SOHO, STEREO and Hinode Observations of Polar Coronal Holes at Solar Minimum

1. SOHO, STEREO and Hinode Observations of Polar Coronal Holes at Solar Minimum Barbara Thompson, NASA GSFC David Alexander, Rice University Jonathan Cirtain, NASA MSFC Craig DeForest, SWRI Sarah Gibson, UCAR Donald Hassler, SWRI Michael Kaiser, NASA GSFC Antonia Savcheva, SAO

2. What do solar polar observations accomplish? • Solar polar observations can lead to significant advances in our understanding of the Sun-Earth system. • As yet, the polar regions of the Sun remain a largely untapped resource in our attempts to understand the Sun and its influence on the near Earth environment.

3. What do solar polar observations accomplish? • 3D Solar Wind and Heliospheric Structure: 360o view of streamer belt, global structure of corona, fast/slow wind interactions • Coronal Hole Structure and Phenomena: polar coronal holes, unipolar regions, jets, polar plumes • Solar Activity: formation and evolution of solar structures, solar irradiance • Sun’s Influenceon Earth: angular momentum loss, cosmic-ray modulation • Helioseismology: solar dynamo, polar fields, meridional circulation, field reversal FUTURE PLANS

4. 3D Heliospheric Structure • The 3D structure of the corona and inner heliosphere is tremendously complex, even at solar minimum. • The interactions of fast/slow solar wind, the expansion into the heliosphere, and rotational effects are most directly studied with a solar minimum corona. • Additionally, the Sun is not as likely to change for an entire solar rotation during solar minimum, and CMEs are not as likely to disturb the structure.

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7. Whole Heliosphere Interval Originating from Carrington Rotation 2068 March 20 - April 16, 2008 • What is WHI? • Internationally coordinated observing and modeling effort • What are WHI’s science goals? • Characterize the 3-D solar minimum heliosphere • Trace the effects of solar structure and activity through the solar wind to the Earth and other planetary systems • How does WHI work? • Will involve ground- and space-based observations • From the solar interior, through geospace, to the heliopause • Over 200 international scientists already participating

8. How to get involved? http://ihy2007.org/WHI

9. STEREO 3D Solar wind flows Movies quickly demonstrate that the flow of solar wind structures can be traced out to 1 AU. Tomographic techniques give us new insight into the azimuthal topology.

10. Coronal Hole Structure and Phenomena: Polar Plumes • A major highlight of the SOHO mission was the ability to view polar plumes, using multiple imagers and spectrometers, out to 30 solar radii. • The structure of these high-contrast features allowed us to trace the magnetic field emanating from poles, and variability in the emission has served as a diagnostic of wave activity.

11. SOHO plume maps out to 30 solar radii allow us to trace the “super-radial expansion” and thus the fast solar wind. Tracking variations in plume structure have revealed quasi-periodic oscillations, traveling 75 - 150 km/sec, with periods of 10-15 minutes (indicative of slow-mode MHD waves). Deforest et al., 1997, 2000

12. Coronal Hole Structure and Phenomena: Polar Jets • Polar jets are small-scale eruptive events that provide insight into magnetic evolution and reconnection, Alfvénic properties, and mass loading of the solar wind. • They are relatively small phenomena that rapidly evolve in 3 dimensions. Prior to STEREO and Hinode observations, the resolution in time and space were, for the most part, insufficient to explore the underlying physics.

13. Hinode’s high-resolution spectral observations have allowed new insights into polar jets. Observations of nearly 10 jets/hour bring us closer to understanding the role of small-scale structure and magnetic reconnection in solar coronal energetics. Jet expansion speeds (~ 35 km/sec) indicate the rate of pressure-driven expansion and serve as a diagnostic of the pressure of the fast solar wind. Alfvénic fluctuations have been observed at both at slow- and fast-mode speeds, enabling us to further sample MHD parameters on the pole, and providing insights into astrophysical jets. Hinode XRT observations (Cirtain et al, 2007; Savcheva et al, 2007)

14. “A twisted mess:” Combining Hinode’s observations with STEREO’s 3D imaging capability, we obtain a more clear view of a jet’s motion, including rapid rotation about the magnetic axis.

15. What can “direct” polar observations bring us? There are several major limitations to our observations of the poles thus far: We only see a small fraction of the pole at a given time. We view the poles from a high inclination angle - nearly perpendicular. Our view of the footpoints and the magnetic field is severely restricted. Complementary in situ observations are extremely valuable but have been scarce.

16. Solar Activity as viewed from the poles will enable the following: • Long-term evolution of the Sun’s magnetic field: in particular, meridional flows & field reversal • A greater understanding of the poloidal component of solar field, believed responsible for much of the interplanetary magnetic field • Formation, evolution and demise of solar structures (e.g. active regions, helmet streamers) • Global dynamics and rotation of the Sun • “True” solar radiance: establishing a 3D determination of the energetic input to the Earth’s atmosphere and ionosphere

17. CMEs and Space Weather New observations from out of the ecliptic plane provided by Solar Orbiter will yield unique information about the coronal effects of CMEs and solar flares in addition to providing continuous coverage of Earth-directed disturbances. A mission to the poles will enable unprecedented exploration of the: • initiation and evolution of Earth-directed CMEs • global extent of CMEs and propagation of CME disturbance around Sun • acceleration and transport of solar energetic particles

18. Helioseismology: the origin of the solar cycle and the Sun’s magnetic field Continuous coverage of the polar regions will provide otherwise unattainable information about the large-scale solar dynamo. Observations above the poles will enable unprecedented exploration of the • internal structure of the Sun • “pure” small-scale fields with no AR contamination • polar magnetic field and field dynamics • polar convective patterns • interaction of convection, rotation and magnetic field • solar rotation at the poles • polar vortex and circumpolar jet streams • reversal of the large-scale magnetic field

19. Conclusions • SOHO, STEREO and Hinode data possess three chief improvements over previous polar observations: extended fields of view, improved resolution (temporal, spatial, spectral), and multiple vantage points. • They have provided several new and exciting insights into the poles of the Sun. The 3D structure of the heliosphere, and the flow of both fast and slow solar wind, are become more and more clear. • Coronal hole phenomena, such as jets and polar plumes, also providing insight into the solar structure in which they are embedded. • The new frontier: observations from above the poles will enable a rich variety of solar and Sun-Earth system studies unavailable from the ecliptic plane. It will significantly advance our knowledge of key solar phenomena, our understanding of the Sun as a star and our forecasting abilities for the Sun’s interaction with the Earth.