Solar Physics from Unconventional Viewpoints

We explore new opportunities for solar physics that could be realized by future missions providing sustained observations from vantage points away from the Sun-Earth line. These include observations from the far side of the Sun, at high latitudes including over the solar poles, or from near-quadrature angles relative to the Earth (e.g., the Sun-Earth L4 and L5 Lagrangian points). Such observations fill known holes in our scientific understanding of the three-dimensional, time-evolving Sun and heliosphere, and have the potential to open new frontiers through discoveries enabled by novel viewpoints.


INTRODUCTION
Observations from satellite missions have transformed the field of solar physics. High-resolution 11 observations with near-continuous temporal coverage have greatly extended our capability for studying In this paper we will present the rich variety of science that is enabled by multi-vantage observations. 23 In section 2 we review past and present missions that have ventured away from the SEL. In section 3 we 24 discuss the science enabled by extra-SEL vantages, including solar dynamo studies, solar atmospheric 25 global connections, and solar wind evolution and transient interactions. In section 4 we discuss the benefits 26 of multi-vantage observations for space-weather monitoring, both for the Earth and other planets, and in 27 section 5 we consider the discovery space opened by new viewpoints. In section 6 we identify the remaining 28 key gaps in our heliospheric great observatory related to extra-SEL observations, and discuss how these 29 may be filled through future opportunities-including both planned near-term missions and game-changing 30 missions for the next decade and beyond. Finally, in section 7 we present our conclusions. The quadrature views are shown as blue patches (intersecting the L4 and L5 Lagrange points); the far-side view as a pink patch (intersecting the L3 Lagrange point); and the polar views as green patches. Sample STEREO orbit is shown in orange, Helios orbits in red, and Ulysses orbit in light blue. The Helios orbits are relative to the Earth, i.e., Heliocentric Earth Ecliptic (HEE) coordinates, the other orbits are in Heliocentric Inertial coordinates (Thompson, 2006).

Open Science Question
What are the solar surface/interior flow and magnetic field patterns vs. longitude, latitude and depth, and how do they constrain dynamo models? can be used to address such limitations by providing independent measurements for validation and/or    Extra-SEL observations thus complement existing SEL observations in multiple ways (Table 2). They

Open Science Question
What is the structure of the global coronal/heliospheric magnetic field? What are the source regions of the solar wind? How is magnetic energy stored/released in eruption, and what is role of helicity/topology? How do local and global solar magnetic fields interact?

Measurements Needed
(  The CME is in the HI1-B FOV at this time. The Earth is bright and saturates a few columns in the HI2-B FOV. The faint broad front of a preceding CME can be discerned in HI2-B.

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As discussed in Section 2, STEREO revolutionized heliospheric physics by providing consistent, spatially 223 resolved imaging of the inner heliosphere with coverage from the Sun to 1 AU. When viewing from the 224 extra-SEL, the STEREO heliospheric imagers can observe CIRs and CMEs en route to Earth without 225 interruption (Figure 4), enabling studies of their evolution and interactions, and naturally feeding an 226 increasingly sophisticated space-weather research field (see further discussion in Section 4.

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While the STEREO mission demonstrates that we can indeed image the solar wind and its major 228 components, it has left us with many unanswered questions. With regards to CMEs, heliospheric imaging 229 analyses provide some indications of CME rotational evolution en route to Earth (e.g., Isavnin et al.,

SPACE-WEATHER SIGNIFICANCE OF EXTRA-SUN-EARTH-LINE OBSERVATIONS
In addition to all that may be gained scientifically from extra-SEL observations, there are clear benefits to 268 such measurements for space-weather prediction and monitoring.

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Of utmost importance, extra-SEL configurations seem to be our best option for quantifying the magnetic 295 field entrained in the CME. We note that from the poles, the line-of-sight field is B z , the North-South coronal IR spectropolarimetry has the potential to observe line-of-sight magnetic field strength at the core 300 of an erupting CME (Figure 6 (Fan, 2018).

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In addition to improved forecast capability, extra-SEL measurements are the best means for monitoring 302 evolution of Earth-intersecting CMEs (and CIRs) via heliospheric imaging, as discussed in Section 3.3.

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The STEREO HIs demonstrated that it is possible to spatially resolve and track CMEs to 1 AU (Davis  with signal-to-noise ratio > 3, based on a 1.5 meter telescope,12" spatial resolution, and 5 minute integration. This is essentially the same as Fan (2018) Figure 12c, except without the background scatter included in that paper appropriate to ground-based telescopes. d) Same except for 20 cm telescope and 60" resolution. e) Same except for 10 cm telescope and 124" resolution. Note that the sign and strength of the pre-eruptive core field is captured for all.
detail the interaction between CMEs and establish whether CMEs distort, rotate, and/or change propagation 308 direction.

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For many of these measurements, the optimal viewpoint lies outside the ecliptic plane (see Table 4. tracking of the CME core, extra-SEL magnetic observations and data-constrained models could 325 lead to a comprehensive system for forecasting CME impact (e.g., Savani et al., 2015Savani et al., , 2017 Line-of-sight measurements of southward-directed magnetic field yes (5; 9) no no Improved irradiance measurements no yes (3; 10) no Table 4. Space-weather prediction and modeling enabled by extra-SEL observations

DISCOVERY SPACE
Opportunities for discoveries by extra-SEL spacecraft abound. Previous experience shows that these include unique ways to find and observe celestial objects such as comets and asteroids: Ulysses, for example, Cassini ( Figure 7C-D), direct observations of the poles is likely to reveal far more complex and beautiful 347 structure than anything we have been able to piece together to date.

FUTURE MISSIONS AND GAP ANALYSIS
We have shown that extra-SEL observations have potential for transformative progress in a range of science areas. Tables 1 -4 describe the measurements needed from the various extra-SEL vantages that would 350 lead to such progress. We now discuss plans for future missions, and consider how the gaps in our current 351 observational capability may be filled. context for the in-situ payload but has no disk imaging (for thermal reasons).

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It is undeniable that both missions will provide unique data and views of the coronal and heliospheric  Table 4). We now consider how next-generation extra-SEL mission concepts might address the remaining gaps between the observational capabilities of our existing and planned missions, and the 385 outstanding science questions raised in this paper.

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Variations of those concepts were submitted for the Next Generation Solar Physics Mission call for ideas.

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The majority of these white papers are not in the published literature, but a subset, with emphasis on 391 helioseismology science, is discussed by (Sekii et al., 2015). Here, we present some representative concepts 392 for mission architectures that emphasize sustained measurements, and so fill most, if not all, of the gaps 393 indicated by