Transition region, corona, and solar wind in coronal holes

Abstract

Previous wave‐driven solar wind models (Hollweg, 1978) have been extended by including a new hypothesis for the nonlinear wave dissipation. The hypothesis is that the waves dissipate via a turbulent cascade at the rate given by (1) and the waves evolve according to (16). A subhypothesis is that the relevant correlation length scales as the distance between magnetic field lines. This hypothesis allows us to treat the corona and the solar wind on an equal footing; unlike in previous wave‐driven models, we do not assume that the coronal heating takes place below the base of the model. The models exhibit the correct qualitative features, viz., a steep temperature rise (the transition region) to a maximum coronal temperature in excess of 106 K, and a substantial solar wind mass flux in excess of 3.5×108 cm−2 s−1 at 1 AU. However, the model fails in detail. Parameters that yield a high‐speed flow at 1 AU have base pressures that are too low; parameters that yield correct base pressures have low solar wind flow speeds. However, the model “comes close.” Thus although we have not shown that the initial hypothesis is consistent with available data, we feel that there are sufficient uncertainties both in the model and in the data to preclude outright rejection of the hypothesis altogether.

Time-dependent hydrodynamical simulations of slow solar wind, coronal inflows, and polar plumes

“…In this model, the coronal hole is heated by the damping of Alfvén waves via a turbulent cascade; the turbulent heating rate is inversely proportional to the transverse correlation length L ⊥ , which in turn varies as B −1/2 (cf. Hollweg 1986).…”

“…where κ 0 = 10 −6 (in cgs units), but we modify it in two ways: a) at distances larger than 5 solar radii we limit the flux to 2/3 the Spitzer-Härm value, the width of the transition being 1 solar radius, so to prevent it from being larger than its collisionless estimation (see, e.g., Hollweg 1976Hollweg , 1986); b) we use an additional numerical conductive term of the form k num ∂ 2 T /∂r 2 such that k num ≈ c S ·Δz 10 2 at the transition region (hereafter TR), where c S is the sound speed and Δz the width of the TR. This term is negligible compared to the Spitzer-Härm term everywhere except around the Transition Region, where it moderates the conductive flux and helps in keeping the mesh size not too small (see e.g, Linker et al 2001, for comparable prescriptions).…”

“…Because it has different compositional properties and shows greater temporal and spatial variability, its sources are often assumed to lie outside the coronal holes that produce highspeed wind, with closed field regions being a favored choice (see Zurbuchen 2007, and references therein). However, an alternative viewpoint is that both high-and low-speed winds come from coronal holes (defined as open field regions), and that it is the rate of flux-tube expansion (Levine et al 1977;Wang & Sheeley 1990;Arge & Pizzo 2000;Poduval & Zhao 2004;Whang et al 2005) and/or the location of the coronal heating (Leer & Holzer 1980;Hammer 1982;Hollweg 1986;Withbroe 1988;Wang 1993Wang , 1994aSandbaek et al 1994;Cranmer et al 2007) that controls the wind speed at 1 AU. Thus, the slow wind tends to be highly variable because it emanates from just inside the boundaries of large coronal holes and from the small, rapidly evolving holes that form near active regions at sunspot maximum.…”

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.…”

Cyclotron resonance in coronal holes: 2. A two‐proton description

“…In section 5 we discuss the assumed power law power spectrum, the specification of the resonant wavenumber, and the self-consistent evolution of the wave level taking into account the energy absorbed by the resonant particles. Sections 6 and 7 contrast the nondispersive and dispersive cases, respec- Hollweg [1986] for a discussion of the caveats associated with this Ansatz.) The resonant particles absorb the energy which is cascaded to the small resonant scales.…”