ACE News #20
August 19, 1998

Dissipation of Magnetic Turbulence in the Interplanetary Medium

This ACE News is a progress report on efforts to address a long-standing question of cosmic ray modulation and turbulence theory: What is responsible for dissipating the magnetic energy in the solar wind, at what length scales does dissipation occur, and where does the energy go? The top panel of the figure shows the magnetic field power spectrum from 0.001 to 0.5 Hz for a fairly typical interval of ACE data. The upper curve in the top panel is the total power (the trace of the power spectral matrix) contained in all 3 components of the fluctuating vector. The lower curve in the top panel is the power contained in the fluctuations of |B|. The fact that the |B| spectrum is an order of magnitude lower than the trace shows that the fluctuations are primarily transverse to the mean field and therefore non-compressive.

The magnetic field in the solar wind originates at the Sun and extends to the furthest reaches of the heliosphere. It provides the guiding structure for the transport of solar and galactic cosmic rays, it determines the low-frequency collective behavior of the plasma, and it is a significant consideration in the evolution of the thermal particle populations. However, the magnetic fluctuations also evolve and are dissipated. There is growing evidence that magnetic energy associated with the largest spatial scales is transported to small scales where it is dissipated and heats the background ions and electrons. The region of the spectrum shown in the plot from < 0.001 to 0.2 Hz, where the power spectrum follows a -5/3 power-law form, is called the inertial range and it is in this region where the transfer of magnetic energy is thought to occur.

At frequencies just greater than the proton gyrofrequency, fpc = 0.17 Hz, the spectrum steepens to form the dissipation range. In the case shown, the dissipation range has a power-law index of -3.7. Very similar behavior is seen in hydrodynamics where the steepening of the spectrum is associated with dissipation. Traditional thinking about magnetic dissipation in the solar wind would assert that the resonant ion cyclotron instability is responsible, but this would leave a strong polarization signature in the spectrum.

The bottom panel of the figure shows the normalized magnetic helicity spectrum, which is the spatial counterpart to polarization and which is constrained to have -1 < sigmaM(f) < +1. If energy dissipation were the result of proton cyclotron resonance, then a strong polarization signature (e.g., |sigmaM(f)| -> 1) should be present in the helicity spectrum at dissipation-range frequencies. While in some instances a modest signal is present, virtually none is present in this case.

Our ongoing research, using Voyager, WIND, and now most recently ACE data, has shown that processes other than proton cyclotron damping are also present in the dissipation range. Most notably, electron Landau damping, wherein particles change energy by interacting with the phase velocity of the waves, consumes on average about 1/2 of the energy dissipated, heating the thermal electrons. Our studies have shown that the inferred energy dissipation is about equal to what is needed to obtain agreement with the temperature profiles of thermal particles in the solar wind.

Contributed by Chuck Smith, Bartol Research Institute.

See The MAG Home Page at Bartol Research Institute for more information about the MAG instrument.

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Last modified 19 August 1998, Andrew Davis
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