Input for Phil’s SOHO-17 talk...

1. Input for Phil’s SOHO-17 talk...

2. UVCS / SOHO • 1979–1995: Rocket flights and Shuttle-deployed Spartan 201 laid groundwork. • 1996–present: Solar and Heliospheric Observatory (SOHO), with 12 instruments probing solar interior to outer heliosphere. • The Ultraviolet Coronagraph Spectrometer (UVCS) measures plasma properties of coronal protons, ions, and electrons between 1.5 and 10 solar radii. slit field of view: • Mirror motions select height • Instrument rolls indep. of spacecraft • 2 UV channels: LYA & OVI • 1 white-light polarimetry channel

3. 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. (e.g., Kohl et al. 1997,1998)

4. Coronal holes: over the 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:

5. Alfven wave’s oscillating E and B fields ion’s Larmor motion around radial B-field Ion cyclotron waves in the corona? • UVCS observations have rekindled theoretical efforts to understand heating and acceleration of the plasma in the (collisionless?) acceleration region of the wind. • Ion cyclotron waves (10 to 10,000 Hz) suggested as a natural energy source that can be tapped to preferentially heat & accelerate heavy ions. • Dissipation of these waves produces diffusion in velocity space along contours of ~constant energy in the frame moving with wave phase speed:

6. But where do ion cyclotron waves come from? • Alfven waves with frequencies > 10 Hz have not yet been observed in the corona or solar wind, but ideas for their origin abound . . . . (1) Base generation by, e.g., “microflare” reconnection in the lanes that border convection cells (e.g., Axford & McKenzie 1997). Problem: low Z/A ions consume base-generated wave energy before it can be absorbed by, e.g., O+5, He+2, p+ (Cranmer 2000, 2001; Tu & Marsch 2001) (2) Secondary generation: low-frequency Alfven waves may be converted into cyclotron waves gradually in the corona. Problem: Turbulence produces mainly high-kperp fluctuations (which still have low freq’s). Instabilities require stronger nonlinearity than may exist for low-freq Alfven waves.

7. MHD turbulence • It is highly likely that somewhere in the outer solar atmosphere the fluctuations become turbulent and cascade from large to small scales: • With a strong background field, it is easier to mix field lines (perp. to B) than it is to bend them (parallel to B). • Also, the energy transport along the field is far from isotropic: Z– Z+ Z– (e.g., Dmitruk et al. 2002)

8. An Alfvén wave heating model • Cranmer & van Ballegooijen (2005) built a model of the global properties of non-WKB (low-frequency!) Alfvenic turbulence along an open flux tube. • Background plasma properties (density, flow speed, B-field strength) are fixed empirically; wave properties are modeled with virtually no “free” parameters. • Lower boundary condition: observed horizontal motions of G-band bright points.

9. 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)

10. 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 . . .

11. Anisotropic MHD cascade • Can MHD turbulence generate ion cyclotron waves? Many models say no! • Simulations & analytic models predict cascade from small to large k ,leaving k ~unchanged.“Kinetic Alfven waves” with large k do not necessarily have high frequencies. • In a low-beta plasma, KAWs are Landau-damped, heating electrons preferentially! • Cranmer & van Ballegooijen (2003) modeled the anisotropic cascade with advection & diffusion in k-space and found some k “leakage” . . . probably not enough!

12. something else? But does turbulence generate cyclotron waves? freq. • Preliminary models say “probably not” in the extended corona. (At least not in a straightforward way!) horiz. wavenumber How then are the ions heated & accelerated? • Impulsive plasma micro-instabilities that locally generate high-freq. waves (Markovskii 2004)? • Non-linear/non-adiabatic KAW-particle effects (Voitenko & Goossens 2004)? • Larmor “spinup” in dissipation-scale current sheets (Dmitruk et al. 2004)? • KAW damping leads to electron beams, further (Langmuir) turbulence, and Debye-scale electron phase space holes, which heat ions perpendicularly via “collisions” (Ergun et al. 1999; Cranmer & van Ballegooijen 2003)? cyclotron resonance-like phenomena MHD turbulence

13. Two recent possibilities for “something else!” • Markovskii et al. (2006): high k_perp turbulence generates strong horizontal shears that are unstable to generation of ion cyclotron waves. (May also explain in-situ steepening of frequency spectra.) • Chandran (2006): nonlinear couplings between Alfven and Fast-mode waves. Fast waves cascade ~isotropically in k-space) and can “feed” into Alfven wave power at high k_parallel (high frequency).

14. 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 & where the various components of the variable slow solar wind are produced (e.g. “LASCO 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 . . .

15. Future Diagnostics: more ions • Observing emission lines of additional ions (i.e., more charge & mass combinations) in the acceleration region of the solar wind would constrain the specific kinds of waves and the specific collisionless damping modes. Example prediction of “new” ion line widths (Cranmer 2002, astro-ph/0209301)