A Nonlinear Model for Dynamics in the Expanding Accelerating Solar Wind

Abstract:

One of the outstanding problems in astrophysics is the origin of stellar coronae, winds, and, more generally, the ubiquitous existence in the universe of hot million degree (or more) plasmas. The solar corona and wind provide an accessible environment to understand plasma heating and acceleration, and this is one of the main goals of the upcoming NASA mission Solar Probe Plus, which will arrive closer to the Sun (10 R_s), within the acceleration region than any previous spacecraft.

Alfvén waves, which can easily propagate along magnetic field lines from the cooler photosphere to the hot corona and above, are thought to provide a possible mechanism to supply the energy required to heat and boost the solar wind, through turbulent dissipation and pressure. In-situ observations show that a nonlinear cascade of Alfvén waves, mainly propagating outward, is taking place, and that it evolves with heliocentric distance. In spite of the well defined observational signatures, the evolution of such Alfvénic turbulence in the solar wind is still a matter under debate, as neither linear theory nor numerical simulations can account for the observed properties. In particular, the effects of the expansion of the underlying solar atmosphere are a crucial element which must be taken into account, since the observed decrease in overall rms energies is best accounted for by expansion effects.

Here we present a model to study the dynamics of a plasma parcel embedded in a radially accelerating solar wind, all the way from the acceleration region to the inner heliosphere, called the Accelerating Expanding Box. This model takes describes the radial evolution of turbulence and structures as they are observed in the expanding solar wind in a relatively simple way. As a first application, we show how expansion affects the onset and the radial evolution of the decay of large amplitude Alfvén waves through interaction with magnetoacoustic waves, the parametric decay instability.