Post on 18-Dec-2015
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
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
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,
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
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).
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
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.
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
Expectations from flux-tube expansion
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
Adjusting the field strength?
Unmodified MAS fields :Add photospheric & chromospheric fields,
and multiply MAS fields by 4 :
Black curves: Cranmer et al. (2007)
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
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...
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
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...
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
extra slides . . .
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
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?
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
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
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
Cranmer et al. (2007): other results
Ulysses
SWICS
Helios
(0.3-0.5 AU)
Ulysses
SWICS
ACE/SWEPAM ACE/SWEPAM
Wang & Sheeley (1990)
Testing the S-Web Idea with Time-Steady Turbulence S. R. Crarnmer, Oct. 27-28, 2011
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.