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ABSTRACT: It has long been known that the energy in velocity and magnetic field
fluctuations in the solar wind is not in equipartition. In this paper, we
present an analysis of 5 years of Wind data at 1 AU to investigate the reason
for this. The residual energy (difference between energy in velocity and
magnetic field fluctuations) was calculated using both the standard
magnetohydrodynamic (MHD) normalization for the magnetic field and a kinetic
version, which includes temperature anisotropies and drifts between particle
species. It was found that with the kinetic normalization, the fluctuations are
closer to equipartition, with a mean normalized residual energy of sigma_r =
-0.19 and mean Alfven ratio of r_A = 0.71. The spectrum of residual energy, in
the kinetic normalization, was found to be steeper than both the velocity and
magnetic field spectra, consistent with some recent MHD turbulence predictions
and numerical simulations, having a spectral index close to -1.9. The local
properties of residual energy and cross helicity were also investigated,
showing that globally balanced intervals with small residual energy contain
local patches of larger imbalance and larger residual energy at all scales, as
expected for non-linear turbulent interactions.
04/2013;
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ABSTRACT: We use a statistically significant set of measurements to show that the
field-aligned electron heat flux $q_\parallel$ in the solar wind at 1 AU is
consistent with the Spitzer-H\"{a}rm collisional heat flux $q_{sh}$ for
temperature gradient scales larger than a few mean free paths $L_T \gtrsim 3.5
~\lambda_{fp}$. This represents about 65% of the measured data and corresponds
primarily to high $\beta$, weakly collisional plasma ('slow solar wind'). In
the more collisionless regime $\lambda_{fp}/L_T \gtrsim 0.28$, the electron
heat flux is limited to $q_\parallel/q_0 \sim 0.3$, independent of mean free
path, where $q_0$ is the 'free-streaming' value; the measured $q_\parallel$
does not achieve the full $q_0$. This constraint $q_\parallel/q_0 \sim 0.3$
might be attributed to wave-particle interactions, an interplanetary electric
potential, or inherent flux limitation. We also show a $\beta_e$ dependence to
these results that is consistent with a local radial electron temperature
profile $T_e \sim r^{-\alpha}$ that is a function of the thermal electron beta
$\alpha = \alpha(\beta_e)$ and that the $\beta$ dependence of the collisionless
regulation constraint is not obviously consistent with a whistler heat flux
instability. We discuss the results in a broader astrophysical context.
03/2013;
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ABSTRACT: New measurements using radio and plasma-wave instruments in interplanetary
space have shown that nanometer-scale dust, or nanodust, is a significant
contributor to the total mass in interplanetary space. Better measurements of
nanodust will allow us to determine where it comes from and the extent to which
it interacts with the solar wind. When one of these nanodust grains impacts a
spacecraft, it creates an expanding plasma cloud, which perturbs the
photoelectron currents. This leads to a voltage pulse between the spacecraft
body and the antenna. Nanodust has a high charge/mass ratio, and therefore can
be accelerated by the interplanetary magnetic field to speeds up to the speed
of the solar wind: significantly faster than the Keplerian orbital speeds of
heavier dust. The amplitude of the signal induced by a dust grain grows much
more strongly with speed than with mass of the dust particle. As a result,
nanodust can produce a strong signal, despite their low mass. The WAVES
instruments on the twin Solar TErrestrial RElations Observatory spacecraft have
observed interplanetary nanodust particles since shortly after their launch in
2006. After describing a new and improved analysis of the last five years of
STEREO/WAVES Low Frequency Receiver data, a statistical survey of the nanodust
characteristics, namely the rise time of the pulse voltage and the flux of
nanodust, is presented. Agreement with previous measurements and interplanetary
dust models is shown. The temporal variations of the nanodust flux are also
discussed.
03/2013;
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V. Krasnoselskikh,
M. Balikhin,
S. N. Walker,
S. Schwartz,
D. Sundkvist,
V. Lobzin,
M. Gedalin, S. D. Bale,
F. Mozer,
J. Soucek,
Y. Hobara,
H. Comisel
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ABSTRACT: The physics of collisionless shocks is a very broad topic which has been
studied for more than five decades. However, there are a number of important
issues which remain unresolved. The energy repartition amongst particle
populations in quasiperpendicular shocks is a multi-scale process related to
the spatial and temporal structure of the electromagnetic fields within the
shock layer. The most important processes take place in the close vicinity of
the major magnetic transition or ramp region. The distribution of
electromagnetic fields in this region determines the characteristics of ion
reflection and thus defines the conditions for ion heating and energy
dissipation for supercritical shocks and also the region where an important
part of electron heating takes place. All of these processes are crucially
dependent upon the characteristic spatial scales of the ramp and foot region
provided that the shock is stationary. The earliest studies of collisionless
shocks identified nonlinearity, dissipation, and dispersion as the processes
that arrest the steepening of the shock transition. Their relative role
determines the scales of electric and magnetic fields, and so control the
characteristics of processes such as of ion reflection, electron heating and
particle acceleration. The purpose of this review is to address a subset of
unresolved problems in collisionless shock physics from experimental point of
view making use multi-point observations onboard Cluster satellites. The
problems we address are determination of scales of fields and of a scale of
electron heating, identification of energy source of precursor wave train, an
estimate of the role of anomalous resistivity in energy dissipation process by
means of measuring short scale wave fields, and direct observation of
reformation process during one single shock front crossing.
03/2013;
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ABSTRACT: We motivate the importance of studying kinetic scale turbulence for
understanding the macroscopic properties of the heliosphere, such as the
heating of the solar wind. We then discuss the technique by which kinetic scale
density fluctuations can be measured using the spacecraft potential, including
a calculation of the timescale for the spacecraft potential to react to the
density changes. Finally, we compare the shape of the density spectrum at ion
scales to theoretical predictions based on a cascade model for kinetic
turbulence. We conclude that the shape of the spectrum, including the ion scale
flattening, can be captured by the sum of passive density fluctuations at large
scales and kinetic Alfven wave turbulence at small scales.
09/2012;
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ABSTRACT: We present a measurement of the spectral index of density fluctuations between ion and electron scales in solar wind turbulence using the EFI instrument on the ARTEMIS spacecraft. The mean spectral index at 1 AU was found to be -2.75±0.06, steeper than predictions for pure whistler or kinetic Alfvén wave turbulence but consistent with previous magnetic field measurements. The steep spectra are also consistent with expectations of increased intermittency or damping of some of the turbulent energy over this range of scales. Neither the spectral index nor the flattening of the density spectra before ion scales were found to depend on the proximity to the pressure anisotropy instability thresholds, suggesting that they are features inherent to the turbulent cascade.
Physical Review Letters 07/2012; 109(3):035001. · 7.37 Impact Factor
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ABSTRACT: Kinetic plasma theory is used to generate synthetic spacecraft data to
analyze and interpret the compressible fluctuations in the inertial range of
solar wind turbulence. The kinetic counterparts of the three familiar linear
MHD wave modes---the fast, Alfven, and slow waves---are identified and the
properties of the density-parallel magnetic field correlation for these kinetic
wave modes is presented. The construction of synthetic spacecraft data, based
on the quasi-linear premise---that some characteristics of magnetized plasma
turbulence can be usefully modeled as a collection of randomly phased, linear
wave modes---is described in detail. Theoretical predictions of the
density-parallel magnetic field correlation based on MHD and Vlasov-Maxwell
linear eigenfunctions are presented and compared to the observational
determination of this correlation based on 10 years of Wind spacecraft data. It
is demonstrated that MHD theory is inadequate to describe the compressible
turbulent fluctuations and that the observed density-parallel magnetic field
correlation is consistent with a statistically negligible kinetic fast wave
energy contribution for the large sample used in this study. A model of the
solar wind inertial range fluctuations is proposed comprised of a mixture of a
critically balanced distribution of incompressible Alfvenic fluctuations and a
critically balanced or more anisotropic than critical balance distribution of
compressible slow wave fluctuations. These results imply that there is little
or no transfer of large scale turbulent energy through the inertial range down
to whistler waves at small scales.
06/2012;
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ABSTRACT: The twin STEREO and the Wind spacecraft make remote multipoint measurements of interplanetary radio sources of solar origin from widely separated vantage
points. One year after launch, the angular separation between the STEREO spacecraft reached 45°, which was ideal for locating
solar type III radio sources in the heliosphere by three-spacecraft triangulation measurements from STEREO and Wind. These triangulated source locations enable intrinsic properties of the radio source, such as its beaming characteristics,
to be deduced. We present the first three-point measurements of the beaming characteristics for two solar typeIII radio bursts
that were simultaneously observed by the three spacecraft in December of 2007 and in January of 2008. These analyses suggest
that individual typeIII bursts exhibit a wide beaming pattern that is approximately beamed along the direction tangent to
the Parker spiral magnetic field line at the source location.
Solar Physics 04/2012; 259(1):255-276. · 2.78 Impact Factor
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O. C. St. Cyr,
M. L. Kaiser,
N. Meyer-Vernet,
R. A. Howard,
R. A. Harrison, S. D. Bale,
W. T. Thompson,
K. Goetz,
M. Maksimovic,
J.-L. Bougeret,
D. Wang,
S. Crothers
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ABSTRACT: Early in the STEREO mission observers noted that the white-light instruments of the SECCHI suite were detecting significantly
more spacecraft-related “debris” than any previously flown coronagraphic instruments. Comparison of SECCHI “debris storms”
with S/WAVES indicates that almost all are coincident with the most intense transient emissions observed by the radio and
plasma waves instrument. We believe the debris is endogenous (i.e., from the spacecraft thermal blanketing), and the storms appear to be caused by impacts of large interplanetary dust grains
that are detected by S/WAVES. Here we report the observations, compare them to interplanetary dust distributions, and document
a reminder for future spacebased coronagraphic instrument builders.
Solar Physics 04/2012; 256(1):475-488. · 2.78 Impact Factor
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S. D. Bale,
M. A. Balikhin,
T. S. Horbury,
V. V. Krasnoselskikh,
H. Kucharek,
E. Möbius,
S. N. Walker,
A. Balogh,
D. Burgess,
B. Lembège,
E. A. Lucek,
M. Scholer,
S. J,
M. F. Thomsen
Space Science Reviews 04/2012; 118(1):161-203. · 3.61 Impact Factor
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D. Burgess,
E. A. Lucek,
M. Scholer, S. D. Bale,
M. A. Balikhin,
A. Balogh,
T. S. Horbury,
V. V. Krasnoselskikh,
H. Kucharek,
B. Lembège,
E. Möbius,
S. J. Schwartz,
M. F. Thomsen,
S. N. Walker
Space Science Reviews 04/2012; 118(1):205-222. · 3.61 Impact Factor
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ABSTRACT: The nature of small-scale turbulent fluctuations in the solar wind is investigated using a comparison of Cluster magnetic and electric field measurements to predictions arising from models consisting of either kinetic Alfvén waves or whistler waves. The electric and magnetic field properties of these waves from linear theory are used to construct spacecraft-frame frequency spectra of (|δE|/|δB|) s/c and (|δB |/|δB|) s/c , allowing for a direct comparison to spacecraft data. The measured properties of the small-scale turbulent fluctuations, found to be inconsistent with the whistler wave model, agree well with the prediction of a spectrum of kinetic Alfvén waves with nearly perpendicular wavevectors.
The Astrophysical Journal Letters 01/2012; 745(1):L9. · 5.53 Impact Factor
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ABSTRACT: Temperature anisotropy in the solar wind results from a combination of mechanisms of anisotropic heating (e.g., cyclotron-resonant heating and dissipation of kinetic Alfvén waves) and cooling (e.g., Chew-Goldberger-Low double-adiabatic expansion). In contrast, anisotropy-driven instabilities such as the cyclotron, mirror, and firehose instabilities limit the allowable departure of the plasma from isotropy. This study used data from the Faraday cups on the Wind spacecraft to examine scalar temperature and temperature components of protons. Plasma unstable to the mirror or firehose instability was found to be about 3-4 times hotter than stable plasma. Since anisotropy-driven instabilities are not understood to heat the plasma, these results suggest that heating processes are more effective than cooling processes at creating and maintaining proton temperature anisotropy in the solar wind.
Physical Review Letters 11/2011; 107(20):201101. · 7.37 Impact Factor
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ABSTRACT: We present a measurement of the scale-dependent, three-dimensional structure
of the magnetic field fluctuations in inertial range solar wind turbulence with
respect to a local, physically motivated coordinate system. The Alfvenic
fluctuations are three-dimensionally anisotropic, with the sense of this
anisotropy varying from large to small scales. At the outer scale, the magnetic
field correlations are longest in the local fluctuation direction, consistent
with Alfven waves. At the proton gyroscale, they are longest along the local
mean field direction and shortest in the direction perpendicular to the local
mean field and the local field fluctuation. The compressive fluctuations are
highly elongated along the local mean field direction, although axially
symmetric perpendicular to it. Their large anisotropy may explain why they are
not heavily damped in the solar wind.
09/2011;
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ABSTRACT: The propagation of Langmuir waves in plasmas is known to be sensitive to
density fluctuations. Such fluctuations may lead to the coexistence of wave
pairs that have almost opposite wave-numbers in the vicinity of their
reflection points. Using high frequency electric field measurements from the
WIND satellite, we determine for the first time the wavelength of intense
Langmuir wave packets that are generated upstream of the Earth's electron
foreshock by energetic electron beams. Surprisingly, the wavelength is found to
be 2 to 3 times larger than the value expected from standard theory. These
values are consistent with the presence of strong inhomogeneities in the solar
wind plasma rather than with the effect of weak beam instabilities.
07/2011;
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ABSTRACT: We use a large, statistical set of measurements from the Wind spacecraft at 1
AU, and supporting synthetic spacecraft data based on kinetic plasma theory, to
show that the compressible component of inertial range solar wind turbulence is
primarily in the kinetic slow mode. The zero-lag cross correlation C(delta n,
delta B_parallel) between proton density fluctuations delta n and the
field-aligned (compressible) component of the magnetic field delta B_parallel
is negative and close to -1. The typical dependence of C(delta n,delta
B_parallel) on the ion plasma beta_i is consistent with a spectrum of
compressible wave energy that is almost entirely in the kinetic slow mode. This
has important implications for both the nature of the density fluctuation
spectrum and for the cascade of kinetic turbulence to short wavelengths,
favoring evolution to the kinetic Alfven wave mode rather than the (fast)
whistler mode.
06/2011;
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ABSTRACT: The propagation of Langmuir waves in plasmas is known to be sensitive to density fluctuations. Such fluctuations may lead to the coexistence of wave pairs that have almost opposite wave-numbers in the vicinity of their reflection points. Using high frequency electric field measurements from the WIND satellite, we determine for the first time the wavelength of intense Langmuir wave packets that are generated upstream of the Earth's electron foreshock by energetic electron beams. Surprisingly, the wavelength is found to be 2 to 3 times larger than the value expected from standard theory. These values are consistent with the presence of strong inhomogeneities in the solar wind plasma rather than with the effect of weak beam instabilities.
Annales Geophysicae. 01/2011;
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ABSTRACT: We present the first survey of electric field data using the ARTEMIS spacecraft in the solar wind to study inertial range turbulence. It was found that the average perpendicular spectral index of the electric field depends on the frame of measurement. In the spacecraft frame it is –5/3, which matches the magnetic field due to the large solar wind speed in Lorentz transformation. In the mean solar wind frame, the electric field is primarily due to the perpendicular velocity fluctuations and has a spectral index slightly shallower than –3/2, which is close to the scaling of the velocity. These results are an independent confirmation of the difference in scaling between the velocity and magnetic field, which is not currently well understood. The spectral index of the compressive fluctuations was also measured and found to be close to –5/3, suggesting that they are not only passive to the velocity but may also interact nonlinearly with the magnetic field.
Astrophys. J. Lett. 01/2011; 737:L41.
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09/2010;
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ABSTRACT: A three-dimensional (3-D), self-consistent code is employed to solve for the static potential structure surrounding a spacecraft in a high photoelectron environment. The numerical solutions show that, under certain conditions, a spacecraft can take on a negative potential in spite of strong photoelectron currents. The negative potential is due to an electrostatic barrier near the surface of the spacecraft that can reflect a large fraction of the photoelectron flux back to the spacecraft. This electrostatic barrier forms if (1) the photoelectron density at the surface of the spacecraft greatly exceeds the ambient plasma density, (2) the spacecraft size is significantly larger than local Debye length of the photoelectrons, and (3) the thermal electron energy is much larger than the characteristic energy of the escaping photoelectrons. All of these conditions are present near the Sun. The numerical solutions also show that the spacecraft's negative potential can be amplified by an ion wake. The negative potential of the ion wake prevents secondary electrons from escaping the part of spacecraft in contact with the wake. These findings may be important for future spacecraft missions that go nearer to the Sun, such as Solar Orbiter and Solar Probe Plus. Comment: 25 pages, 7 figures, accepted for publication in Physics of Plasmas
06/2010;
