落入行星的大型固体尘埃颗粒的组成，而 Mitchell 等人。研究了较小的尘埃纳米颗粒，并展示了它们如何与行星的高层大气相互作用。最后，韦特等人。确定了下落物质中的分子，并直接测量了土星大气的成分。科学，这个问题 p。eaat5434, 页。eaat1962, p. eaat2027，第 eaat3185, 页。eaat2236, 页。eaat2382 介绍 从 2017 年 4 月 26 日开始，卡西尼号任务的大结局阶段使航天器通过土星大气层与其最内环（D 环）内缘之间的间隙 22 次，最终于 2017 年 9 月 15 日坠入大气层。这一阶段为研究土星的内部磁场和行星及其环之间的电磁环境提供了机会。内部磁场是对宿主行星内部结构、动力学和演化的诊断。行星高导电层中的旋转对流运动被认为通过磁流体动力学 (MHD) 发电机过程维持磁场。土星的内部磁场令人费解，因为它相对于自旋轴的高度对称性，自先驱者 11 号飞越以来就为人所知。这种对称性阻碍了对土星深部内部旋转速率的准确确定，并挑战了我们对 MHD 发电机过程的理解，因为 Cowling 定理排除了通过有源发电机维持完美轴对称磁场。基本原理 卡西尼磁通门磁力计能够以每秒 32 个矢量和高达 44,000 nT 的时间分辨率测量磁场，这大约是 Grand Finale 轨道期间遇到的峰值场强的两倍。卡西尼号星载相机和陀螺仪的组合提供了推断磁场矢量分量所需的姿态确定。磁层中电流的外部场被逐个轨道地明确建模。结果土星的磁赤道，磁场变得平行于自旋轴，从行星赤道向北偏移 2808.5 ± 12 公里，证实了土星磁场的南北不对称性质。去掉自旋轴随距离的系统性变化后，土星磁赤道位置的峰峰值“纵向”变化<18 km，表明磁轴与自旋轴在0.01°以内。尽管在纵向上没有结构，但土星的内部磁场在纬度方向上具有许多不同特征长度尺度的变化。当在球谐空间中表示时，需要至少 9 次的内部轴对称磁矩来描述纬度结构。由于在 Grand Finale 轨道期间存在不完整的纬度覆盖，这可能导致解中的非唯一性，因此使用正则化反演技术构建了高达 11 次球谐函数的土星内部磁场模型。该模型与卡西尼号测量结果相匹配并保留了最小的内部磁能。在所有近点（最接近土星的地方）上测量到比径向和子午分量小两个数量级的方位角场分量。该组件的陡坡和 D 形环内边缘的磁映射表明该组件的外部起源。结论 卡西尼大结局的观测证实了土星内部磁场的极端轴对称性。这意味着存在强烈的纬向流动（差异旋转）和围绕土星深层发电机的稳定分层。场中快速的纬度变化表明，第二个浅发电机由来自深发电机、小规模螺旋运动和靠近表面的半导体区域中的深带状流动的背景场维持。一些高强度磁矩可能是由行星深层发电机内强烈的高纬度磁通量集中引起的。近点方位角场源自一个强大的半球间电流系统，该电流系统沿着土星和 D 环内边缘之间的磁场线流动，其强度与与土星极光相关的高纬度场对齐电流 (FAC) 的强度相当. 卡西尼号磁力计在大结局轨道期间观测结果的子午视图。覆盖在航天器轨迹上的是从第一次总决赛轨道测得的方位角场，揭示了高纬度极光 FAC 和低纬度半球间 FAC 系统。源自浅层内部的一致小尺度轴对称内部磁场结构显示为行星内的磁力线。一个暂定的深稳定层和一个更深的发电机层，覆盖在中央核心上，显示为虚线半圆。A-、B-、C-和D-环被标记，磁场线显示为实线。RS 是土星的半径，Z 是到行星赤道的距离，ρ 是到自旋轴的垂直距离，Bϕ 是磁场的方位角分量。2017 年期间，卡西尼磁通门磁力计在 22 个高度倾斜的 Grand Finale 轨道期间，在 1-bar 表面上方约 2550 ± 1290 公里处对土星磁场进行了原位测量。这些观测改进了土星内部磁场的极端轴对称性，并显示了磁赤道从行星物理赤道向北的位移。观察到与高度（> 3）轴对称磁矩相对应的持久小规模磁结构。这表明在土星内部的半导体区域中存在次级浅层发电机作用。一些高度磁矩可能是由行星深层发电机内的高纬度磁通量集中引起的。一个强场对齐电流 (FAC) 系统位于土星和其 D 形环的内边缘之间，