Progress Report: Testing the S-Web Idea with “Time-Steady” Turbulence Steven R. Cranmer...

Progress Report:Testing the S-Web Idea with“Time-Steady” Turbulence

Steven R. CranmerHarvard-Smithsonian Center for Astrophysics

Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011

A turbulence-driven solar wind?• There’s lots of evidence that ample wave/turbulence power exists to heat the corona.

• A likely scenario is that the Sun produces MHD waves that propagate up open flux tubes, partially reflect back down, and undergo a turbulent cascade until they are damped at small scales, causing heating.

• Cranmer et al. (2007) explored the wave/turbulence paradigm with self-consistent 1D models, and found a wide range of agreement with observations.

Z+

Z–

Z–

(e.g., Matthaeus et al. 1999)

Ulysses 1994-1995


Turbulent heating scales with field strength• Mean field strength in low corona:

• If the regions below the merging height can be treated with approximations from “thin flux tube theory,” then:

B ~ ρ1/2 Z± ~ ρ–1/4 L┴ ~ B–1/2

B ≈ 1500 G (universal?)

f ≈ 0.002–0.1B ≈ f B ,. .

..

. . . and since Q/Q ≈ B/B , the turbulent heating in the low corona scales directly with the mean magnetic flux density there (e.g., Pevtsov et al. 2003; Schwadron et al. 2006; Kojima et al. 2007; Schwadron & McComas 2008).

..

• Thus,


What else does turbulent heating scale with?

• In other words, what effects do small-scale transverse gradients in field strength, density, or flow velocity have on the solar wind?

• When interacting with supergranular “magnetic carpet” motions, these regions will undergo more changes in topology/connectivity than will more homogeneous regions (Antiochos et al. 2011).

• A local increase in interchange reconnection (jet-like) events may generate additional wave energy in the open-field regions.

• Sharp gradients enable mixing between Alfvén, fast, slow mode waves (e.g., Stein 1971; Kaghashvili 2007). Maybe they also enhance Alfvén wave reflection, which would boost the turbulent heating rate.

• Small enough gradients can introduce kinetic-scale instabilities (e.g., Markovskii 2001; Mecheri & Marsch 2008).


Increase the complexity of the field . . .• Jon Linker sent me a latitudinal cut of field lines threading a MAS model from

the 2008 eclipse: one field line per degree from north pole to south pole.


Expectations from flux-tube expansion


Adjusting the field strength?

Unmodified MAS fields :Add photospheric & chromospheric fields,

and multiply MAS fields by 4 :

Black curves: Cranmer et al. (2007)


Extremely preliminary results• The Cranmer et al. (2007) ZEPHYR code finds a steady-state solar wind solution

along one field line in approx. 30 minutes of CPU time on my workstation...


Discussion

• I’ll circulate a more complete version of the preliminary results plot in a few days. (If the internal energy convergence is bad at the equator, I’ll have to run it again with different iteration parameters...)

• Are there “interesting” QSLs included in the MAS latitudinal cut presented above? It would be interesting to see a corresponding plot of squashing factor vs. latitude.

• In parallel, I’m working with Aad van Ballegooijen in running the ZEPHYR code on field lines mapped using the PFSS technique from SOLIS and HMI magnetograms. Preliminary results show a much clearer anti-correlation between wind speed and flux-tube expansion factor... not sure why...


extra slides . . .


Other questions to address (from last meeting)

• Origin of lowest-frequency (1/f) waves seen at 1 AU:

The self-consistent product of a turbulent cascade?

Spacecraft passage through “spaghetti-like” flux tubes rooted on the solar surface? (Borovsky 2008)

• Coronal heating from MHD turbulence:

Does damping of turbulence produce the right mixture of collisionless kinetic effects?

How can we better constrain the frequency spectrum of waves/turbulence in the corona? (crucial for non-WKB reflection)

• Do reconnection/loop-opening events generate enough mass, momentum, & energy to power the solar wind?


What processes drive solar wind acceleration?• No matter the relative importance of RLO events, we do know that waves and

turbulent motions are present everywhere... from photosphere to heliosphere.

• How much can be accomplished by only WTD processes? (Occam’s razor?)

Hinode/SOT

G-band bright points

SUMER/SOHO

Helios & Ulysses

UVCS/SOHO

Undamped (WKB) wavesDamped (non-WKB) waves


Cranmer et al. (2007): other results

Ulysses

SWICS

Helios

(0.3-0.5 AU)

Ulysses

SWICS

ACE/SWEPAM ACE/SWEPAM

Wang & Sheeley (1990)


How are ions preferentially heated?

• MHD turbulence may have some kind of “parallel cascade” that gradually produces ion cyclotron waves in the corona and solar wind.

• When MHD turbulence cascades to small perpendicular scales, the small-scale shearing motions may be unstable to generation of cyclotron waves (Markovskii et al. 2006).

• Dissipation-scale current sheets may preferentially spin up ions (Dmitruk et al. 2004).

• If MHD turbulence exists for both Alfvén and fast-mode waves, the two types of waves can nonlinearly couple with one another to produce high-frequency ion cyclotron waves (Chandran 2005).

• If nanoflare-like reconnection events in the low corona are frequent enough, they may fill the extended corona with electron beams that would become unstable and produce ion cyclotron waves (Markovskii 2007).

• If kinetic Alfvén waves reach large enough amplitudes, they can damp via wave-particle interactions and heat ions (Voitenko & Goossens 2006; Wu & Yang 2007).

• Kinetic Alfvén wave damping in the extended corona could lead to electron beams, Langmuir turbulence, and Debye-scale electron phase space holes which could heat ions perpendicularly (Matthaeus et al. 2003; Cranmer & van Ballegooijen 2003).

UVCS results (mainly in coronal holes) have spurred a lot of theoretical work . . . but observations still haven’t allowed the exact mechanisms to be pinned down!

• CPI is a large-aperture ultraviolet coronagraph spectrometer that has been proposed to be deployed on the International Space Station (ISS).

• The primary goal of CPI is to identify and characterize the physical processes that heat and accelerate the plasma in the fast and slow solar wind.

• CPI follows on from the discoveries of UVCS/SOHO, and has unprecedented sensitivity, a wavelength range extending from 25.7 to 126 nm, higher temporal resolution, and the capability to measure line profiles of He II, N V, Ne VII, Ne VIII, Si VIII, S IX, Ar VIII, Ca IX, and Fe X, never before seen in coronal holes above 1.3 solar radii.

• 2011 September 29: NASA selected CPI as an Explorer Mission of Opportunity project to undergo an 11-month Phase A concept study.

Progress Report: Testing the S-Web Idea with “Time-Steady” Turbulence Steven R. Cranmer...

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