Progress in Interplanetary ScintillationProgress in Interplanetary Scintillation
Bill Coles, University of California at San Diego
A. The Solar Wind:
B. Radio Scattering:
C. Observations:
D. Recent progress:
The Solar Wind
1. The existence of the solar wind could have been inferred from the shape of helmet streamers.
2. It could also have been inferred from measurements of the aurora.
3. It was inferred from observations of the direction of the ionic comet-tails.
Sun
Solar Wind
driftingintensitypattern
incidentplanewave
compactradio
source
receivingantennas
baseline
Plan view of an ecliptic observation
driftingphase
pattern
angularspectrumof plane waves
Velocity vs Latitudeover Solar Cycle
UCSD Nagoya
Dennison & Hewish, 1966
Hewish & Symonds, 1967
Solar Maximum
Anisotropy vs Solar Distance
Model AR(R) of plasma
expected AR(R) for radio wave
The vertical bars indicate variation not statistical error
Helios (equatorial)Ulysses(polar)
VLBA (polar)
Grall et al., VLA par (polar)
Harmon and Coles (mean)
Paetzold & Bird (polar)
VLA perp
Woo & Armstrong(mean)
Scale Dependence of Anisotropy
These characteristics of the solar wind microstructure have been known for 20 years. They lead John Harmon to propose that the micro-structure was caused by obliquely propagating Alfven waves because these waves would satisfy all four of the properties discussed:
1. They would cause radial elongation of the structure2. The elongation would decrease with distance3. The spectrum would be flatter than Kolmogorov4. The waves would damp at the ion inertial scale.
The problem is that these waves would also cause the intensity diffraction pattern to move outwards with respect to the flow at the group velocity of the waves VA. For quite some time we did not think this was compatible with the observations.
We now believe that the velocity observations are compatible with these waves.
Cross Correlationof Intensity in Fast Wind
10 RS at VLBA
-solar minimum -half the baselines shown-slow and fast peaks clear-best fit model not unique
Implication
Variations in angular scattering are not obviously correlated with variations in density.
Angular scattering is a column integral of density2, whereas white light brightness is a column integral of density.
Apparently scattering near the Sun is dominated by small but dense structures which are invisible in white light because their contribution to integrated density is negligible, however they contribute to scattering because they contribute significantly to the integral of density2.
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