Solar Cycle 24 Group B
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Fast Wind / Plumes
This working group will assess current knowledge of how polar plumes and coronal holes are heated, and how they relate to the solar wind. Polar plumes have been an on-again, off-again candidate source for the solar wind; because the plasma topology is simpler than for the quiet sun or active regions, this portion of the corona is a convenient laboratory for general coronal heating mechanisms; and recent improvements in resolution, vantage point, and theoretical modeling may significantly change our understanding of how plumes originate and relate to the solar wind.
Specific questions for discussion include:
- Are plumes fundamentally caused by magnetic reconnection at the footpoint, and how does that reconnection proceed?
- Can we shed new light on the debate over whether plumes are significant sources of solar wind?
- What has been learned recently about the time scales and associated mechanisms for plume formation and maintenance?
- What is the relationship between jets, plumes, and helical structure in the open coronal hole?
- What steps are needed to generate realistic numerical models of plume formation and evolution?
- Jonathan Cirtain
- Craig DeForest
- Yang Liu
- James McLaughlin
- Karin Muglach
- Nour-Eddine Raouafi
- Ilan Roth
- Leif Svalgaard
- Luca Teriaca
- Marco Velli
- Yi-Ming Wang
Discussion of Casalbuoni et al (1999) - an extension of the results of Parker (1964). Paper concludes that adding an ad hoc heating profile is no better than specifying the temperature profile. So how important is expansion profile if you're allowed to make up your own temperature profile?
Discussion of Borovsky (2008) - argument that the inner heliosphere is filled with a network of entangled magnetic flux tubes and that these tubes are fossil structures that originate at the solar surface. Marco doesn't agree with this interpretation of this observation result.
Discussion of Verdini & Velli (2007) - propagation and dissipation of Alfvénic turbulence in a model solar atmosphere.
There are persistent, bright patches in polar regions seen in Nobeyama 17 GHz synoptic limb charts. Furthermore, the Nobeyama 17 GHz brightness temperature is a good proxy for WSO solar polar magnetic field strength (but WSO only has a 3-inch aperture!).
What is interesting is that these Nobeyama bright points match the polar field strength so well. This suggests that something is going on. Critically, are these the footpoints of plumes? Something is sitting there for a long time - what are these?
So we have Radio emission at 17 GHz (corresponding to brightness temperature of chromosphere) matching the magnetic field, and these bright patches are not behaving like a polar cap should.
Alternatively, are these polar faculae? This proxy for the polar magnetic field may turn out to be better than using magnetograms.
'Magnetic Elements at Higher Latitudes'. Comparing MDI (LOS) magnetograms and Hinode vertical field, not every flux concentration matches up. Hence, some of the strong flux concentrations seen in MDI have horizontal/transverse contributions.
NB: MDI uses Ni line, Hinode uses Fe I 630 nm, but both should have similar formation heights.
Wilhelm claims there is no velocity in plumes. [enter talk/slides here] Luca says there is velocity in plumes.
It seems like there are outflow motions associated with plumes. Are they always there? Luca's results improve on others' work by making a more careful analysis (e.g. using nonlinear distortion removal).
NB: Although plumes are cooler, they do have hot DEM tails.
NB: Yi-Ming Wang: Simulations show that there would be velocity in initial formation of the plume.
Polar X-ray jets: 7-10 per hour, repeated runs of 'HOP 3' (can search for the data through the Harvard webpage). Wave motions apparent in jets, typical period 180s. The movement/shifting of the jet can give an indirect measurement for what the flux distribution has to be at the lower boundary.
[Question] Why didn't we see these with TRACE? [Answer] Exposure times were too long.
Savcheva et al (2007) presents statistics for jets: average duration ~ 800 seconds, average size ~ 0.8Mm. Upper bound on the size of jet-width-distribution ~ 12Mm. Preference for transverse velocity component: evidence of dynamo process? Perhaps even something like a Hale-like law for jets? Upper bound on jet duration distribution 2500 seconds. This implies large jets have more flux, and large jets have longer durations. Most jets are 700km wide and last 700seconds. This is indicative of flux cancellation.
There is no preference to jets spatial distribution. Although, given N-E's comments later, this may need to be looked at again.
Cirtain et al (2007) demonstrates high speed outflows. Two velocity components: ~800km/s and ~200km/s. Jets have temperature 4-5MK, not 2-3MK as previously reported. For example, can see them very clearly in Ca XVII. But can see some jets in EUVI: 60-80% of jets in XRT are NOT seen in EUVI.
Filament eruptions are also seen, and are sometimes reported as jets. These filament eruptions show rotation.
Shibata model also allows cool chromospheric material to be dragged along. We can see this sometimes: 75,000K material along side 3MK material has been seen.
Discussion of Tsuneta et al (2008).
"Coronal Temperature Anisotropies; Plumes; Jets & Polar Magnetic Fields": (2nd Hinode Meeting).
Concerning plasma flows in plumes: O IV broadening indicates contribution of interplumes, otherwise contribution of plumes.
The work uses the Banaszkiewicz et al (1998) coronal field model.
Vplume = f(r) * Vinterplume
Plumes considered as homogeneous cylinders. Plumes are cooler than interplume up to 3-4 RSun. After this point, plume warms up and disappears / merges with rest of plume. Details can be found in Raouafi et al (2007).
Thus, there must be outflows in plumes, in agreement with Luca (i.e. not v=0).
They merge at a certain height, where the speeds come together (about 30RSun?). So at this point, the contribution to the fast solar wind is the same, i.e. the distinction between plume and interplume is gone.
However, Richard Wu claims to have seen plumes above 45 RSun - unresolved problem.
Plumes may be preferentially rooted away from the poles (Raouafi et al 2007). Low down, we see many plumes. High up, we see very few. Plumes expand super-radially, faster than the poles themselves. The beta=1 layer occurs at about 4 RSun , but plumes can continue to expand up to 10 RSun. The more a plume is inclined towards you, the more the parallel component enters into the LOS. When the plume is at 45 degrees, there are contributions from both the transverse and parallel components. This is not considered in the model of (Raouafi et al 2007), as the velocity f(r) is isotropic. So do we only see plumes towards the edge of the polar hole (and not actually at the top)? Remember that at higher latitudes, we don't observe the same area, due to the LOS effect.
Looking at the latitude distribution of polar flux, we see more flux elements further away from the pole. These have a uniform distribution from the polar cap (edge of polar cap extends to about 75-80 degrees). The density of relatively large flux elements is greater away from the pole.
So if there are no plumes between 80-90 degrees, this is saying something deep about how flux is emerging (within a few degrees of the poles).
Testable idea / prediction: there are plumes near poles, and plumes are related to jets. Thus, if there is a preference to plumes spatial distribution, there should be a preference to jets spatial distribution. Ideas of plume suppression?
Plumes have internal structure / filamentary structure, but this is an open question. Questions for creation mechanism - lots of reconnection events?
How do we get a steady plume when we have impulsive heating / filamentary structure?
The finest scale structure is probably of the order 1-2 minutes.
NB: There are 7-10 jets per hour, all the time, inside a polar coronal hole.
Proton solar wind distribution function leads to ANISOTROPIC + BEAM (Marsh et al 1982, Helios data). This leads to motivation for cyclotron wave interpretation. But Marco believes a proper model of lower frequency waves with explain this. Also, Cranmer et al (1999) reports on broad line profiles in O VI. Again, this leads to an interpretation that we don't like. Rebuttal put forward by Raoaufi & Solanki (2004a,b;2006a,6;2007), concluding that there is no need for large anisotropies.
Maximum energy estimates in a plume
[CDeF]: Clare Parnell and Andrew Haynes conducted experiments of 3D reconnection, taking a footpoint and driving it into another.
[MV]: Surely there is a maximum energy, you are killing a flux fragment, so integral of B^2/8/pi dV is finite.
[CDeF]: No, also energy from compressing the field. Thus, for plumes fudge factor would be how much do you squash/collide the fields.
[MV]: Thus, in addition to B^2/8/pi term, you would have to consider the work done on the field.
How to make a plume
To make a plume, we need two things: flux emergence and unipolar flux concentration (emerging bipole and existing open field). The concentration of flux at the poles is dominated by weak flux (everywhere). Need network elements and bipolar emergence, but flux concentrations that are unipolar exist closer to the polar hole. Out of phase: At solar max, there is a large number of emerging bipoles. At solar min, there is lots of open field. But we need both of these to make a plume!
Can we get some leverage on how plumes are heated? Can sustain plumes by injecting energy: this can be steady or impulsive (low rate reconnection).
Idea: If plumes are so dense, radiative cooling time will be much faster. But we see plumes for a long time, so heating must be sustained.
Luca: When we observe a plume, we are looking through plume and interplume, so we must be careful.
There are 30 - 50 plumes (per polar cap) at any given time, occupying about 5% of a polar cap.
Density measurements are still a problem (depends on how good atomic data is) - unclear if plumes are denser then interplume by 20% or up to a factor of 10.
TRACE observations of plumes in DeForest (2007)
On acceleration mechanisms: You can't make a fast solar wind purely based on the temperature profile; you need to add something extra to push it along.
Plumes are hazy structures with no clear edges.
How important are drift mechanisms in the solar wind. Do they mix plume and interplume material?
It is possible to make sinograms of coronal holes. So as a plume rotates, is make a sine wave. But...we don't see any vertical lines in 3 years of data!
Key science questions that need answering
- What distinguishes plumes and interplumes? If reconnection driven, why plumes not everywhere in light of the recent Hinode results that tiny magnetic field emergence is ubiquitous? Answer: height of X-point?
- What sustains plumes? (We think it is driven reconnection because the plume time scale is long compared to the plasma cooling time scale and comparable to the cancellation time scale - but this needs to be verified with detailed observations at high resolution)
- What is the detailed 3D structure of plumes? Curtain versus tube debate still exists. Answers in stereoscopic data? Tomography (e.g. N. Barbey Ph.D. thesis)? (Plumes appear to be flux tubes but may have broad aspect ratios; curtains may exist within each plume)
- How are plumes accelerated? Wave input? Reconnection driven? All models require heat *and* momentum input high in the corona, generally assumed to be from alfven waves propagating upward from the surface, but (despite results from Tomczyk et al. and De Pontieu et al.) the energy has not yet been explicitly detected.
- How far out into the corona do they extend? Mixing? Diffusion? Can we use the HI instruments? (Important for heliospherics to know how/where plumes fade into the “background” wind - current imaging detection record is still 30 Rs but HI might do better.)
- Leif Svalgaard mystery: There are persistent, bright patches (“radio dots”) in polar regions seen in Nobeyama 17 GHz synoptic limb charts. What is interesting is that these Nobeyama bright points match the polar field strength so well. Critically, are these the footpoints of plumes? There are other problems with the dots - e.g. why do they shine radio only in the near-horizontal direction?
- Jet/Plumes spatial distribution and Plume suppression: Plumes are preferentially rooted away from the pole (Raouafi et al. 2007). No plumes between 80-90 degrees. But no preference to jets spatial distribution? (This is a reasonably solid result and is seen *after* compensation for the area effect at longitudes close to 90°; if true it is very interesting for the dynamo.)
Summary talk (James McLaughlin)
12-December-2008. To be added...