7 research outputs found
Radial evolution of the wave-vector anisotropy of solar wind turbulence between 0.3 and 1 AU
We present observations of the power spectral anisotropy in wave-vector space
of solar wind turbulence, and study how it evolves in interplanetary space with
increasing heliocentric distance. For this purpose we use magnetic field
measurements made by the Helios-2 spacecraft at three positions between 0.29
and 0.9 AU. To derive the power spectral density (PSD) in -space based on single-satellite measurements is a challenging task not
yet accomplished previously. Here we derive the spectrum
(, ) from the spatial
correlation function by a
transformation according to the projection-slice theorem. We find the so
constructed PSDs to be distributed in k-space mainly along a ridge that is more
inclined toward the than axis, a new result
which probably indicates preferential cascading of turbulent energy along the
direction. Furthermore, this ridge of the distribution is found
to gradually get closer to the axis, as the outer scale length of
the turbulence becomes larger while the solar wind flows further away from the
Sun. In the vicinity of the axis, there appears a minor
spectral component that probably corresponds to quasi-parallel Alfv\'enic
fluctuations. Their relative contribution to the total spectral density tends
to decrease with radial distance. These findings suggest that solar wind
turbulence undergoes an anisotropic cascade transporting most of its magnetic
energy towards larger , and that the anisotropy in the inertial
range is radially developing further at scales that are relatively far from the
ever increasing outer scale
Proton Heating in Solar Wind Compressible Turbulence with Collisions between Counter-propagating Waves
Magnetohydronamic turbulence is believed to play a crucial role in heating
the laboratorial, space, and astrophysical plasmas. However, the precise
connection between the turbulent fluctuations and the particle kinetics has not
yet been established. Here we present clear evidence of plasma turbulence
heating based on diagnosed wave features and proton velocity distributions from
solar wind measurements by the Wind spacecraft. For the first time, we can
report the simultaneous observation of counter-propagating magnetohydrodynamic
waves in the solar wind turbulence. Different from the traditional paradigm
with counter-propagating Alfv\'en waves, anti-sunward Alfv\'en waves (AWs) are
encountered by sunward slow magnetosonic waves (SMWs) in this new type of solar
wind compressible turbulence. The counter-propagating AWs and SWs correspond
respectively to the dominant and sub-dominant populations of the imbalanced
Els\"asser variables. Nonlinear interactions between the AWs and SMWs are
inferred from the non-orthogonality between the possible oscillation direction
of one wave and the possible propagation direction of the other. The associated
protons are revealed to exhibit bi-directional asymmetric beams in their
velocity distributions: sunward beams appearing in short and narrow patterns
and anti-sunward broad extended tails. It is suggested that multiple types of
wave-particle interactions, i.e., cyclotron and Landau resonances with AWs and
SMWs at kinetic scales, are taking place to jointly heat the protons
perpendicularly and parallel