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:



Marco Velli

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.

Leif Svalgaard

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.

Yang Liu

'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.

Luca Teriaca

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.

Jonathan Cirtain

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).

Nour-Eddine Raouafi

"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?


Filamentary Structure

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.

Cyclotron Waves

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.

General Comments

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

Summary talk (James McLaughlin)

12-December-2008. To be added...

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