1. 2005 Joint SPD/AGU Assembly, SP33A–02
On the Incompatibility between UVCS/SOHO Observations of Polar Coronal Holes and Isotropic Oxygen Velocity Distributions
S. R. Cranmer, A. V. Panasyuk, and J. L. Kohl Harvard-Smithsonian Center for Astrophysics
2005 Joint SPD/AGU Assembly, SP33A–02

2. UVCS results: background
Ultraviolet spectroscopy probes properties of ions in the wind’s acceleration region.
In June 1996, the first measurements of heavy ion (e.g., O5+) line emission in the extended corona revealed surprisingly wide line profiles . . .
Off-limb profiles: T > 200 million K !
On-disk profiles: T = 1–3 million K

3. UVCS results: O VI over polar coronal holes

4. Doppler dimming
Off-limb photons formed by resonant scattering of solar-disk photons:
Profile width depends on line-of-sight component of velocity distribution (i.e., perp. temperature and projected component of wind flow speed);
Total intensity depends on the radial component of velocity distribution:
If atoms are flowing in the same direction as the incoming disk photons “Doppler dimming”

5. 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.

6. 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 ... R < 1 implies anisotropy?
Raouafi & Solanki (2004) suggested that it may be possible to model the O VI observations with an isotropic velocity distribution.

7. Modeling all possible O5+ parameters
To clear up confusion, we performed an exhaustive search of the O5+ parameter space:
For each observed height:
Fixed: T u
ui : km/s
T i : MK
Te : MK (constant)
ne (r): Doyle et al. (1999)
dui/dr mass flux conservation
“uproj” (Banaszkiewicz
et al. 1998)
surfaces of const. line width: 200, 600 km/s

8. Search for allowable anisotropies
0.15 1 2 5 10 20 50
100
Result of search: Raouafi & Solanki (2004) did find a new “corner” of parameter space that approaches the measurements . . .
The 3D “data cubes” were searched for the minimum ratio (T / T ) that could produce a specific line width & intensity ratio at a given height.
Contours of anisotropy ratio for the data cube at 2.5 solar radii.
Whether the line width is 400 or 600 km/s matters crucially!

9. UVCS results: spectral line properties

10. Minimum allowed anisotropies

11. UVCS results: spectral line properties

12. UVCS results: spectral line properties

13. Minimum allowed anisotropies

14. Minimum allowed anisotropies

15. Conclusions
Above ~2.5 solar radii in coronal holes, UVCS/SOHO observations still require:
Actual anisotropy ratios may be even larger!
Temperature anisotropy of this kind is strong evidence for ion cyclotron resonance.
Work is ongoing to analyze UVCS coronal hole ion properties over the solar cycle . . .
(see Friday poster SH51A–03)
(see Friday poster SP51B–07)
Upcoming missions (SDO, STEREO, Solar-B) will help build a more complete picture, but we really need next-generation UVCS and LASCO.

16. 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:
Alfven wave’s oscillating
E and B fields
ion’s Larmor motion around radial B-field

17. 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–

18. How are ions heated preferentially?
Variations on “Ion cyclotron resonance:”
Additional unanticipated frequency cascades (e.g., Gomberoff et al. 2004)
Fermi-like random walks in velocity space when inward/outward waves coexist (heavy ions: Isenberg 2001; protons: Gary & Saito 2003)
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)
Other ideas:
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)
Collisionless velocity filtration of suprathermal tails (Pierrard et al. 2004)

19. 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.