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日球层作为一个宜居性的星球层(habitable astrosphere),认识日球层的现状乃至其过去与未来,都将为人类在茫茫银河系和宇宙中探索宜居性星球层提供重要的参考线索。如同认识地球空间环境那样,要对不同方位的磁层进行原位探测,磁层顶鼻尖、侧翼边界层、磁尾等一个都不能落,这样才能认清地球空间环境的全貌,摸清地球空间受太阳风影响动态演化的机理。
日球层顶,作为从一个空间(行星际)跨越到另一个空间(恒星际空间)的分水岭(见图11),能够从不同的方位穿过它,都将是人类探索茫茫宇宙空间的壮举,也会带领领略日球层顶不同方位(鼻尖、尾部、极区)的特色与差异。关键探测要素(特别是拾起离子)的缺位,已经成为恒星际空间探路者(“旅行者1号”和“旅行者2号”飞船)永远的遗憾,但也为后来者继续探秘外日球层太阳风与侵入星际风的相互作用机理与效应提供了继续前行的动力。
在浩瀚的日球层空间这个舞台上,离子作为主角之一,会不停地变换角色,比如从拾起离子膨胀冷却下来与背景太阳风离子混合,拾起离子被可压缩湍动能化形成超热离子,超热离子在终止激波处进一步加速形成异常宇宙线。要追踪离子在不同角色之间的串位,需要在宽能段范围里(比如携带一套宽能段离子能谱仪:可包括法拉第杯、静电分析仪、和固体半导体探测器等)测量它们(见表1)。
表 1针对外日球层太阳风与侵入星际介质流的宽能段粒子测量的建议探测载荷和方案
Table 1.Possible payloads and schemes suggested for the measurement of wide-energy-band ions in the outer heliosphere and the intrusive interstellar medium flow
载荷 能量范围 探测对象 可选方案 太阳风法拉第杯 10 eV~12 keV 原初太阳风离子(H+,He++) 法拉第杯(FC) 太阳风静电分析仪 100 eV~20 keV 原初太阳风离子;部分星际风拾起离子
(H+,He+)静电分析仪(ESA) 拾起离子质谱仪 5 eV~80 keV 星际风拾起离子(H+,He+,N+,O+);
部分超热离子静电分析仪(ESA)+飞行时间技术
(TOF)+ 固态半导体(SSD)超热离子谱仪 30 keV~5 MeV 湍动加速的超热离子 (双层)固态半导体SSD “新视野号”航天器作为外日球层探测任务因为没有携带磁强计,所以无法真正开展与“场–粒子耦合作用”这一基本物理过程有关的“超热离子受可压缩湍动能化加速形成”的前沿课题。因此“探测行星际乃至恒星际中微弱的磁场及其湍动”应该成为未来太阳系边际探测任务设计中所面对的众多探测要素中的必选项。专门针对太阳系边际开展抵达探测的设想,是驱动深空探测能力迈上新台阶的重要源泉,也被认为将是我国建设航天强国的重要标志[39]。
Broad Energy-Band Ions in the Ourter Heliosphere and Their Coupling with Turbulence
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摘要:围绕“日球层太阳风及其与侵入星际风的作用”这个主题,从目前认知、前沿问题、探测建议等3方面展开论述。日球层中的离子包括:原初太阳风离子、源于星际风的拾起离子、超热离子。其中拾起离子和超热离子有来自局地星际质侵入流的贡献。深空飞船探测到双模态遍布日球层:日球层内边界、太阳风、行星际湍动、超热离子能谱等都存在双模态特征。前沿问题有3个方面:①从未到达的领域,即黄道面里的日球层尾部以及高纬的外日球层;②曾经到达的领域,但是没有探测的物理量,例如外日球层中的拾起离子;③曾经到达并探测,但是形成机制不明,例如超热离子的幂律谱及其双模态。针对上述前沿问题,本文提出如下的建议:①设计不同的飞行路径,朝向不同的方位进行探测;②携带宽能段的离子谱仪器包涵盖原初太阳风等离子体、拾起离子和超热离子;③搭载高灵敏度磁强计测量外日球层的可压缩磁湍动。Abstract:This paper focuses on the theme of “solar wind in the heliosphere and its interaction with the invading interstellar wind”, and discusses it from three aspects: current cognition, frontier problems, and exploration suggestions. The ions in the heliosphere include primary solar wind ions, pick-up ions converted partially from interstellar wind, and super-thermal ions. Among them, pick-up ions and super-thermal ions have their contribution from the local interstellar medium flow. The deep-space spacecraft have detected the prevalence of two modes in the heliosphere: the inner boundary of the heliosphere, the solar wind, the interplanetary turbulence, and the energy spectrum of the super-thermal ions. There are three types of cutting-edge issues: ① the territory never reached, that is, the tail of the heliosphere in the ecliptic plane and the outer heliosphere at high latitudes; ② the territory that has been reached, but some key variables have not been detected, such as the picked-up ions in the outer heliosphere; ③ the territory and variables that have been reached and detected, but the formation mechanism is unknown, such as the power-law spectrum and dual-mode of the super-thermal ions. To address these problems, we put forward the following suggestions: ① to design different flight paths and detect in different directions; ② to carry ion spectra instruments with wide energy band, covering the primary solar wind plasma, the pick-up ions, and the super-thermal ions; ③ to carry high sensitivity magnetometer to measure the compressible magnetic turbulence in the outer heliosphere.
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Key words:
- outer heliosphere/
- interstellar wind/
- particle spectrum
Highlights● This paper points out that the broad energy-band ion spectrum and its coupling with the turbulence is one of the key windows to understand the interaction process between the heliosphere and the local interstellar medium. ● This work provides a brief review of the current cognition of the heliosphere, summarizes three related cutting-edge issues, and proposes the corresponding exploration suggestions for the future program. -
图 9“新视野号”飞船上SWAP载荷测量到的离子计数率随单位电荷能量的变化剖面和根据“新视野号”和“旅行者号”飞船探测,以及模型假设,所画出的不同压强随日心距离的变化以及相互之间大小随单位电荷能量的变化剖面
Fig. 9Profile of ion count rate versus energy per charge measured by SWIP payload onboard New Horizons spacecraft, consisting of solar wind protons, solar wind helium ions,pickup protons and pickup helium ions and changes of different pressures with the heliocentric distance, according to the measurements from New Horizons and Voyage,as well as the model prediction
表 1针对外日球层太阳风与侵入星际介质流的宽能段粒子测量的建议探测载荷和方案
Table 1Possible payloads and schemes suggested for the measurement of wide-energy-band ions in the outer heliosphere and the intrusive interstellar medium flow
载荷 能量范围 探测对象 可选方案 太阳风法拉第杯 10 eV~12 keV 原初太阳风离子(H+,He++) 法拉第杯(FC) 太阳风静电分析仪 100 eV~20 keV 原初太阳风离子;部分星际风拾起离子
(H+,He+)静电分析仪(ESA) 拾起离子质谱仪 5 eV~80 keV 星际风拾起离子(H+,He+,N+,O+);
部分超热离子静电分析仪(ESA)+飞行时间技术
(TOF)+ 固态半导体(SSD)超热离子谱仪 30 keV~5 MeV 湍动加速的超热离子 (双层)固态半导体SSD -
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