204 research outputs found
Parallel-propagating Fluctuations at Proton-kinetic Scales in the Solar Wind are Dominated by Kinetic Instabilities
We use magnetic helicity to characterise solar wind fluctuations at
proton-kinetic scales from Wind observations. For the first time, we separate
the contributions to helicity from fluctuations propagating at angles
quasi-parallel and oblique to the local mean magnetic field, . We
find that the helicity of quasi-parallel fluctuations is consistent with
Alfv\'en-ion cyclotron and fast magnetosonic-whistler modes driven by proton
temperature anisotropy instabilities and the presence of a relative drift
between -particles and protons. We also find that the helicity of
oblique fluctuations has little dependence on proton temperature anisotropy and
is consistent with fluctuations from the anisotropic turbulent cascade. Our
results show that parallel-propagating fluctuations at proton-kinetic scales in
the solar wind are dominated by proton temperature anisotropy instabilities and
not the turbulent cascade. We also provide evidence that the behaviour of
fluctuations at these scales is independent of the origin and macroscopic
properties of the solar wind.Comment: Accepted for publication in ApJL. 6 Pages, 3 figures, 1 tabl
Coherent deflection pattern and associated temperature enhancements in the near-Sun solar wind
Measurements of transverse magnetic field and velocity components from Parker
Solar Probe have revealed a coherent quasi-periodic pattern in the near-Sun
solar wind. As well as being Alfv\'enic and arc-polarised, these deflections
were characterised by a consistent orientation and an increased proton core
temperature, which was greater parallel to the magnetic field. We show that
switchbacks represent the largest deflections within this underlying structure,
which is itself consistent with the expected outflow from interchange
reconnection simulations. Additionally, the spatial scale of the deflections
was estimated to be around \,Mm on the Sun, comparable to the jetting
activity observed at coronal bright points within the base of coronal plumes.
Therefore, our results could represent the in situ signature of interchange
reconnection from coronal bright points within plumes, complementing recent
numerical and observational studies. We also found a consistent relationship
between the proton core temperature and magnetic field angle across the Parker
Solar Probe encounters and discussed how such a persistent signature could be
more indicative of an in situ mechanism creating a local increase in
temperature. In future, observations of minor ions, radio bursts and remote
sensing images could help further establish the connection between reconnection
events on the Sun and signatures in the solar wind
Recommended from our members
Co-producing and evaluating an innovative eHealth intervention for family carers of people with psychosis - the EFFIP Project (E-support for Families & Friends of Individuals affected by Psychosis)
Recommended from our members
Co-producing and evaluating an innovative eHealth intervention for family carers of people with psychosis – the EFFIP Project
Paper presented at European Academy of Nursing Science Summer Conference, 2019, 10-11 July, Lisbon, Portugal
Dependence of Solar Wind Proton Temperature on the Polarization Properties of Alfvénic Fluctuations at Ion-kinetic Scales
We use fluctuating magnetic helicity to investigate the polarization properties of Alfvénic fluctuations at ion-kinetic scales in the solar wind as a function of β p , the ratio of proton thermal pressure to magnetic pressure, and θ vB , the angle between the proton flow and local mean magnetic field, B 0. Using almost 15 yr of Wind observations, we separate the contributions to helicity from fluctuations with wavevectors, k, quasi-parallel and oblique to B 0, finding that the helicity of Alfvénic fluctuations is consistent with predictions from linear Vlasov theory. This result suggests that the nonlinear turbulent fluctuations at these scales share at least some polarization properties with Alfvén waves. We also investigate the dependence of proton temperature in the β p -θ vB plane to probe for possible signatures of turbulent dissipation, finding that it correlates with θ vB . The proton temperature parallel to B 0 is higher in the parameter space where we measure the helicity of right-handed Alfvénic fluctuations, and the temperature perpendicular to B 0 is higher where we measure left-handed fluctuations. This finding is inconsistent with the general assumption that by sampling different θ vB in the solar wind we can analyze the dependence of the turbulence distribution on θ kB , the angle between k and B 0. After ruling out both instrumental and expansion effects, we conclude that our results provide new evidence for the importance of local kinetic processes that depend on θ vB in determining proton temperature in the solar wind
Reactivity of Gold Hydrides: O2 Insertion into the Au–H Bond
Dioxygen reacts with the gold(I) hydride (IPr)AuH under insertion to give the hydroperoxide, (IPr)AuOOH, a long-postulated reaction in gold catalysis and the first demonstration of O2 activation by Au-H in a well-defined system. Subsequent condensation gave the peroxide (IPr)Au-OO-Au(IPr) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene). The reaction kinetics are reported, as well as the reactivity of Au(I) hydrides with radical scavengers
PS-FCN: A Flexible Learning Framework for Photometric Stereo
This paper addresses the problem of photometric stereo for non-Lambertian
surfaces. Existing approaches often adopt simplified reflectance models to make
the problem more tractable, but this greatly hinders their applications on
real-world objects. In this paper, we propose a deep fully convolutional
network, called PS-FCN, that takes an arbitrary number of images of a static
object captured under different light directions with a fixed camera as input,
and predicts a normal map of the object in a fast feed-forward pass. Unlike the
recently proposed learning based method, PS-FCN does not require a pre-defined
set of light directions during training and testing, and can handle multiple
images and light directions in an order-agnostic manner. Although we train
PS-FCN on synthetic data, it can generalize well on real datasets. We further
show that PS-FCN can be easily extended to handle the problem of uncalibrated
photometric stereo.Extensive experiments on public real datasets show that
PS-FCN outperforms existing approaches in calibrated photometric stereo, and
promising results are achieved in uncalibrated scenario, clearly demonstrating
its effectiveness.Comment: ECCV 2018: https://guanyingc.github.io/PS-FC
The In Situ Signature of Cyclotron Resonant Heating
The dissipation of magnetized turbulence is an important paradigm for
describing heating and energy transfer in astrophysical environments such as
the solar corona and wind; however, the specific collisionless processes behind
dissipation and heating remain relatively unconstrained by measurements. Remote
sensing observations have suggested the presence of strong temperature
anisotropy in the solar corona consistent with cyclotron resonant heating. In
the solar wind, in situ magnetic field measurements reveal the presence of
cyclotron waves, while measured ion velocity distribution functions have hinted
at the active presence of cyclotron resonance. Here, we present Parker Solar
Probe observations that connect the presence of ion-cyclotron waves directly to
signatures of resonant damping in observed proton-velocity distributions. We
show that the observed cyclotron wave population coincides with both flattening
in the phase space distribution predicted by resonant quasilinear diffusion and
steepening in the turbulent spectra at the ion-cyclotron resonant scale. In
measured velocity distribution functions where cyclotron resonant flattening is
weaker, the distributions are nearly uniformly subject to ion-cyclotron wave
damping rather than emission, indicating that the distributions can damp the
observed wave population. These results are consistent with active cyclotron
heating in the solar wind
Enhanced proton parallel temperature inside patches of switchbacks in the inner heliosphere
Context. Switchbacks are discrete angular deflections in the solar wind magnetic field that have been observed throughout the helio-sphere. Recent observations by Parker Solar Probe(PSP) have revealed the presence of patches of switchbacks on the scale of hours to days, separated by ‘quieter’ radial fields. Aims. We aim to further diagnose the origin of these patches using measurements of proton temperature anisotropy that can illuminate possible links to formation processes in the solar corona. Methods. We fit 3D bi-Maxwellian functions to the core of proton velocity distributions measured by the SPAN-Ai instrument onboard PSP to obtain the proton parallel, Tp,‖, and perpendicular, Tp,⊥, temperature. Results. We show that the presence of patches is highlighted by a transverse deflection in the flow and magnetic field away from the radial direction. These deflections are correlated with enhancements in Tp,‖, while Tp,⊥remains relatively constant. Patches sometimes exhibit small proton and electron density enhancements. Conclusions. We interpret that patches are not simply a group of switchbacks, but rather switchbacks are embedded within a larger-scale structure identified by enhanced Tp,‖that is distinct from the surrounding solar wind. We suggest that these observations are consistent with formation by reconnection-associated mechanisms in the corona
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