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Exploring the Solar Wind with Ultraviolet Light

Exploring the Solar Wind with Ultraviolet Light. Steven R. Cranmer and the UVCS/SOHO Team Harvard-Smithsonian Center for Astrophysics. Outline: Motivation & history How does the Sun “expel” the hot solar wind? Modern space-based observations: SOHO (Solar and Heliospheric Observatory)

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Exploring the Solar Wind with Ultraviolet Light

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  1. Exploring the Solar Windwith Ultraviolet Light Steven R. Cranmer and the UVCS/SOHO TeamHarvard-Smithsonian Center for Astrophysics

  2. Outline: • Motivation & history • How does the Sun “expel” the hot solar wind? • Modern space-based observations: • SOHO (Solar and Heliospheric Observatory) • UVCS (Ultraviolet Coronagraph Spectrometer) Exploring the Solar Windwith Ultraviolet Light Steven R. Cranmer and the UVCS/SOHO TeamHarvard-Smithsonian Center for Astrophysics

  3. In visible light . . .

  4. In ultraviolet light . . .

  5. Solar Physics: A wide-angle view • The Sun is the source of energy for life on Earth, as well as weather & climate. • The Sun is the closest example of a star. • The Sun is a “laboratory without walls” for many basic processes in physics, at regimes (T, P) inaccessible on Earth! • plasma physics • nuclear physics • non-equilibrium thermodynamics • electromagnetic theory • Space weather can affect satellites, power grids, and the safety of orbiting astronauts . . .

  6. The Sun’s Structure Core: • Nuclear reactions fuse hydrogen atoms into helium. Radiation Zone: • Photons bounce around in the dense plasma, taking millions of years to escape the Sun. Convection Zone: • Energy is transported by boiling, convective motions. Photosphere: • Photons stop bouncing, and start escaping freely. Corona: • Outer atmosphere where gas is heated from ~5800K to several million degrees!

  7. The Sun’s outer atmosphere • The solar photosphere radiates like a “blackbody;” its spectrum gives T, and dark “Fraunhofer lines” reveal its chemical composition. • Total eclipses let us see the vibrant outer solar corona: but what is it? • 1870s: spectrographs pointed at corona: • Is there a new element (“coronium?”) • 1930s: Lines identified as highly ionized ions: Ca+12 , Fe+9 to Fe+13 it’s hot! • Fraunhofer lines (not moon-related!) • unknown bright lines

  8. The solar wind • 1860–1950: Evidence slowly builds for outflowing magnetized plasma in the solar system: • 1958: Eugene Parker proposed that the hot corona provides enough gas pressure to counteract gravity and accelerate a “solar wind.” • 1962: Mariner 2 provided direct confirmation! • solar flares  aurora, telegraph snafus, geomagnetic “storms” • comet ion tails point anti-sunward (no matter comet’s motion)

  9. Exploring the solar wind (1970s to present) • Space probes have pushed out the boundaries of the “known” solar wind . . . • Helios 1 & 2: “inner” solar wind (Earth to Mercury) • Ulysses: “outer” solar wind (Earth to Jupiter, also flew over N/S poles!) • Voyager 1 & 2: far out past Pluto: recently passed the boundary between the solar wind and the interstellar medium

  10. The “coronal heating problem” • We still don’t understand the physical processes responsible for heating up the coronal plasma. A lot of the heating occurs in a narrow “shell!” • Most suggested ideas involve 3 general steps: 1. Churning convective motions that tangle up magnetic fields on the surface. 2. Energy is stored in tiny twisted & braided “magnetic flux tubes.” 3. Collisions between ions and electrons (i.e., friction?) release energy as heat. Heating Solar wind acceleration!

  11. L1 orbit provides 24-hour viewing! The SOHO mission • SOHO (the Solar and Heliospheric Observatory) was launched in Dec. 1995 with the goal of solving long-standing mysteries about the Sun. • 12 instruments on SOHO probe: • solar interior (via “seismology”) • solar atmosphere (images, movies, spectra) • solar wind (collect particles, measure fields) • interstellar gas (some light bounces back)

  12. High-resolution UV images of the solar disk

  13. The UVCS instrument on SOHO • 1979–1995: Rocket flights and Shuttle-deployed Spartan 201 laid groundwork. • 1996–present: The Ultraviolet Coronagraph Spectrometer (UVCS) measures plasma properties of coronal protons, ions, and electrons between 1.5 and 10 solar radii. • Combines “occultation” with spectroscopy to reveal the solar wind acceleration region! slit field of view: • Mirror motions select height • Instrument rolls indep. of spacecraft • 2 UV channels: LYA & OVI • 1 white-light polarimetry channel

  14. Several rotations of UVCS + EIT

  15. Energy re-emitted as light Incoming particle Electron absorbs energy What produces “emission lines” in a spectrum? • There are 2 general ways of producing extra photons at a specific wavelength. • Both mechanisms depend on the quantum nature of atoms: “bound” electrons have discrete energies . . . • The incoming particle can be either: • A free electron from some other ionized atom (“collisional excitation”) • A photon at the right wavelength from the bright solar disk (“resonant scattering”) • There is some spread in wavelength

  16. Using lines as plasma diagnostics • The Doppler effect shifts photon wavelengths depending on motions of atoms: • If profiles are shifted up or down in wavelength (from the known “rest wavelength”), this indicates the bulk flow speed along the line-of-sight. • The widths of the profiles tell us about random motions along the line-of-sight (i.e., temperature!) • The total intensity (i.e., number of photons) tells us mainly about the density of atoms, but for resonant scattering there’s also another “hidden” Doppler effect that tells us about the flow speedsperpendicular to the line-of-sight.

  17. On-disk profiles: T = 1–3 million K Off-limb profiles: T > 200 million K ! UVCS results: over the poles (1996-1997) • The fastest solar wind flow is expected to come from dim “coronal holes.” • In June 1996, the first measurements of heavy ion (e.g., O+5) line emission in the extended corona revealed surprisingly wide line profiles . . .

  18. Coronal holes: the impact of UVCS UVCS/SOHO has led to new views of the acceleration regions of the solar wind. Key results include: • The fast solar wind becomes supersonic much closer to the Sun (~2 Rs) than previously believed. • In coronal holes, heavy ions (e.g., O+5) both flow faster and are heated hundreds of times more strongly than protons and electrons, and have anisotropic temperatures. “Collisionless!”

  19. Coronal holes: over the 11-year solar cycle • Even though large coronal holes have similar outflow speeds at 1 AU (>600 km/s), their acceleration (in O+5) in the corona is different! (Miralles et al. 2001) Solar minimum: Solar maximum:

  20. Turbulent heating of the ions • UVCS observations have rekindled theoretical efforts to understand heating and acceleration of the plasma in the (collisionless?) acceleration region of the wind. • Shaking a magnetic field back and forth creates “Alfven waves” that become turbulent and can damp out . . . heating some particles more than others.

  21. Streamers: open or closed? Wang et al. (2000) • High-speed wind: strong connections to the largest coronal holes hole/streamer boundary (streamer “edge”) streamer plasma sheet (“cusp/stalk”) small coronal holes active regions (some with streamer cusps) • Low-speed wind: still no agreement on the full range of coronal sources:

  22. Streamers with UVCS • Streamers viewed “edge-on” look different in H0 and O+5 • Ion abundance depletion in “core” due to grav. settling? • Brightest “legs” show negligible outflow, but abundances consistent with in situ slow wind. • Higher latitudes and upper “stalk” show definite flows (Strachan et al. 2002). • Stalk also has preferential ion heating & anisotropy, like coronal holes! (Frazin et al. 2003)

  23. CMEs • Coronal mass ejections (CMEs) are magnetically driven eruptions from the Sun that carry energetic, twisted strands of plasma into the solar system . . . solar flare prominence eruption

  24. UVCS CME results: Doppler shifts • Images and movies contain much information, but spectroscopy provides more! Intensity Width Shift (Lyman alpha) April 18, 2000 Feb. 12, 2000

  25. GPS cellphones radar interference Practical application: space weather prediction?

  26. Conclusions • SOHO and UVCS results have led to new understanding of the acceleration of the solar wind, and demonstrated the power of spectroscopy to learn things that cannot be gleaned from images alone! Get involved? • We welcome more participation and collaboration with students! • 10 years of data is only beginning to be fully analyzed. • Many web tools and tutorials have been developed, with more coming online all the time. • For more info: http://cfa-www.harvard.edu/~scranmer/

  27. Doppler dimming & pumping • After H I Lyman alpha, the O VI 1032, 1037 doublet are the next brightest lines in the extended corona. • The isolated 1032 line Doppler dims like Lyman alpha. • The 1037 line is “Doppler pumped” by neighboring C II line photons when O5+ outflow speed passes 175 and 370 km/s. • The ratio R of 1032 to 1037 intensity depends on both the bulk outflow speed (of O5+ ions) and their parallel temperature. . . • The line widths constrain perpendicular temperature to be > 100 million K. • R < 1 implies anisotropy!

  28. UVCS CME results: Reconnection physics • On several occasions, narrow brightening in Fe XVIII (Te ~ 6 MK) appears in the probable location of a current sheet. • Lin et al. (2005) also saw Lyman alpha “closing down” in the sheet: one can measure reconnection rate (Vin / Vout )

  29. Thin tubes merge into supergranular funnels Peter (2001) Tu et al. (2005)

  30. Non-WKB Alfvén wave reflection • Above the 600 km merging height, we follow Eulerian perturbations along the axis of the superradial flux tube, with wind (Heinemann & Olbert 1980; Velli 1993):

  31. Resulting wave amplitude (with damping) • Transport equations solved for 300 “monochromatic” periods (3 sec to 3 days), then renormalized using photospheric power spectrum. • One free parameter: base “jump amplitude” (0 to 5 km/s allowed; 3 km/s is best)

  32. Results: polar hole vs. streamer edge • Streamer wave amplitudes are smaller than holes: more damping occurs when waves “spend more time” in the corona (lower Vph). • More damping in the low corona leads to more heating... but the waves “run out of steam” higher up in the extended corona. (QS>QH below 1.4 Rsun!) • 1-fluid temperatures are approximate, but there is general agreement with observations—and with above crit.pt. ideas.

  33. Turbulent heating rate • Anisotropic heating and damping was applied to the model; L = 1100 km at the merging height; scales with transverse flux-tube dimension. • The isotropic Kolmogorov law overestimates the heating in regions where Z– >> Z+ • Dmitruk et al. (2002) predicted that this anisotropic heating may account for much of the expected (i.e., empirically constrained) coronal heating in open magnetic regions . . .

  34. The Need for Better Observations • Even though UVCS/SOHO has made significant advances, • We still do not understand the physical processes that heat and accelerate the entire plasma (protons, electrons, heavy ions), • There is still controversy about whether the fast solar wind occurs primarily in dense polar plumes or in low-density inter-plume plasma, • We still do not know how and where the various components of the variable slow solar wind are produced (e.g., “blobs”). (Our understanding of ion cyclotron resonance is based essentially on just one ion!) UVCS has shown that answering these questions is possible, but cannot make the required observations.

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