a new look at some solar wind turbulence puzzles
DESCRIPTION
A New Look at Some Solar Wind Turbulence Puzzles. D. Aaron Roberts NASA GSFC (SHINE, 2006). The Puzzles. Magnetic vs. velocity spectra; why are they different? Origin of the anisotropic variance of B Large-scale fluctuations; reason for the “Alfv én ratio” ~ 1 Origin of k-space anisotropy - PowerPoint PPT PresentationTRANSCRIPT
A New Look at Some Solar Wind Turbulence Puzzles
D. Aaron RobertsNASA GSFC
(SHINE, 2006)
The Puzzles
• Magnetic vs. velocity spectra; why are they different?
• Origin of the anisotropic variance of B
• Large-scale fluctuations; reason for the “Alfvén ratio” ~ 1
• Origin of k-space anisotropy
• How can various quantities turn with the Parker field?
Spectrum of B; 1AU, many days
Spectrum of V; 1AU, many days
Spectrum of V; 0.3 AU, Helios 2, 6 days40.5 sec data. Slope = 1 (green)
Spectrum of V; 2 AU, Voyager 2, 8.3 hrs12 sec data; Slope = 1.5 (red), = 1.67 (green)
Spectrum of sqrt(rho)V, SW frame; 5 AU, Voyager 2, 44 days,96 sec data; Slope = 1.5 (red), = 1.67 (green)
Br vs Bt, 0.3 AU, 1 day, Alfvénic
Br vs Bt, 1 AU, 1 day, Alfvénic
Br vs Bt, 4 AU, 1 day, (less) Alfvénic
Br vs Bt, 0.3 AU, 1 day, Alfvénic (Slow Wind)
Br vs Bt, 0.3 AU, 1 day, nonAlfvénic
Pmax/Pmin (0.3 AU, Alfvénic) vs interval duration
cos(B, min var); 0.3 AU, Alfvénic
cos(B, min var); 0.3 AU, nonAlfvénic
Vsw
Vsw
Currentsheet
r
B
InflowBoundary B(,,t) &v(, ,t) areapplied
Flux tubes: Br(,);Velocity shear: vr()Waves: B(t) v(t)
“Virtual Sun”
The solutions described below were obtained in spherical coordinates in three dimensions, and at a resolution of 150150150 for r, , and
Br vs Bt simulated; 0.5 AU, initially Alfvénic but quickly evolving
Br vs Bt simulated; ~1 AU
Br vs Bt simulated; ~3 AU
Background
• We use the "OMNI" 1-AU, combined hour-averaged solar wind dataset from J. King (NSSDC/SECAA)
• 40 years of data exist, with 30 years complete enough for spectral analysis (~1/4 million points)
• Here we examine magnetic and plasma quantities (B, V, B, n)
Overview
• There is significant power at all scales from hours to 30 years, with high-frequency power laws and many spectral features at solar rotation, annual, and solar cycle frequencies.
• The radial component of V dominates except at the smallest scales.
|B|: 11-year cycle; little 27-day power; multiple high-f power laws (Blue => 50pt smoothing)
+
–
Vr: 11-year and 27 day cycles; broad, high low-f power; -2 spectrum break to -5/3
++
B
Vn: Relatively featureless power laws; low power at low f
Vt: Strong annual peak, little 27-day power
+
Bt: Strong annual and 27-day peaks and harmonics (due to sectors); high-f power laws.
+ +++++
Bn: Similar to Vn; very little low-f power
Br: 27-day and modulated (split) annual peak. Modulation is from 11/22-year cycle.
++
Ratio of energy in V to B; Dominant Vr, but all --> 0.5-1 at high-f; no “quasi-static” (“nonWKB”)
region
R
N
T
PVr/PVt; PVn/PVt; PBn/PBt: Dominant Vr and ~isotropic transverse components
Vr/Vt
Vn/Vt
Bn/Bt
Alfvénicity, 1 AU
ISEE-3, IMP-8, Interball
B
3-D, Q-2D + Slab, k-space; Fourier code initial condition
ky
kx“Slab”
kz
3-D, Q-2D + Slab, k-space; Fourier code later condition
r (AU)
0.5 1.82
-180
180
Correlation function
Power Spectrum
B
Shear produced2-D correlationfunction, similarto solar windobservations
Conclusions (1 of 2)• The minimum variance of B is nearly along B in
highly Alfvénic regions; turbulence tends to decrease both effects.
• |B| ~ const key to min variance (Barnes, 1981), and the “spherical polarization” follows the field. How?
• WKB and simulations have failed to produce this effect; transverse variability is required, but how?
• Compressive effects don’t help (Hollweg & Lilliequist, 1978).
• The dominant energy in fluctuations from the scale of years to a fraction of a day is contained in the variation in the radial flow speed;
Conclusions (2 of 2)• There is no "quasi-static" regime for solar wind
fluctuations but rather the transverse magnetic and velocity fluctuations are comparable in energy at essentially all scales. Why?
• Neither quasi-2-D turbulence or slab waves will turn with the Parker field; only a nonlinear coupling of the two (or other means) will accomplish this. The “two component” model does not reflect this.
• Shear can easily turn k, but not B.• Velocity and magnetic fluctuations evolve at different
rates, and with different spectra. Turbulence theory?