326 research outputs found
Anisotropy of Imbalanced Alfvenic Turbulence in Fast Solar Wind
We present the first measurement of the scale-dependent power anisotropy of
Elsasser variables in imbalanced fast solar wind turbulence. The dominant
Elsasser mode is isotropic at lower spacecraft frequencies but becomes
increasingly anisotropic at higher frequencies. The sub-dominant mode is
anisotropic throughout, but in a scale-independent way (at higher frequencies).
There are two distinct subranges exhibiting different scalings within what is
normally considered the inertial range. The low Alfven ratio and shallow
scaling of the sub-dominant Elsasser mode suggest an interpretation of the
observed discrepancy between the velocity and magnetic field scalings. The
total energy is dominated by the latter. These results do not appear to be
fully explained by any of the current theories of incompressible imbalanced MHD
turbulence.Comment: 5 pages, 2 figure
Power and spectral index anisotropy of the entire inertial range of turbulence in the fast solar wind
We measure the power and spectral index anisotropy of high speed solar wind
turbulence from scales larger than the outer scale down to the ion gyroscale,
thus covering the entire inertial range. We show that the power and spectral
indices at the outer scale of turbulence are approximately isotropic. The
turbulent cascade causes the power anisotropy at smaller scales manifested by
anisotropic scalings of the spectrum: close to k^{-5/3} across and k^{-2} along
the local magnetic field, consistent with a critically balanced Alfvenic
turbulence. By using data at different radial distances from the Sun, we show
that the width of the inertial range does not change with heliocentric distance
and explain this by calculating the radial dependence of the ratio of the outer
scale to the ion gyroscale. At the smallest scales of the inertial range, close
to the ion gyroscale, we find an enhancement of power parallel to the magnetic
field direction coincident with a decrease in the perpendicular power. This is
most likely related to energy injection by ion kinetic modes such as the
firehose instability and also marks the beginning of the dissipation range of
solar wind turbulence.Comment: 5 pages, 4 figures, 1 table, submitted to MNRAS letter
Multiscaling of galactic cosmic ray flux
Multiscaling analysis of differential flux dissipation rate of galactic
cosmic rays (Carbon nuclei) is performed in the energy ranges: 56.3-73.4
Mev/nucleon and 183.1-198.7 MeV/nucleon, using the data collected by ACE/CRIS
spacecraft instrument for 2000 year. The analysis reveals strong
(turbulence-like) intermittency of the flux dissipation rate for the short-term
intervals: 1-30 hours. It is also found that type of the intermittency can be
different in different energy ranges
Self-similar signature of the active solar corona within the inertial range of solar-wind turbulence
We quantify the scaling of magnetic energy density in the inertial range of solar-wind turbulence seen
in situ at 1 AU with respect to solar activity. At solar maximum, when the coronal magnetic field is
dynamic and topologically complex, we find self-similar scaling in the solar wind, whereas at solar
minimum, when the coronal fields are more ordered, we find multifractality. This quantifies the solar-wind
signature that is of direct coronal origin and distinguishes it from that of local MHD turbulence, with
quantitative implications for coronal heating of the solar wind
The scaling properties of dissipation in incompressible isotropic three-dimensional magnetohydrodynamic turbulence
The statistical properties of the dissipation process constrain the analysis
of large scale numerical simulations of three dimensional incompressible
magnetohydrodynamic (MHD) turbulence, such as those of Biskamp and Muller
[Phys. Plasmas 7, 4889 (2000)]. The structure functions of the turbulent flow
are expected to display statistical self-similarity, but the relatively low
Reynolds numbers attainable by direct numerical simulation, combined with the
finite size of the system, make this difficult to measure directly. However, it
is known that extended self-similarity, which constrains the ratio of scaling
exponents of structure functions of different orders, is well satisfied. This
implies the extension of physical scaling arguments beyond the inertial range
into the dissipation range. The present work focuses on the scaling properties
of the dissipation process itself. This provides an important consistency check
in that we find that the ratio of dissipation structure function exponents is
that predicted by the She and Leveque [Phys. Rev. Lett 72, 336 (1994)] theory
proposed by Biskamp and Muller. This supplies further evidence that the cascade
mechanism in three dimensional MHD turbulence is non-linear random eddy
scrambling, with the level of intermittency determined by dissipation through
the formation of current sheets.Comment: 9 pages, 6 figures. Figures embedded in text. Typos corrected in text
and references. Published in Physics of Plasmas. Abstract can be found
at:http://link.aip.org/link/?php/12/02230
Anisotropy of Solar Wind Turbulence between Ion and Electron Scales
The anisotropy of turbulence in the fast solar wind, between the ion and
electron gyroscales, is directly observed using a multispacecraft analysis
technique. Second order structure functions are calculated at different angles
to the local magnetic field, for magnetic fluctuations both perpendicular and
parallel to the mean field. In both components, the structure function value at
large angles to the field S_perp is greater than at small angles S_par: in the
perpendicular component S_perp/S_par = 5 +- 1 and in the parallel component
S_perp/S_par > 3, implying spatially anisotropic fluctuations, k_perp > k_par.
The spectral index of the perpendicular component is -2.6 at large angles and
-3 at small angles, in broad agreement with critically balanced whistler and
kinetic Alfven wave predictions. For the parallel component, however, it is
shallower than -1.9, which is considerably less steep than predicted for a
kinetic Alfven wave cascade.Comment: 4 pages, 4 figures, replaced to match published versio
Mutual information between geomagnetic indices and the solar wind as seen by WIND : implications for propagation time estimates
The determination of delay times of solar wind conditions at the sunward libration point to effects on Earth is investigated using mutual information. This measures the amount of information shared between two timeseries. We consider the mutual information content of solar wind observations, from WIND, and the geomagnetic indices. The success of five commonly used schemes for estimating interplanetary propagation times is examined. Propagation assuming a fixed plane normal at 45 degrees to the GSE x-axis (i.e. the Parker Spiral estimate) is found to give optimal mutual information. The mutual information depends on the point in space chosen as the target for the propagation estimate, and we find that it is maximized by choosing a point in the nightside rather than dayside magnetosphere. In addition, we employ recurrence plot analysis to visualize contributions to the mutual information, this suggests that it appears on timescales of hours rather than minutes
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
Magnetic reconnection as an erosion mechanism for magnetic switchbacks
Context. Magnetic switchbacks are localised polarity reversals in the radial component of the heliospheric magnetic field. Observations from Parker Solar Probe (PSP) have shown that they are a prevalent feature of the near-Sun solar wind. However, observations of switchbacks at 1 au and beyond are less frequent, suggesting that these structures evolve and potentially erode as they propagate away from the Sun. The specific mechanisms at play have not been identified thus far. /
Aims. We search for magnetic switchbacks undergoing magnetic reconnection, characterise them, and evaluate the viability of reconnection as a possible channel for their erosion. /
Methods. We analysed magnetic field and plasma data from the Magnetometer and Solar Wind Analyser instruments aboard Solar Orbiter collected between 10 August and 30 August 2021. During this period, the spacecraft was 0.6–0.7 au from the Sun. Using hodographs and Walén analysis methods, we tested for rotational discontinuities (RDs) in the magnetic field and reconnection-associated outflows at the boundaries of the identified switchback structures. /
Results. We identified three instances of reconnection occurring at the trailing edge of magnetic switchbacks, with properties that are consistent with existing models of reconnection in the solar wind. Based on these observations, we propose a scenario through which reconnection can erode a switchback and we estimated the timescales for these occurrences. For our events, the erosion timescales are much shorter than the expansion timescale. Thus, the complete erosion of all three observed switchbacks would occur well before they reach 1 au. Furthermore, we find that the spatial scale of these switchbacks would be considerably larger than is typically observed in the inner heliosphere if the onset of reconnection occurs close to the Sun. Our results suggest that the onset of reconnection must occur during transport in the solar wind in the cases we consider here. These results suggest that reconnection can contribute to the erosion of switchbacks and may explain the relative rarity of switchback observations at 1 au
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