On the preferential acceleration and heating of solar wind heavy ions

Abstract

We investigate the preferential heating and acceleration of solar wind heavy ions by the resonant cyclotron interaction with parallel‐propagating left‐polarized hydromagnetic waves. We set up a scenario whereby the energy for this interaction is taken from saturated low‐frequency Alfven waves via a cascade to the higher, resonant frequencies. In order to utilize the existing theoretical work, the particles are taken to be thermally isotropic, and the waves are taken to be dispersionless. This scenario is incorporated into a numerical solar wind code describing the flow from an inner radius (taken to be 10 solar radii) to 1 AU. Thus we present the first model of a wave‐driven, three‐fluid, supersonic solar wind. By varying the model parameters we test the ability of the resonant interaction in this model to produce the excess speeds and temperatures of heavy ions that are observed. We find that unrealistically steep wave spectra are required to produce differential speeds of the order of the Alfven speed at 0.3 AU. In the case of He2+, the power spectrum of waves must be steeper than γ ≃ 4.7, where γ is the power law spectral index. Ions of oxygen or iron, with larger mass‐per‐charge ratios, are accelerated more readily than helium, but still require steeper spectra than are observed. This model is also unable to produce the mass‐proportional heavy ion temperatures that are observed. We show that the production of mass‐proportional temperatures is inconsistent with preferential acceleration of heavy ions by this mechanism. The model also produces a heavy ion‐to‐proton temperature ratio at 1 AU which is anticorrelated with solar wind speed, in contradiction to the observed behavior.

Kinetic properties of heavy solar wind ions from Ulysses-SWICS

“…[13] We now discuss these data in the framework of qualitative expectations from models addressing ion interactions in the solar wind [e.g., Dusenbery and Hollweg, 1981;Isenberg and Hollweg, 1983;Marsch et al, 1982]. Recent observations by SOHO [e.g., Cranmer et al, 1999;Tu et al, 1998] indicate preferential heating of heavy ions close to the Sun, especially in coronal-hole-associated regions, emphasizing the importance of electromagnetic waves or turbulence.…”

“…[16] For simplicity, we approximate the wave-particle interaction time scale by t w / (q/m) 2 I(m/q)(dB/B) 2 [Isenberg and Hollweg, 1983]. Here, I(m, q, v k ) stands for the relative wave-intensity resonantly interacting with the particle, B and dB are the average and the turbulent magnetic field.…”

Observations of the Sun at Vacuum-Ultraviolet Wavelengths from Space. Part II: Results and Interpretations

“…These waves exert a wave-pressure-gradient force and can thus accelerate the flow (Belcher 1971;Alazraki and Couturier 1971;Hollweg 1973;Jacques 1978). Also direct acceleration of the ions by cyclotron-resonant absorption of high-frequency waves (Dusenbery and Hollweg 1981;Marsch et al 1982;Isenberg and Hollweg 1983) may occur. The stationary momentum equation including the Alfvén wave pressure, p w , and cyclotron-wave acceleration, a w , is…”

Resonant acceleration and heating of solar wind ions by dispersive ion cyclotron waves

“…With the cascade effect included in one way or another, the Alfv•n waves were successfully used to explain the extended heating of the protons and to generate a fast solar wind [Hollweg, 1986;Hollweg and Johnson, 1988;Tu, 1987 To further apply the mechanism to the preferential acceleration and heating of the alphas and other heavy ions, Isenberg and Hollweg [1983] (hereinafter designated as IH) adopted a three-fluid solar wind model to investigate the preferential acceleration and heating of heavy ions. In that model, the saturation-and-cascade scenario was hypothesized to determine the wave dissipation rate, and the quasi-linear theory of the waveparticle interaction was invoked to distribute the dissipated wave energy among different species.…”