February 24, 2012, 12:00 pm - 1:00 pm
February 24, 12:00 pm - 1:00 pm
Hybrid modeling of the solar wind - ion heating and acceleration by Alfven-cyclotron waves
Dr. Yana Maneva, NASA GSFC/Catholic University of America
In situ measurements by Helios, Ulysses and Wind spacecrafts unambiguously show that minor ions in the fast solar wind are hotter and flow faster than their more abundant lighter companions. Thus the protons are faster and hotter than the electrons, the He++ ions are faster and hotter than the protons, etc. The same properties are obtained by remote sensing of the solar corona, starting with the first SoHO/UVCS observations. In addition both spectroscopic and in situ data shows that all the ion species significantly deviate from thermal equillibrium and have highly anisotropic velocity distribution functions. In order to investigate the preferential anisotropic heating and acceleration of minor ions as observed in the solar corona and the solar wind we have performed 1D and 2D hybrid simulations, where the electrons are treated as an isothermal charge-neutralizing massless fluid and the ions are treated fully kinetically within a particle-in-cell approach. As an energy source for the ion heating and acceleration we consider nonlinear Alfven-cyclotron waves. Resonant damping of ion-cyclotron waves has long been proposed as a plausible scenario for preferential heating of ions in the solar corona and the solar wind, but it is only recently that those waves were actually observed by STEREO close to the Earth. As an initial setup we include either non-resonant Alfven-cyclotron waves bellow the gyrofrequency for the alpha particles or an mhd type broad-band spectra of ion-cyclotron waves slightly above the proton-cyclotron frequency. Both results in a1D homogeneous plasma show that the minor ions are differentially accelerated and preferentially heated up to a more than charge-to-mass temperature ratios, as observed in the fast solar wind regions. The ion species are heated either by the initial wave-spectra or by the corresponding daughter waves born via parametric instabilities of the initial nonlinear mother wave. For all investigated cases the proton velocity distribution function acquires persistent long-lived beams.Occasional alpha beams are formed as well, although with much lower densities relative to the core population. We have investigated the solar wind expansion, which plays an important role as the solar wind travels away form the Sun. We consider the effects of expansion on the wave heating, relative drifts, and initial temperature anisotropy, and find that expansion leads to perpendicular cooling that partially counteracts the effects of heating, thus affecting the heat input required to account for the observed solar wind plasma properties. Preliminary results from 2D hybrid simulations are also presented to investigate the effect of the additional spatial dimension, allowing for oblique wave propagation and additional degree of freedom for the wave-particle interactions.