Interplanetary Scintillations and the Acceleration of the Solar Wind

Steven R. Spangler …. University of Iowa

Radio propagation observations yield information on turbulence in the solar corona and the solar wind. Example of observation is phase scintillations of a VLBI interferometer

Radioastronomical scintillations yield information on density fluctuations in the solar wind. They indicate the form of the density spatial power spectrum.

This is a problem, since the important terms in solar wind turbulence are probably magnetic and velocity fluctuations. As a result, despite 40 years of observations, radio scintillation observations have had relatively few results of major astrophysical significance.

What can we do to make the radio observations more directly constrain theories of solar wind heating and acceleration?

Theory of the Wave-Driven Solar Wind

The presence of turbulence causes acceleration of the solar wind fluid and heating due to dissipation of the turbulence (Hollweg ApJ 181, 547, 1973)

Note that driving terms are determined by magnetic

fluctuations, which we do not directly measure

Inferring Magnetic Fluctuations from Density Fluctuations

The key to making IPS more useful is to determine a relationship between density and magnetic field fluctuations. Spangler and Spitler (Phys. Plasm. 11, 1969, 2004) studied solar wind fluctuations at 1 au and measured the compressibility factor R for 66 time intervals.

Results: median R of 0.46, mean of 0.72

From measurement of R, and assumption of similar power spectra, one can infer the magnetic power spectrum from that

of density

An Empirical Test of Solar Wind Acceleration and Heating Models

Use empirical models for bulk solar wind parameters (as function of heliocentric distance) and IPS-inferred turbulence parameters to test relative magnitude of turbulent heating and acceleration terms

Results: Comparison of Various Terms in Momentum and Energy

Equations

Preview: viability of wave-driven models depends on the assumed density model. Higher density means more material to accelerate and heat.

Solar Wind Acceleration- Density Model 1

Measured solar wind acceleration

Ponderomotive forceGas pressure gradient

Solar Wind Acceleration-Density Model 2

Measured solar wind acceleration

Ponderomotive force

Gas pressure gradient

Turbulent Heating-Density Model 1

Modeled gas heating

Turbulent heating

Turbulent Heating-Density Model 2

Turbulent heating

Modeled gas heating

Conclusions

• Radio scintillations observations, together with empirical n-b conversion, and independent solar wind data, are “not inconsistent with” an important dynamical and thermodynamical role for MHD turbulence.

• Main poorly-constrained quantities are precise value of n-b conversion and density in the solar wind.

• Accurate MHD modeling of solar wind will allow improvements