A Survey of Velocity Distributions of Solar Wind Ions : ACE/SWICS observations

Wednesday, 17 December 2014
Lars Berger, Eckart Marsch and R F Wimmer-Schweingruber, University of Kiel, Kiel, Germany
The state of solar wind ions is generally described by a small set of state variables. In detail these are
total density (or flux), center of mass (or bulk) velocity, and temperature. These variables are obtained
from measured velocity distributions either by fitting a Maxwell-Boltzmann distribution or by calculating
the zeroth, first, and second order moments of the distribution. Obviously the first approach requires a
thermalized distribution to yield meaningful results, while the second approach is universally valid.
However, in both cases the shape of the distribution can not be reproduced by the derived parameters.
From observations of solar wind protons and alpha particles it is known that the assumption of a thermalized
distribution is not valid, at least for the majority of observations. For heavy solar wind ions most observations
are severely limited by statistics and do not even allow to distinguish whether the distribution is thermal or not.
This often insufficient characterization of the solar wind VDF severely limits the information which can be
obtained for more detailed studies, especially about microscopic kinetic physics and the associated
wave-particle interactions. These naturally yield deviations from Maxwell-Boltzmann distributions.
To address this problem we have analyzed ten years worth of data from the Solar Wind Ion Composition Spectrometer (SWICS)
and the Magnetometer (MAG) on the Advanced Composition Explorer (ACE). From our analysis we obtained reduced 1D
velocity spectra in 12 minute cadence for some 40 solar wind ions, from protons and alpha particles up to iron.
Using the magnetic field vector information we were able to study periods where the reduced 1D spectra represent
the parallel and perpendicular shape of the velocity distributions. We present our results and discuss
them in the aforementioned context.