Overview of the Solar Polar Orbit Telescope Project for Space Weather Mission ∗

The Solar Polar ORbit Telescope (SPORT) project for space weather mission has been under intensive scientific and engineering background studies since it was incorporated into the Chinese Space Science Strategic Pioneer Project in 2011. SPORT is designed to carry a suite of remote-sensing and in-situ instruments to observe Coronal Mass Ejections (CMEs), energetic particles, solar high-latitude magnetism, and the fast solar wind from a polar orbit around the Sun. The first extended view of the polar regions of the Sun and the ecliptic enabled by SPORT will provide a unique opportunity to study CME propagation through the inner heliosphere, and the solar high-latitude magnetism giving rise to eruptions and the fast solar wind. Coordinated observations between SPORT and other spaceborne/ground-based facilities within the International Living With a Star (ILWS) framework can significantly enhance scientific output. SPORT is now competing for ∗ Supported by the Strategic Priority Research Program on Space Science (XDA04060801, XDA04060802, XDA04060803, XDA04060804) of Chinese Academy of Sciences, the Specialized Research Fund for State Key Laboratory of China, the Chinese National Science Foundation (41374175, 41204129), and the CAS/SAFEA international Partnership Program for Creative Research Teams Received February 21 E-mail: mxiong@spaceweather.ac.cn 246 Chin. J. Space Sci. 2016, 36(3) official selection and implementation during China’s 13th Five-Year Plan period of 2016—2020.


Introduction
The heliosphere represents a uniquely accessible domain to study fundamental physical processes common to space and laboratory plasmas that cannot be studied from astronomical distances or reproduced on Earth.Understanding the causal connections between the Sun and the heliosphere is of fundamental importance to space physics and space weather [1−2] .The Sun, interplanetary space, and Earth can be viewed as key elements of an interconnected system.The inner heliosphere is permeated with the supersonic and magnetized solar wind from the Sun.
The solar wind consists of dominant protons and electrons as well as minor heavy ions.The solar wind plasma is collisionless, and waves/turbulence are ubiquitous [3−6] .The solar wind is the only available plasma "laboratory" where turbulence can be studied without interference from spatial boundaries and in the important domain of very large magnetic Reynolds numbers.
Coronal Mass Ejections (CMEs), carrying largescale expulsions of plasma and magnetic field from the corona, frequently disturb the background solar wind flow [7−9] .The source regions of CMEs on the photosphere usually have highly-sheared magnetic field lines.A sudden disruption of the complex magnetic configuration leads to a large-scale expulsion of plasma and magnetic field from the corona.Such a catastrophic process typically develops in the low corona within 10∼15 minutes.CMEs can accelerate rapidly during the early stages of their formation and reach speeds of up to 3000 km•s −1 .As a result, the accelerated, heated, and magnetized CMEs generally carry a total energy of 10 32 erg into interplanetary space.
CMEs are the main driver of interplanetary and geo-magnetic disturbances [10−11] .Multiple CMEs moving at different speeds collide and merge, smoothing out the flow and removing information about their relative origins [12−18] .CMEs are also the main driver of hazardous space weather effects [19] : (i) CMEs are often associated with a sustained southward magnetic field, which allows a strong coupling between the solar wind and the magnetosphere; (ii) CMEs can generate interplanetary shocks, a key source of energetic particles and radio bursts.CMEs may also be of astrophysical interest since they may be the dominant way that stars shed both magnetic flux and magnetic helicity that are built up as a result of the stellar dynamo [20−21] .
An international effort to understand the heliosphere and its space weather transients should be taken with a fleet of spacecraft carrying remote-sensing observations at visible, Extreme Ultra-Violet (EUV), and X-ray wavelengths, as well as in-situ measurements of interplanetary plasmas, particles, and fields.
Coordinated remote-sensing and in-situ observations can track the propagation, evolution, and possible interactions of CMEs.Stereoscopic white-light imaging of a large portion of the inner heliosphere is successfully realized by the twin Solar TErrestrial RElations Observatory (STEREO) spacecrafts within the ecliptic [22−24] , but still unavailable from viewpoints of polar orbits.The injection of the spacecraft to a solar polar orbit is difficult.Tremendous energy is needed for a spacecraft to escape from the ecliptic, so the gravity assist from Jupiter is necessary to bend the flight path of the spacecraft.As an out-of-ecliptic mission, the International Solar Polar Mission (ISPM) was thereby proposed in the 1970s [25] .For the ISPM, two spacecrafts were to be built by National Aeronautics and Space Adminis-tration (NASA) and European Space Agency (ESA), respectively.One would be sent over Jupiter, then under the Sun.The other would fly under Jupiter, then over the Sun.All solar imaging instruments of the ISPM were to be accommodated on the NASA spacecraft.However, due to cutbacks, the NASA spacecraft was canceled in 1981.For the remaining ESA spacecraft, NASA provided a Radioisotope Thermoelectric Generator (RTG) and the launch services.
Then, the ISPM was recast as Ulysses, due to the indirect and untried flight path.Ulysses was first launched with a space shuttle in 1990, then swung by Jupiter, and finally reached an inclination of 80 • .Till now, Ulysses is the sole out-of-ecliptic mission to orbit the Sun and study the solar wind at all latitudes [26−29] .Science and Technology Corporation [30] .SPORT was successfully incorporated into the first round of background research of the Chinese Space Science Strategic Pioneer Project in 2011 [31]   The three-dimensional density visualization of an interplanetary CME is generated by a time-dependent tomographic reconstruction algorithm at University of California, San Diego [32] 1 Scientific Objectives

A spacecraft mission, named the
The definition of science objectives and the prescription of payload configuration are a priority to develop the SPORT mission.The determination of science goals justifies the exploration significance and scientific merits of the SPORT mission, and also provides guidance for optimizing technical parameters of the payload design.The SPORT mission is specifically designed to target the unsolved mysteries of solar and heliospheric physics and potential application to space weather.The SPORT mission addresses the following four top-level scientific questions.
(1) Characterize CME propagation through, and interaction with, the inner heliosphere, in particular a global view of the longitudinal dimension that is so far integrated by all observations: • Tracking kinematical motions of Earthdirected CMEs and probing these underlying dynamics; • Coordinated studies between remote-sensing imaging and in-situ measurements of Earthdirected CMEs; • Panoramic imaging of background solar wind streams near the ecliptic such as Corotating Interacting Regions (CIRs) and the slow solar wind.
What are the physical processes involved in the triggering, formation, propagation, and evolution of CMEs in the inner heliosphere?How do CMEs interact with the medium in the inner heliosphere, and affect its global structures and dynamics?How do the mass and magnetic field distribute in the longitudinal dimension, and whether/how are CMEs deflected in the longitudinal direction?
(2) Discover solar high-latitude magnetism associated with eruptions and solar cycle variation: • Causal link between the solar polar magnetic field and the solar cycle variation; • Evolution of solar high-latitude magnetic configuration before and after CME occurrence; The CMEs are the largest transient and explosive events in the solar system.Our understanding of CME evolution and propagation has been significantly improved since the launch of the STERE-O spacecraft in 2006 [22] .The trajectories of CMEs in the corona and interplanetary space can be charted in three dimensions, using simultaneous and stereoscopic imaging by STEREO telescopes [33−36] .
Remote-sensing and in-situ observations can be combined to obtain a global picture of CMEs and their consequences in the heliosphere.STEREO data have supported detailed comparison both of in-situ measurements with remote-sensing observations [19,37−38] and of heliospheric Magnetohydrodynamic (MHD) simulations with realistic observations [39−41] .However, some basic questions concerning CMEs remain outstanding: (i) What are the physical processes involved in the triggering, formation, propagation, and evolution of CMEs in the inner heliosphere?(ii) How do CMEs interact with interplanetary medium, and affect the structures and dynamics of the heliosphere?(iii) How do the mass and magnetic field distribute in the longitudinal dimension, and whether/how are CMEs deflected in the longitudinal direction?
The solar high-latitude magnetism plays a pivotal role in regulating the 11-year solar cycle [42−46] .The magnetic flux from high latitudes extends high enough in the solar atmosphere to be dragged out into the heliosphere by the solar wind.In contrast, magnetic flux from the low latitudes closes in the lower layers of the solar atmosphere.Further, controlled by interchange reconnection, these open field lines can reconnect and change their connection across the solar surface from time to time.The transport of magnetic flux at high latitudes and the properties of the polar magnetic field are paramount to answer the cyclic nature of solar magnetic activity [47−48] .Both the emerging magnetic flux and the ubiquitous convective flow underneath the photosphere shake and tangle magnetic field lines threading the corona.The In particular, the mystery of polar corona holes is quite outstanding because all remote observations are made from the ecliptic so far [45,49] .
The solar wind originates from the chromospheric networks [50−52] , starts flowing out of the corona in magnetic funnels [53] , and fills the entire heliosphere [54] .The fast solar wind (about 700 km•s −1 and comparatively steady) outflows from large-scale regions of a single magnetic polarity in polar coronal holes [27][28]55] .The slow solar wind (300∼ 500 km•s −1 ), which emanates from magnetically complex regions at low latitudes and the periphery of coronal holes, permeates the ecliptic [56] . The baance between the fast and slow solar winds is modulated by the 11-year solar cycle.At solar maximum, this stable bimodal configuration gives way to a more complex mixture of slow and fast streams at all heliospheric latitudes, depending on the distribution of open and closed magnetic regions and the highly tilted magnetic polarity inversion line.The Heliospheric Current Sheet (HCS) is warped and deformed by a malalignment effect between the Sun's rotation axis and the Sun's magnetic axis, and the effect is even more prominent at solar maximum [57−59] . Fst and slow winds carry embedded turbulent fluctuations, and these fluctuations also display different properties depending on different solar origins.A statistical analysis of the fluctuating fields also reveals pervasive fine-scale structures (e.g., discontinuities and pressure balanced structures) [60−61] .Such fluctuations are considered to be responsible for the difference in heating and acceleration between different solar wind streams.Local kinetic processes dissipate the turbulent fluctuations and heat the plasma [61−65] . TheSPORT mission is expected to discover (i) which magnetic and plasma structures in the low solar atmosphere does the fast solar wind come from, and (ii) how do the kinetic properties of the fast wind connect with the solar source regions?Energetic particle radiation produced by solar eruption also fills the heliosphere.Energetic articles accelerated in the corona and inner heliosphere are scattered by inhomogeneities in the interplanetary magnetic field during their transport in the heliosphere [66−68] .Such a scattering process affects the particle flux at 1 AU.The turbulence scattering in the solar wind has spatial scales from millions of kilometers to below the electron gyro-radius.CMEdriven shocks can produce relativistic particles on time scales of minutes [69−70] , and many CMEs can convert around 10% of their kinetic energy into energetic particles.However, the longitudinal structure of CME is well constrained from ecliptic observations, and its extent has a large impact on the acceleration of energetic particles [71−72] .As a polar orbit mission, SPORT is very suitable to address the following puzzles of energetic particles: (i) How are energetic particles accelerated, transported, and distributed in the inner heliosphere?(ii) What is the causal relation between energetic particles and solar eruptions?(iii) How does an ENA imager improve our knowledge about acceleration and transport of energetic particles?
Overall, the SPORT mission focuses on CMEs, high-latitude magnetism, the fast solar wind, and energetic particles.Most of previous observations are made within the ecliptic.The out-of-ecliptic Ulysses spacecraft did not have remote imaging instruments [26] .Scientific breakthroughs in the fields of solar physics, heliospheric physics, and space weather are expected, when both remote-imaging and in-situ observations from a solar polar orbit become available.

Scientific Payloads
The SPORT scientific objectives determine the choice of scientific payloads.Space physics research is largely driven by space-based measurements, the correct interpretation of these measurements requires not only an understanding of the physics of what is being measured, but also an understanding of the experimental techniques used to obtain the measurements.
The SPORT makes in-situ measurements of the solar wind plasma, electromagnetic fields, electromagnetic waves, and energetic particles from a solar polar orbit.Moreover, these in-situ measurements are connected back to their source regions on the Sun, through simultaneous remote imaging observations out of the ecliptic.
Carrying both in-situ and remote-sensing instruments at high heliospheric latitudes, SPORT addresses a central question of heliophysics: How does the Sun create and control the heliosphere?Table 1 maps the science questions to the required observations.(iv) interface between the instruments and the platform (command/control, power supply, etc).
Information-gathering in space is an expensive, demanding scientific endeavor that requires unique instrument design and fabrication.A suite of SPORT payloads is expected to detect the radiation, particles, waves, and fields in the inner heliosphere.The solar EUV imager aboard SPORT is designed to operate at two wavelengths of 121.6 nm and 13.1 nm.Ultraviolet coronagraph observations of the extended solar corona provide detailed empirical descriptions of coronal holes, streamers, and CMEs.In particular, the core of CMEs made of cool prominence material can be imaged in the Lyman-α line of hydrogen atom at 121.6 nm.The simultaneous coronagraph imaging at two-channel wavelengths of visible light and 121.6 nm is realized via a combination of broad-band coating on the mirrors and spectral band-pass filters.The solar atmosphere from the photosphere to the corona can be continuously monitored for detecting largescale solar eruptive events, using a set of EUV imager and coronagraph [23] .A white-light Heliospheric Imager (HI) can image both the quasi-steady flow and transient disturbances in the solar wind over a large portion of the inner heliosphere [24,73−76] .The longitudinal structures of solar storms are not observable from the ecliptic.Remote-sensing imagers aboard SPORT will provide the first-ever images of the solar polar regions and the ecliptic from an out-ofecliptic viewpoint.Using these imaging suite, SPORT can address outstanding scientific questions, including (i) the radial dependence of CME-driven shocks and associated particle populations, (ii) the evolution of CMEs and CIRs in the inner heliosphere, (iii) the structure and turbulence within solar wind streams and their implication for the origin and evolution of the solar wind, (iv) sources, acceleration mechanisms, and transport processes of solar energetic particles.In addition, the particle detectors for the solar wind species can provide comprehensive in-situ measurements of the solar wind plasma including high time-resolution velocity distributions and composition of solar wind ions and electrons up to suprathermal energies [77−78] .Using suprathermal particles to trace interplanetary magnetic field lines, the interplanetary magnetic connectivity can be inferred.Using both particle and wave detectors, the solar wind can be comprehensively diagnosed for its properties such as the speed, mass flux, composition, magnetic field, charge states, and waves/turbulence.A three-dimensional solar-interplanetary MHD model based on a Conservation Element/Solution Element scheme [79−82] has been extensively used in synthesi-   zing and investigating the observable signatures during the SPORT mission lifetime.
The SPORT mission has formed teams for different payloads, working on the design, fabrication, and test of the payloads.Each SPORT payload working team consists of both scientists and engineers, seeking a combination of expertise in science and engineering.
Below is a list of some completed instrument prototypes.

Radio Imager (SARI)
Radio signatures from interplanetary CMEs as results of thermal free-free emission and non-thermal gyro-synchrotron emission are thought to be observable [83−84] .A SARI with passive interference is designed to capture radio pictures of CMEs and trace their interplanetary propagation, using 8 long booms to perform a clock-scanning sampling of radio emission [85−87] .A 4+4 arm configuration of the 8 booms is helpful to increase the radiometric resolution and keep the system balance.Figure 2

Prototype of White-light HI
Heliospheric imaging fills the observation gap between near-Sun coronagraph imaging and in-situ measurements [24] .The angle between the Sun and a target, such as a CME, as viewed from an observer, is termed elongation ε.Interplanetary structures and transients have been viewed by means of Thomson-scattered sunlight [40,75,88] .The brightness difference between the Sun and a target (to be observed) at a large elongation is many orders of magnitude.Specifically, large CMEs at ε = 45 • have optical intensities that are of 10 −14 B s order [89] , where B s is the mean solar brightness.To image interplanetary CMEs, the faint and transient signals of CMEs must be separated from many other intense and stable sources: background light from the Sun, zodiacal light, and star light.Instrument specifications for a HI aboard a deep-space spacecraft require careful design that takes into account the stray-light impacts of the imager bus, imager appendages, and other instruments [24,90−91] .A prototype of the SPORT/HI is nearly finished, as shown in Figure 3.The SPORT/HI is composed of four stops, two occulters, three groups of lenses, and a Lyot spot.A toothed occulter and diaphragm can be used to suppress stray light because they diffract much less light in the central area than a circular disk [92] .The SPORT/HI is essentially a wide-field coronagraph, following the overall optical design of the SOHO/LASCO/C2 coronagraph [93] .The SPO-RT/HI has a brightness sensitivity of 10

Prototype of Solar EUV Imager
An EUV imager with its normal-incidence multilayer-coated optics can probe spectral emission lines at different temperatures, and diagnose the lower solar atmosphere on a global scale [94] .An EUV imager  [92−93] aboard SPORT is proposed to operate at two wavelengths, i.e., 121.6 nm and 13.1 nm.The emission at 121.6 nm comes from the top of the chromosphere and the bottom of the transition layer, whereas that at 13.1 nm originates from the high corona.The SPORT/EUV imager is composed of mirrors, filters, and a focal system [95−96] .The EUV imager has a FOV of 45 arcmin, a CCD array of 2048×2048 pixels, and an angular resolution of 1.4 arcsec per pixel.
The diameters of the primary and secondary mirrors are 100 mm and 38 mm, respectively.Distance between the primary and secondary mirrors is 520 mm.
The structure model and optical mirrors are shown in Figure 4.

Magnetograph
A full-disk photospheric magnetograph can be used to study the solar magnetic field and surface motions, and identify precursors of solar disturbances for space-weather forecasts [97−98] .The solar vector magnetograph aboard SPORT is designed on basis of multiple Lyot-type birefringent filters [99] , as shown in Figure 5 The four Stokes parameters (I, Q, U, V ) of the polarized sunlight from the photosphere are retrieved via a rapid polarization modulation [99−100] .The polarization modulation is realized through a coordinated

Spacecraft Orbit
The orbit design is crucial to achieve the scientific The design and optimization of the SPORT orbit are based on the successful lessons learned from the previous Ulysses [26] and the planned Solar Orbiter [101] missions.SPORT is designed to have a solar polar orbit with a gravity assist from Jupiter, similar to Ulysses [102] .In addition, the gravity of planets in the inner heliosphere can also be considered for spacecraft orbit design.Using multiple Venus flybys, the planned Solar Orbiter mission is believed to reach an inclination angle of 36 • [101] .Without remote imaging instruments, Ulysses has a mass of 370 kg, a power of 285 W, and an orbit inclination of 80 • [26] .
In contrast to Ulysses, SPORT is much heavier in the spacecraft mass and lower in the inclination angle.
For remote sensing studies of CMEs at lower heliospheric latitudes, the SPORT's inclination angle of  Because of the rocket capability, the total mass of In order to prolong the imaging observation time, further multiple gravity assists from Venus or Earth are explored to shrink the solar polar orbit within 2 AU and increase the inclination angle towards 70 • .As shown in Figure 6, Table 3, and   5. Specially, SPORT must be a three-axis stabilized spacecraft with a sufficient  pointing accuracy; SPORT must satisfy electromagnetic cleanliness such that the imaging, particle, and wave detectors can accurately measure relevant physical parameters; SPORT must combine China's existing tracking infrastructure and worldwide facilities to achieve the requirements of tracking, operations, and data downlink.The SPORT payload configuration is designed with two platform models and illustrated in Figure 7.Because of high orbit inclination, the total mass of SPORT has to be no more than 1050 kg.If a lower orbit inclination is acceptable, SPORT can accommodate more propellant for orbit maneuver and/or more mass for payloads.
The design of the spacecraft platform includes many aspects such as the structure, mechanisms, thermal control, material, propulsion, electronic components, and so on.The SPORT measurements to achieve the scientific objectives place the following requirements on the spacecraft engineering.

Advanced Active Thermal Control
SPORT, travelling at an elliptic orbit around the Sun,  lunar mission in late 2013 [103] .But the mixed use of solar panel and RTG is unprecedented and challenging for the Chinese industry.

Deployable in Space
The long booms aboard SPORT has low mass, low stiffness, weak damping, and dense vibration modes.
There is a strong coupling between boom vibration and attitude control.The stability of the flexible deployable booms and its coupling dynamics with the spacecraft platform are being analyzed.Thermal deformation of the booms should be minimized to maintain high accuracies of the pointing and stability.Composite material such as carbon fiber is considered to be used to make these booms.

For a Flexible Probe
The antenna booms and large solar panels make SPO-RT as a flexible probe.Moreover, high pointing accuracy of 20 (3σ) is demanded by the performance of scientific instruments.Now, the attitude dynamics of the flexible probe, coupling with vibration modes of solar panels and deployed booms, is intensively studied.A suitable controller is being designed to control the attitude of platform and suppress the vibration of deployed booms and solar panels.

Deep Space Communication
The distance between the spacecraft antenna and terrestrial receiver could be as far as 9×10 8 km.Even for a minimal configuration of the scientific instruments, the data rate reaches 45 kbit•s −1 .In order to transmit the data from SPORT, key microwave devices, such as dielectric resonator oscillator with low phase noise, low power amplifier, etc., are being fabricated, and advanced technology of weak signal acquisition and adaptive phase-locked receiver are being developed.

International Cooperation
Space science is an extremely costly enterprise and the budgets will always be limited, so the heliospheric missions should be coordinated to maximize the science return.Only by working together and coordinating efforts, the international space science community can make the most out of the limited resources.
SPORT, together with Solar Probe Plus, Solar Orbiter, and InterHelio-Probe, are now incorporated together under an ILWS framework for the purpose of coordinated exploitation of the inner heliosphere [104] .
Coordinated observations between the SPORT and other spaceborne/ground-based facilities would greatly enhance scientific outputs.Each of these missions had a specific focus, being part of an overall strategy of coordinated solar and heliospheric research [104] .For example, a white-light HI aboard    ESA [112] ; Yinghuo-1 was launched through the Russian Phobos-Grunt mission -although the spacecraft failed to leave Earth's orbit [113] .The SPORT team benefits greatly from the invaluable legacy in- Solar Polar ORbit Telescope (SPORT), is now under comprehensive scientific and engineering background studies in China.The SPORT mission focuses on CMEs, polar coronal holes, the fast solar wind, and energetic particles.As depicted in Figure 1, out-of-ecliptic imaging from SPORT can map the ecliptic in fine detail, and track the morphological and kinematical properties of Earth-directed CMEs.Scientific breakthroughs in the fields of CMEs, the fast solar wind, and high-latitude magnetism are expected, when both remote-imaging and in-situ observations from a solar polar orbit become available.The SPORT mission was first proposed in 2004 by the National Space Science Center (NSSC) of the Chinese Academy of Sciences (CAS), and later jointly studied by the University of Science and Technology of China, the National Astronomical Observatories of CAS, as well as the China Aerospace . The Strategic Pioneer Project fosters the next generation of Chinese space science missions, aiming to deepen our understanding of the universe and Earth through independent and cooperative science programs.The roadmap of the SPORT mission includes: (i) key technology and engineering feasibility studies from 2008 to 2011, with the support of China National Space Administration, (ii) background scientific and engineering studies from 2011 to 2015, with the support of CAS, (iii) engineering implementation expected to start in 2016, if selected, (iv) tentative launch in March 2020.The science definition, orbit design, payload fabrication, and international cooperation constitute the key elements of the current SPORT blueprint.This paper presents an overview of the SPO-RT mission and the accomplishment of the ongoing SPORT project, and demonstrates the scientific importance of the mission for space weather and solar physics.The scientific objectives, scientific payloads, spacecraft orbit, spacecraft platform, and international cooperation of the SPORT mission are respectively elaborated in Sections 1∼5.The status and vision of the SPORT mission within the International Living With a Star (ILWS) framework are summarized and discussed in Section 6.

Fig. 1
Fig. 1 An out-of-ecliptic view of the Sun and interplanetary space from the SPORT spacecraft.

•( 3 )( 4 )•
Dynamic responses of the global solar atmosphere to the local CME initialization in the low corona.How does the high-latitude magnetic field constrain the global coronal magnetic field configuration?How does the high-latitude magnetism give rise to eruptions (in particular the relationship between pseudostreamers and sympathetic eruptions)?How does the polar magnetic field regulate solar cycle variation?Investigate the origin and properties of the fast solar wind: • Source regions of the fast solar wind in the polar coronal holes and magneto-fluid properties of the nascent solar wind; • Kinetic-scale waves and turbulence in the fast solar wind; • Solar wind flows and transients around coronal streamers.What is the structure and magnetism of the source regions that produce the fast solar wind?How do the kinetic properties of the fast solar wind connect with in-situ fluctuations/turbulence? How do the kinetic properties of the fast solar wind connect with the solar source regions?Understand the acceleration, transport, and distribution of energetic particles in the corona and heliosphere: • Spectrum, species, and flux of energetic particles in the high-latitude heliosphere; • Heliocentric height and spatial location of CME-driven shock formation, energetic particle acceleration across the shock front, and ensuing electromagnetic radiation from the shock front; Physical link between solar eruption and energetic particles.What are the physical mechanisms of the acceleration, transport, and distribution of energetic particles in the corona and heliosphere?What is the causal relation between energetic particles and solar eruptions?In particular, how does an Energetic Neutral Atom (ENA) imager improve our knowledge about acceleration and transport of energetic particles?
photosphere, i.e., magnetic shearing and/or rotation, can eventually result in an extremely structured and highly dynamic region above the sunspots.A super active solar region is likely to produce several CMEs within one day.It is still unclear what determines the amount of open magnetic flux from the Sun, and how open magnetic field lines are distributed at the solar surface.In particular, the mystery of polar corona the payloads.Final selection of the payloads is decided by the following factors: (i) capability of current and near-future Chinese instrument technology; (ii) resources of weight, power, and data rate allocated by spacecraft platform engineering; (iii) international assistance on the full or part of instrument hardware; is so far integrated by all observations full-disk photospheric magnetic field full-disk EUV imaging white-light imaging for the corona and heliosphere radio burst detecting in-situ measurements of plasma and magnetic field interplanetary radio imaging (optional) discover solar high-latitude magnetism associated with eruptions and solar cycle variation full-disk photospheric magnetic field full-disk EUV imaging white-light imaging of the corona investigate the origin and properties of the fast solar wind full-disk photospheric magnetic field full-disk EUV imaging white-light imaging of the corona and heliosphere in-situ measurements of plasma, magnetic field, and electric field in-situ measurements of solar wind turbulence understand the acceleration, transport, and distribution of energetic particles radio burst detecting white-light imaging for the corona and heliosphere in-situ measurements of energetic ions, electrons, and neutral atoms in-situ measurements of solar wind turbulence gives the complete system design of the SARI, including antenna, receiver, and central unit.The SARI has a central frequency of 150 MHz, a bandwidth of 20 MHz, a polarization of both circular and H-V linear patterns, a Field of View (FOV) of ±25 • , an angular resolution of 2 • , a radiometric sensitivity of 1 K, and an image refreshing time of 30 min.Image retrieval and post-processing algorithms are developed, and parts of electronic components of the SARI hardware is tested.

Fig. 2
Fig. 2 System design (a) and ground experiment (b) of a synthetic aperture radio imager −14 B s , a FOV of ±20 • , a CCD array of 2048×2048 pixels, a bandwidth of 630∼730 nm, and an angular resolution of 1.2 arcmin per pixel.The optical axis of the SPO-RT/HI points towards the Sun, whereas that of the STEREO/HI is off the Sun-spacecraft line.The Sun is externally occulted in the center of the SPORT/ HI FOV.Accordingly, the SPORT/HI FOV covers 8∼72 R s (24∼215 R s ), when the SPORT is at 1 AU (3 AU).In contrast to the STEREO/HI, the technical difficulty of cutting the stray-light is much greater for the SPORT/HI.

Fig. 3
Fig. 3 Optical system design (a) and prototype (b) of a heliospheric imager.The imager consists of external diaphragm A0, external occulter D1, entrance aperture A1, objective lens O1, field stop A2, internal occulter D2, field lens O2, Lyot stop A3, relay lens with Lyot spot O3 and focal plane F[92−93] . The SPORT magnetograph instrument consists of three subsystems of optical imaging, optical polarimeter, and CCD readout.The diameter, focal length, and FOV of the magnetograph are 80 mm, 1000 mm, and 33 arcmin, respectively.The working wavelength of the optical subsystem is centered around 532.4 nm with a Full Width at Half Maximum (FWHM) of 0.1 Å.The incidence light inside the birefringent filter is less than 1.2 • .Other technical parameters of the optical subsystem include the total transmission (> 8%), off-band stray light (< 15%), and working temperature (25±0.01• ).

Fig. 4
Fig. 4 Structure model (a) and primary (b)/secondary (c) mirrors of an EUV imager objectives of SPORT.The most critical parameter of the SPORT orbit is the out-of-ecliptic inclination angle.Other factors, such as the launch vehicle, launch time window, and gravity assist, should also be considered for the orbit design.The capability of the launcher and the weight of the payloads significantly restrict the inclination angle.In order to realize the scientific objectives, the following requirements have to be satisfied: (i) the inclination angle of a desired polar orbit should be no smaller than 60 • ; (ii) the periapsis of the polar orbit to the Sun should be within 0.5∼1 AU; (iii) the orbit should be small enough to have a maximal time of imaging observations; (iv) the launch time should be within a solar maximum, when solar activities such as CMEs are much more frequent.

Fig. 5
Fig. 5 Optical system design (a) and mechanic prototype (b)∼(d) of a vector magnetograph on basis of birefringent filters

4
Spacecraft Platform Resources needed to build and operate the spacecraft, such as mass, power, memory, and telemetry rate are always at a premium.The very worthy desire for more science per dollar has increased the demand on instrument performance while reducing resources available to the designer.Independent designs of the spacecraft are being carried out by the Beijing Institute of Spacecraft System Engineering and the Shanghai Institute of Satellite Engineering which have delivered more than 90 spacecrafts since 1970.The general parameters of the proposed SPORT spacecraft platform are given in Table

Fig. 7
Fig. 7 SPORT platform models independently designed by the Beijing Institute of Spacecraft System Engineering (a) (c) and the Shanghai Institute of Satellite Engineering (b) (d).Because of the high risks and technology difficulties, a synthetic aperture radio imager is likely to be excluded from the SPORT payload list.Accordingly, the SPORT platform designs are simplified from the models (a) (b) to (c) (d)

6Fig. 8
Fig. 8 Planned orbits of four future spacecraft missions ecliptic phase, the gravity-assisted maneuvers at Venus will be used for maximal inclination of the orbit to the ecliptic with a possible second spacecraft, InterHelio-Probe will be transformed into a PEP (Polar Ecliptic Patrol) mission mechanisms of the solar corona heating and acceleration of the solar wind fine structure and dynamics of the solar atmosphere nature and global dynamics of the solar flares and CMEs and their influence on the heliosphere and space weather energetic particles powerfully and changeably accelerated by the Sun solar atmosphere in polar and equatorial regions Solar Polar Imager (NASA) use a solar sail to leave the ecliptic, and reach high inclination angle of about 75 • heliocentric orbit at 0.5 AU observe solar activities from a new perspective of solar polar orbit helioseismology and magnetic fields of polar regions polar view of corona, CMEs, and total solar irradiance link high-latitude solar wind and energetic particles to coronal sources latitude magnetism, fast solar wind, and energetic particles from a polar orbit around the Sun.SPORT is intended to be the first mission that carries remotesensing instruments (complemented with in-situ detectors) from a polar orbit around the Sun, the first mission that could image interplanetary CMEs at radio wavelengths from space, and the first mission that could measure solar high-latitude magnetism leading to solar eruptions and fast solar wind.SPORT would provide a unique opportunity to study CME propa-gation through the inner heliosphere from a vantage point at high latitudes and investigate solar highlatitude magnetism giving rise to solar eruptions and fast solar wind.Tentative payloads aboard SPORT include a solar EUV imager (121.6 and 13.1 nm), a solar vector magnetograph, a large-angle coronagraph (whitelight and 121.6 nm), a HI, an energetic neutral atom imager, a solar wind plasma analyzer, a fluxgate magnetometer, a radio burst detector, a low-frequency electromagnetic wave detector, and an energetic particle and composition analyzer.The tradeoff between the polar orbit inclination and the payload mass has to be considered in terms of cost-performance balance.Risks should also be carefully taken into account, in particular for a synthetic aperture radio imager.If more payload mass is allowable, additional instruments such as the synthetic aperture radio imager could be mounted on the SPORT platform.The current background studies of the SPORT mission aim at being competitively selected and officially approved during China's 13th Five-Year Plan period of 2016-2020.If successfully approved, the SPORT mission will be boosted from the background phase to the engineering phase.Meanwhile, the development of the SPORT mission continues with the goal of a launching around 2020.The scientists and engineers should be coordinated to ensure scientific merits and technique feasibilities for each payload.All risks in instruments, platform, and rocket should be avoided or minimized within a reliable technology readiness level.International collaboration is vital for the development of the SPORT mission.China has an opendoor policy regarding international collaboration in space science.Both of the earlier Chinese spacescience missions had a strong component of international collaboration: Double Star was a joint mission between China's National Space Administration and herited from developing the past Chinese missions of Double Star and Yinghuo-1.However, there is still a lack of trial-and-error development and space flight experience in China, especially at the payload level.International partnerships for instrumentation designs and scientific collaborations are both needed and welcome.Heliospheric satellite missions should be coordinated within the ILWS framework.An ILWS task group on coordinating future missions of the Solar Probe Plus, Solar Orbiter, SPORT, and InterHelio-Probe was established in September 2013.The SPO-RT mission is complementary to other inner heliospheric missions.The concurrent science operations of the Solar Orbiter, Solar Probe Plus, SPORT, and InterHelio-Probe missions will offer a truly unique epoch in heliospheric science.While each mission will achieve its own important science objectives, coordination of these spacecraft will be capable of doing the multi-point measurements required to address many mysteries in heliophysics such as the coronal origin of the solar wind plasma and magnetic field or the way in which the solar transients drive the heliospheric variability.

Table 2
summarizes the mass, power, and date rate of