64 research outputs found
The multi-scale nature of the solar wind
The solar wind is a magnetized plasma and as such exhibits collective plasma
behavior associated with its characteristic spatial and temporal scales. The
characteristic length scales include the size of the heliosphere, the
collisional mean free paths of all species, their inertial lengths, their
gyration radii, and their Debye lengths. The characteristic timescales include
the expansion time, the collision times, and the periods associated with
gyration, waves, and oscillations. We review the past and present research into
the multi-scale nature of the solar wind based on in-situ spacecraft
measurements and plasma theory. We emphasize that couplings of processes across
scales are important for the global dynamics and thermodynamics of the solar
wind. We describe methods to measure in-situ properties of particles and
fields. We then discuss the role of expansion effects, non-equilibrium
distribution functions, collisions, waves, turbulence, and kinetic
microinstabilities for the multi-scale plasma evolution.Comment: 155 pages, 24 figure
Nature of stochastic ion heating in the solar wind: testing the dependence on plasma beta and turbulence amplitude
The solar wind undergoes significant heating as it propagates away from the
Sun; the exact mechanisms responsible for this heating are not yet fully
understood. We present for the first time a statistical test for one of the
proposed mechanisms, stochastic ion heating. We use the amplitude of magnetic
field fluctuations near the proton gyroscale as a proxy for the ratio of
gyroscale velocity fluctuations to perpendicular (with respect to the magnetic
field) proton thermal speed, defined as . Enhanced proton
temperatures are observed when is larger than a critical value
(). This enhancement strongly depends on the proton plasma
beta (); when only the perpendicular proton
temperature increases, while for increased
parallel and perpendicular proton temperatures are both observed. For
smaller than the critical value and no
enhancement of is observed while for minor increases
in are measured. The observed change of proton temperatures
across a critical threshold for velocity fluctuations is in agreement with the
stochastic ion heating model of Chandran et al. (2010). We find that
in 76\% of the studied periods implying that
stochastic heating may operate most of the time in the solar wind at 1 AU.Comment: Accepted for publication in The Astrophysical Journal Letter
Astrophysical gyrokinetics: Turbulence in pressure-anisotropic plasmas at ion scales and beyond
We present a theoretical framework for describing electromagnetic kinetic
turbulence in a multi-species, magnetized, pressure-anisotropic plasma.
Turbulent fluctuations are assumed to be small compared to the mean field, to
be spatially anisotropic with respect to it, and to have frequencies small
compared to the ion cyclotron frequency. At scales above the ion Larmor radius,
the theory reduces to the pressure-anisotropic generalization of kinetic
reduced magnetohydrodynamics (KRMHD) formulated by Kunz et al. (2015). At
scales at and below the ion Larmor radius, three main objectives are achieved.
First, we analyse the linear response of the pressure-anisotropic gyrokinetic
system, and show it to be a generalisation of previously explored limits. The
effects of pressure anisotropy on the stability and collisionless damping of
Alfvenic and compressive fluctuations are highlighted, with attention paid to
the spectral location and width of the frequency jump that occurs as Alfven
waves transition into kinetic Alfven waves. Secondly, we derive and discuss a
general free-energy conservation law, which captures both the KRMHD free-energy
conservation at long wavelengths and dual cascades of kinetic Alfven waves and
ion entropy at sub-ion-Larmor scales. We show that non-Maxwellian features in
the distribution function change the amount of phase mixing and the efficiency
of magnetic stresses, and thus influence the partitioning of free energy
amongst the cascade channels. Thirdly, a simple model is used to show that
pressure anisotropy can cause large variations in the ion-to-electron heating
ratio due to the dissipation of Alfvenic turbulence. Our theory provides a
foundation for determining how pressure anisotropy affects the turbulent
fluctuation spectra, the differential heating of particle species, and the
ratio of parallel and perpendicular phase mixing in space and astrophysical
plasmas.Comment: 59 pages, 6 figures, accepted for publication in Journal of Plasma
Physics (original 28 Nov 2017); abstract abridge
Ion-Driven Instabilities in the Inner Heliosphere II: Classification and Multi-Dimensional Mapping
Linear theory is a well developed framework for characterizing instabilities
in weakly collisional plasmas, such as the solar wind. In the previous
instalment of this series, we analyzed ~1.5M proton and alpha particle Velocity
Distribution Functions (VDFs) observed by Helios I and II to determine the
statistical properties of the standard instability parameters such as the
growth rate, frequency, the direction of wave propagation, and the power
emitted or absorbed by each component, as well as to characterize their
behavior with respect to the distance from the Sun and collisional processing.
In this work, we use this comprehensive set of instability calculations to
train a Machine Learning algorithm consisting of three interlaced components
that: 1) predict if an interval is unstable from observed VDF parameters; 2)
predict the instability properties for a given unstable VDF; and 3) classify
the type of the unstable mode. We use these methods to map the properties in
multi-dimensional phase space to find that the parallel-propagating,
proton-core-induced Ion Cyclotron mode dominates the young solar wind, while
the oblique Fast Magnetosonic mode regulates the proton beam drift in the
collisionally old plasma
Strong Preferential Ion Heating is Limited to within the Solar Alfvén Surface
The decay of the solar wind helium-to-hydrogen temperature ratio due to Coulomb thermalization can be used to measure how far from the Sun strong preferential ion heating occurs. Previous work has shown that a zone of preferential ion heating, resulting in mass-proportional temperatures, extends about 20-40 R-circle dot from the Sun on average. Here we look at the motion of the outer boundary of this zone with time and compare it to other physically meaningful distances. We report that the boundary moves in lockstep with the Alfven point over the solar cycle, contracting and expanding with solar activity with a correlation coefficient of better than 0.95 and with an rms difference of 4.23 R-circle dot. Strong preferential ion heating is apparently predominately active below the Alfven surface. To definitively identify the underlying preferential heating mechanisms, it will be necessary to make in situ measurements of the local plasma conditions below the Alfven surface. We predict that the Parker Solar Probe (PSP) will be the first spacecraft to directly observe this heating in action, but only a couple of years after launch as activity increases, the zone expands, and PSP's perihelion drops.Wind grant [NNX14AR78G]; NASA HSR grant [NNX16AM23G]Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
A Modified Version of Taylor's Hypothesis for Solar Probe Plus Observations
The Solar Probe Plus (SPP) spacecraft will explore the near-Sun environment,
reaching heliocentric distances less than . Near Earth,
spacecraft measurements of fluctuating velocities and magnetic fields taken in
the time domain are translated into information about the spatial structure of
the solar wind via Taylor's "frozen turbulence" hypothesis. Near the perihelion
of SPP, however, the solar-wind speed is comparable to the Alfv\'en speed, and
Taylor's hypothesis in its usual form does not apply. In this paper, we show
that, under certain assumptions, a modified version of Taylor's hypothesis can
be recovered in the near-Sun region. We consider only the transverse,
non-compressive component of the fluctuations at length scales exceeding the
proton gyroradius, and we describe these fluctuations using an approximate
theoretical framework developed by Heinemann and Olbert. We show that
fluctuations propagating away from the Sun in the plasma frame obey a relation
analogous to Taylor's hypothesis when and , where is the component of the spacecraft velocity
perpendicular to the mean magnetic field and () is the
Elsasser variable corresponding to transverse, non-compressive fluctuations
propagating away from (towards) the Sun in the plasma frame. Observations and
simulations suggest that, in the near-Sun solar wind, the above inequalities
are satisfied and fluctuations account for most of the fluctuation
energy. The modified form of Taylor's hypothesis that we derive may thus make
it possible to characterize the spatial structure of the energetically dominant
component of the turbulence encountered by SPP.Comment: 5 pages, 1 figure, accepted in ApJ Lette
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