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  • ReceivedFeb 25, 2020
  • AcceptedApr 14, 2020
  • PublishedMay 13, 2020

Abstract

The concept of the Solar Ring mission was gradually formed from L5/L4 mission concept, and the proposal of its pre-phasestudy was funded by the National Natural Science Foundation of China in November 2018 and then by theStrategic Priority Program of Chinese Academy of Sciences in space sciences in May 2019. Solar Ring mission will be thefirst attempt to routinely monitor and study the Sun and inner heliosphere from a full $360$-degreeperspective in the ecliptic plane. The current preliminary design of the Solar Ring mission is to deploy six spacecraft, grouped in threepairs, on a sub-AU orbit around the Sun. The two spacecraft in each groupare separated by about $30^\circ$ and every two groups by about $120^\circ$. This configuration with necessaryscience payloads will allow us toestablish three unprecedented capabilities: (1) determine the photospheric vector magneticfield with unambiguity, (2) provide $360$-degree maps of the Sun and the inner heliosphere routinely, and (3) resolve the solar wind structuresat multiple scales and multiple longitudes. With these capabilities, the Solar Ring mission aims to addressthe origin of solar cycle, the origin of solar eruptions, the origin ofsolar wind structures and the origin of severe space weather events. The successful accomplishment of the missionwill advance our understanding of the star and the space environment that hold our life and enhance our capability ofexpanding the next new territory of human.


Acknowledgment

This work was supported by the Strategic Priority Program of CAS (Grant Nos. XDB41000000 and XDA15017300), the National Natural Science Foundation of China (NSFC) (Grant No. 41842037). WANG YuMing, SHEN ChengLong, GUO JingNan, ZHANG QuanHao, LIU Kai, LI XiaoLei, LIU Rui and WANG Shui are also supported by the CAS Key Research Program of Frontier Sciences (Grant No. QYZDB-SSW-DQC015), and the NSFC (Grant Nos. 41774178, 41761134088, 41750110481 and 11925302), JI HaiSheng by the NSFC (Grant No. 11790302), and Xia LiDong and Huang ZhengHua by the NSFC (Grant No. 41627806). We thank Dr. J. Zhao from Stanford University for reading the manuscript and providing suggestions.


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  • Figure 1

    (a) Schematic diagram of the Solar Ring mission. The six pink dots roughly denote the positions of the six spacecraft, which run on a sub-AU orbit. (b)A sketch map illustrates the stereoscopic angles of the spacecraft and their coverage in the longitude.

  • Figure 2

    (a)The HMI vector magnetogram of the active region NOAA 12192. The gray color scales the longitudinal component of the magnetic field, and the red/blue arrows denote the transversal component. (b), (c)Scatter plots of the longitudinal and transversal components of the magnetic field in the central region marked by the cyan box in (a). (d), (e) The same scatter plots, but in the region near the edge marked by the yellow box in (a). The horizontal axes in (b)–(e) mark the serial numbers of the data points in the data set, having no physical meaning. The magnetic field within the active region is highly structured, but that in the quiescent region is more close to noise. The horizontal pink lines denote the 90-percentile of the magnetic field strength, suggesting the noise level.

  • Figure 3

    A cartoon of the local photospheric region, illustrating some issues affecting the accuracy of the inversion of magnetic field from dual perspectives. The black arrows $a$ and $b$ indicate the observational paths. The photosphere is a non-uniform layer with different optical depths.

  • Figure 4

    (a)Three-color composite EUV image combined from SDO/AIA 211 Å, 193 Å, and 171 Å channels on 1 August 2010. Coronal magnetic field lines extrapolated using a potential field source surface (PFSS) model are superimposed, showing the magnetic connections among different regions. Letters denote the locations of the eruptive events during 1–2 August 2010. (b)GOES 1–8 Å light curve with the same denoted letters. Adapted from ref. [34].

  • Figure 5

    (a) The running-difference images of the heliosphere taken by HI-1 cameras on board the STEREO A and B spacecraft at 21:29 UT on 3 April 2010. A CME was captured. (b) The correlation coefficient (cc) map of the heliosphere at the same time, inferred from the HI-1 images through CORAR method. The CME is reconstructed in the high cc region. The yellow, cyan, orange and blue balls denote the Sun, Mercury, Venus and Earth. Adapted from the paper [59].

  • Figure 6

    A cartoon showing the large-scale magnetic flux rope, or called magnetic cloud, in the heliosphere. The magnetic field lines are twisted in the magnetic cloud as indicated by the color-coded lines. The reconnection site implies the erosion process. A shock exists if the magnetic cloud propagates fast. Adapted from ref. [72].

  • Figure 7

    The fluxes of energetic particles recorded by (a GOES at Earth, (b Radiation Assessment Detector (RAD [80]) at Mars and (c STEREO A during 10–16 September 2017. The pink region indicates the shock (and CME) arrival, and the cyan region the stream interaction region. The positions of the Earth, Mars and STEREO A on 10 September 2017 are plotted too. A complex solar eruption caused the enhancement of the fluxes in a wide range. Adapted from ref. [81].

  • Figure 8

    The elliptical orbits (cyan, light green and pink) of the three groups of the spacecraft with the perihelion of $0.75$ AU and the aphelion of $1$ AU. In this scheme, the separation angle among the three groups oscillates around $120^\circ$ and the angle between the spacecraft in each group oscillates around $30^\circ$.

  • Figure 9

    The data transmission rate as a function of the distance between the spacecraft and Earth. Different lines show the rate for the telescope with different size and different power.

  • Table 1   Science objectives and required measurements
    Solar
    magnetic Solar White- Solar Solar
    Science Scientific questions Strategy field & EUV light Radio wind wind Energetic
    objectives global images images emissions magnetic plasma particles
    Doppler field
    velocity
    Originof solarcycle How does the global magnetic flux emerge, transport and dissipate? Trace the global magnetic fluxes at multiple scales. $\surd$
    What is the solar internal structure? Analyze the global oscillation modes. $\surd$
    Originof solareruptions How is the energy accumulated and released, and how is an eruption triggered? Trace the evolution of source region and combine measured magnetic field, radio emissions and energetic particles to estimate some key parameters, e.g., the magnetic energy and helicity, and key processes. $\surd$ $\surd$ $\surd$ $\surd$
    How are the coronal structures reconstructed, and what kind of structures are formed and ejected into heliosphere? Extrapolate coronal field and compare with observed coronal plasma structures in EUV, compare erupted signatures in EUV and white-light images. $\surd$ $\surd$ $\surd$
    Origin of solar wind structures Where does an solar wind structure come from? What's its topology and magnetic connection with the Sun? How does a solar wind structure evolve in the heliosphere in terms of its propagation direction, velocity, topology, etc? Use white-light images from multiple perspectives to recognize and reconstruct solar wind structures in interplanetary space and trace their evolution. Associate the imaging data of solar wind structures to em in-situ data at 1 AU to confirm their properties, and trace back to the Sun to obtain the properties of their sources. $\surd$ $\surd$ $\surd$ $\surd$ $\surd$ $\surd$ $\surd$
    Origin of severe space weather events What are the primary factors causing major geomagnetic storms and/or solar energetic particle events? Investigate em in-situ data, including magnetic field, solar wind plasma and energetic particles, at different longitudes to assess the effects of various factors on the space weather. $\surd$ $\surd$ $\surd$ $\surd$
    What are the properties of the source regions of the drivers of severe space weather? How can we make an accurate forecast of the space weather effects of solar eruptions? Use imaging data of the heliosphere and the Sun to identify the source regions of the space-weather-effecting solar wind structures and to study the relationship between the solar eruptions and space weather events. $\surd$ $\surd$ $\surd$ $\surd$ $\surd$ $\surd$ $\surd$
  • Table 2   Main tasks and preliminary technical specifications of payloads (to be continued)
    Payloads Main tasks Preliminary technical specifications
    Spectral imager for magnetic field and Measure photospheric vector magnetic field Mass: $\leq30$ kg
    helioseismology (SIMS) to learn the global transportation of magnetic Power consumption: $\leq40$ W
    flux; measure global Doppler velocity to Data rate: $\leq30$ Mbps (@peak time)
    learn the global oscillations. Field of view: $32'\times32'$ (@1 AU)
    Effective pixels: no less than $4096\times4096$
    Spectral resolution: better than $0.04~\AA$
    Temporal resolution of longitudinal component: $1$ min, $1$ h
    Temporal resolution of transversal component: $2$ min, $1$ h
    Multi-band imager for EUV emissions Obtain the global EUV images of solar Mass: $\leq30$ kg
    (MIE) disk at three wavelength bands, Power consumption: $\leq60$ W
    corresponding to relatively cool, warm Data rate: $\leq21$ Mbps (@peak time)
    and hot temperatures, respectively, to Field of view: $42'\times42'$ (@1 AU)
    learn the morphology, topology, Effective pixels: no less than $4096\times4096$
    connectivity and emission measure of Wavelength bands: $304~\AA$, $171~\AA$, $131~\AA$
    various plasma structures. Temporal resolution: $10$ s, $1$ min, $1$ h
    Wide-angle coronagraph (WAC) Obtain white-light images of the solar Mass: $\leq25$ kg
    wind structures traveling through the Power consumption: $\leq40$ W
    outer corona and inner heliosphere to Data rate: $<1$ Mbps (@peak time)
    learn their kinematic properties; get Field of view: $\pm12^\circ$
    total brightness and the variations to Occulting disk: $\pm2^\circ$
    learn the density distribution in 3D. Effective pixels: no less than $4096\times4096$
    Temporal resolution: $1$ min, $1$ h
    Radio investigator (WAVES) Measure the electric field intensity Mass: $\leq15$ kg
    induced by the radio emissions from Power consumption: $\leq16$ W
    the Sun to recognize the radio bursts Data rate: $0.5$ kbps
    and get the location of the driving Frequency range: $5$ kHz–$30$ MHz
    source and its kinematic properties. Frequency channels: no less than 160
    Temporal resolution: better than $30$ s
    GP mode: $0.2$ s for each channel/antenna configuration
    Flux-gate magnetometer (FGM) Measure the em in-situ magnetic field at 1 Mass: $\leq2$ kg
    AU to learn the variations during solar Power consumption: $\leq3$ W
    wind structures and the distribution in Data rate: $\leq8$ kbps
    longitude. Maximum measuring range: $\pm65000$ nT
    Dynamic measurement range: $2000$ nT
    Resolution: better than $0.01$ nT
    Noise level: better than $0.01$ nT/$\sqrt{\text{Hz}}$
    Zero drift: better than $0.01$ nT/$^\circ~C$
    Sampling rate: $0.1$ Hz, $128$ Hz
    Solar wind plasma analyzer (SPA) Measure the em in-situ solar wind plasma Mass: $\leq7$ kg
    at 1 AU; obtain the velocity, density, Power consumption: $\leq~20$ W
    temperature and composition of the Data rate: $\leq50$ kbps
    solar wind to learn the variations Field of view: $180^\circ$ (azimuthal angle) $\times$ $\pm45^\circ$ (polar angle)
    during solar wind structures and the Angular resolution: better than $12^\circ$ (azimuthal) $\times$ $15^\circ$ (polar)
    distribution in longitude. Temporal resolution: 4–64 s (adjustable)
    Ions
    $\bullet$ Energy range: 0.1–25 keV
    $\bullet$ Energy resolution: better than 12%
    $\bullet$ Energy channels: no less than 64
    $\bullet$ Mass range: 0–60 amu
    $\bullet$ Mass resolution: better than 18%
    Electrons
    $\bullet$ Energy range: 0.05–10 keV
    $\bullet$ Energy resolution: better than 12%
    $\bullet$ Energy channels: no less than 64
    High-energy particle detector (HiPD) Measure energetic particles in multiple Mass: $\leq1$ kg
    energies to obtain the intensity and Power consumption: $\leq~1$ W
    spectrum of a solar energetic particle Data rate: $\leq1$ kbps
    event and to learn its driver and the Field of view: $55^\circ$ cone
    distribution in longitude. Mass range: 0–60 amu, electrons
    Energy range of
    $\bullet$ Electrons: 0.5–20 MeV
    $\bullet$ Protons: 10–100 MeV
    $\bullet$ Heavy ions: 20–200 MeV/nuc
    Total Mass: $\leq110$ kg, power: $\leq180$ W, data rate: $\leq52.06$ Mbps

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