45 research outputs found
Dearest
Dearest is the examination of what remains of a person, looking to the objects they cherished most while contemplating the inevitability of their certain absence. The work questions the futility of preservation in the measure of time, the failure of memories held in fragile containers, and the decay of the physical body. The materials that compose Dearest are chosen for their innate longevity and their ability to evoke remembrance
Frame Dependence of the Electric Field Spectrum of Solar Wind Turbulence
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
Dependence of solar wind power spectra on the direction of the local mean magnetic field
(Abridged) Wavelet analysis can be used to measure the power spectrum of
solar wind fluctuations along a line in any direction with respect to the local
mean magnetic field. This technique is applied to study solar wind turbulence
in high-speed streams in the ecliptic plane near solar minimum using magnetic
field measurements with a cadence of eight vectors per second. The analysis of
nine high-speed streams shows that the reduced spectrum of magnetic field
fluctuations (trace power) is approximately azimuthally symmetric about B_0 in
both the inertial range and dissipation range; in the inertial range the
spectra are characterized by a power-law exponent that changes continuously
from 1.6 \pm 0.1 in the direction perpendicular to the mean field to 2.0 \pm
0.1 in the direction parallel to the mean field. The large uncertainties
suggest that the perpendicular power-law indices 3/2 and 5/3 are both
consistent with the data. The results are similar to those found by Horbury et
al. (2008) at high heliographic latitudes.Comment: Submitted to: The Astrophysical Journa
Impact of observational uncertainties on universal scaling of MHD turbulence
Scaling exponents are the central quantitative prediction of theories of
turbulence and in-situ satellite observations of the high Reynolds number solar
wind flow have provided an extensive testbed of these. We propose a general,
instrument independent method to estimate the uncertainty of velocity field
fluctuations. We obtain the systematic shift that this uncertainty introduces
into the observed spectral exponent. This shift is essential for the correct
interpretation of observed scaling exponents. It is sufficient to explain the
contradiction between spectral features of the Elsasser fields observed in the
solar wind with both theoretical models and numerical simulations of
Magnetohydrodynamic turbulence
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
Kolmogorov versus Iroshnikov-Kraichnan spectra: Consequences for ion heating in the solar wind
Whether the phenomenology governing MHD turbulence is Kolmogorov or
Iroshnikov-Kraichnan (IK) remains an open question, theoretically as well as
observationally. The ion heating profile observed in the solar wind provides a
quantitative, if indirect, observational constraint on the relevant
phenomenology. Recently, a solar wind heating model based on Kolmogorov
spectral scaling has produced reasonably good agreement with observations,
provided the effect of turbulence generation due to pickup ions is included in
the model. Without including the pickup ion contributions, the Kolmogorov
scaling predicts a proton temperature profile that decays too rapidly beyond a
radial distance of 15 AU. In the present study, we alter the heating model by
applying an energy cascade rate based on IK scaling, and show that the model
yields higher proton temperatures, within the range of observations, with or
without the inclusion of the effect due to pickup ions. Furthermore, the
turbulence correlation length based on IK scaling seems to follow the trend of
observations better
Turbulent Density Spectrum in Solar Wind Plasma
The density fluctuation spectrum in the solar wind reveals a Kolmogorov-like
scaling with a spectral slope of -5/3 in wavenumber space. The energy transfer
process in the magnetized solar wind, characterized typically by MHD
turbulence, over extended length-scales remains an unresolved paradox of modern
turbulence theories, raising the question of how a compressible magnetofluid
exhibits a turbulent spectrum that is characteristic of an incompressible
hydrodynamic fluid. To address these questions, we have undertaken
three-dimensional time dependent numerical simulations of a compressible
magnetohydrodynamic fluid describing super-Alfv\'enic, supersonic and strongly
magnetized plasma fluid. It is shown that a Kolmogorov-like density spectrum
can develop by plasma motions that are dominated by Alfv\'enic cascades whereas
compressive modes are dissipated.Comment: Paper is to appear in Monthly Notices of the Royal Astronomical
Society Main Journa
The velocity and magnetic field fluctuations of the solar wind at 1 AU: Statistical analysis of Fourier spectra and correlations with plasma properties
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95508/1/jgra21877.pd
Scale-dependent Polarization of Solar Wind Velocity Fluctuations at the Inertial and Kinetic Scales
We study the polarization properties of the velocity fluctuations in solar wind turbulence using high-resolution data from the Spektr-R spacecraft. The ratio of perpendicular to parallel velocity fluctuations in the inertial range is smaller than the equivalent ratio for magnetic fluctuations, but gradually increases throughout this range. In the kinetic range, there is a large decrease in the ratio, similar to the magnetic fluctuations. We compare the measurements to numerical solutions for a combination of kinetic Alfvén waves and slow waves, finding that both the slow increase and sharp decrease in the ratio are consistent with a majority population of Alfvén waves and minority population of slow waves in critical balance. Furthermore, the beta-dependence of this scale-dependent ratio can be successfully captured in the model when incorporating a beta-dependent Alfvén to slow wave ratio similar to that observed in the solar wind