82 research outputs found
Observations of hydromagnetic turbulence in the solar wind
MHD turbulence is studied by analyzing magnetic field and plasma observations from Helios-1 and -2 at minimum solar activity. The steady conditions in the plasma flows and the magnetic field sector structure in 1975/1976 facilitate an investigation of the radial evolution of the turbulence from 0.29 to 1AU. In high speed streams the fluctuations in the solar wind velocity v and the magnetic field b are highly correlated (the correction coefficient almost being one), which indicates that the turbulence is mainly Alfvenic in high speed plasma. While some general fluctuation properties remain essentially unchanged from 0.29 to 1AU, power spectral analysis reveals a different frequency composition of the Alfvenic turbulence at different heliocentric distances. At 0.3AU much more 'high' frequency fluctuations contribute to the total power in the magnetic field and velocity fluctuations than at 1AU. The contributions of field magnitude fluctuations are found to be distance and frequency dependent. Magnetic field spectra with an extended frequency range up to 470Hz show certain frequency bands, where the steepness of the spectra is independent of the helicocentric distance
On the origin of the 1/f spectrum in the solar wind magnetic field
We present a mechanism for the formation of the low frequency 1/f magnetic
spectrum based on numerical solutions of a shell reduced-MHD model of the
turbulent dynamics inside the sub-Alfv\'enic solar wind. We assign reasonably
realistic profiles to the wind speed and the density along the radial
direction, and a radial magnetic field. Alfv\'en waves of short periodicity
(600 s) are injected at the base of the chromosphere, penetrate into the corona
and are partially reflected, thus triggering a turbulent cascade. The cascade
is strong for the reflected wave while it is weak for the outward propagating
waves. Reflection at the transition region recycles the strong turbulent
spectrum into the outward weak spectrum, which is advected beyond the
Alfv\'enic critical point without substantial evolution. There, the magnetic
field has a perpendicular power-law spectrum with slope close to the Kolmogorov
-5/3. The parallel spectrum is inherited from the frequency spectrum of large
(perpendicular) eddies. The shape is a double power-law with slopes of -1 and
-2 at low and high frequencies respectively, the position of the break
depending on the injected spectrum. We suggest that the double power-law
spectrum measured by Helios at 0.3 AU, where the average magnetic field is not
aligned with the radial (contrary to our assumptions) results from the
combination of such different spectral slopes. At low frequency the parallel
spectrum dominates with its characteristic 1/f shape, while at higher
frequencies its steep spectral slope (-2) is masked by the more energetic
perpendicular spectrum (slope -5/3).Comment: 5 pages, 4 figures, accepted for publication in ApJL, V2: typo
corrected in eq.1, color figure
Mechanical effect of van der Waals interactions observed in real time in an ultracold Rydberg gas
We present time-resolved spectroscopic measurements of Rydberg-Rydberg
interactions in an ultracold gas, revealing the pair dynamics induced by
long-range van der Waals interactions between the atoms. By detuning the
excitation laser, a specific pair distribution is prepared. Penning ionization
on a microsecond timescale serves as a probe for the pair dynamics under the
influence of the attractive long-range forces. Comparison with a Monte Carlo
model not only explains all spectroscopic features but also gives quantitative
information about the interaction potentials. The results imply that the
interaction-induced ionization rate can be influenced by the excitation laser.
Surprisingly, interaction-induced ionization is also observed for Rydberg
states with purely repulsive interactions
Autoionization of an ultracold Rydberg gas through resonant dipole coupling
We investigate a possible mechanism for the autoionization of ultracold
Rydberg gases, based on the resonant coupling of Rydberg pair states to the
ionization continuum. Unlike an atomic collision where the wave functions begin
to overlap, the mechanism considered here involves only the long-range dipole
interaction and is in principle possible in a static system. It is related to
the process of intermolecular Coulombic decay (ICD). In addition, we include
the interaction-induced motion of the atoms and the effect of multi-particle
systems in this work. We find that the probability for this ionization
mechanism can be increased in many-particle systems featuring attractive or
repulsive van der Waals interactions. However, the rates for ionization through
resonant dipole coupling are very low. It is thus unlikely that this process
contributes to the autoionization of Rydberg gases in the form presented here,
but it may still act as a trigger for secondary ionization processes. As our
picture involves only binary interactions, it remains to be investigated if
collective effects of an ensemble of atoms can significantly influence the
ionization probability. Nevertheless our calculations may serve as a starting
point for the investigation of more complex systems, such as the coupling of
many pair states proposed in [Tanner et al., PRL 100, 043002 (2008)]
Scaling law of the plasma turbulence with non conservative fluxes
It is shown that in the presence of anisotropic kinetic dissipation existence
of scale invariant power law spectrum of plasma turbulence is possible.
Obtained scale invariant spectrum is not associated with the constant flux of
any physical quantity. Application of the model to the high frequency part of
the solar wind turbulence is discussed.Comment: Phys Rev E, accepte
Cascade and Damping of Alfv\'{e}n-Cyclotron Fluctuations: Application to Solar Wind Turbulence Spectrum
With the diffusion approximation, we study the cascade and damping of
Alfv\'{e}n-cyclotron fluctuations in solar plasmas numerically. Motivated by
wave-wave couplings and nonlinear effects, we test several forms of the
diffusion tensor. For a general locally anisotropic and inhomogeneous diffusion
tensor in the wave vector space, the turbulence spectrum in the inertial range
can be fitted with power-laws with the power-law index varying with the wave
propagation direction. For several locally isotropic but inhomogeneous
diffusion coefficients, the steady-state turbulence spectra are nearly
isotropic in the absence of damping and can be fitted by a single power-law
function. However, the energy flux is strongly polarized due to the
inhomogeneity that leads to an anisotropic cascade. Including the anisotropic
thermal damping, the turbulence spectrum cuts off at the wave numbers, where
the damping rates become comparable to the cascade rates. The combined
anisotropic effects of cascade and damping make this cutoff wave number
dependent on the wave propagation direction, and the propagation direction
integrated turbulence spectrum resembles a broken power-law, which cuts off at
the maximum of the cutoff wave numbers or the He cyclotron frequency.
Taking into account the Doppler effects, the model can naturally reproduce the
broken power-law wave spectra observed in the solar wind and predicts that a
higher break frequency is aways accompanied with a greater spectral index
change that may be caused by the increase of the Alfv\'{e}n Mach number, the
reciprocal of the plasma beta, and/or the angle between the solar wind velocity
and the mean magnetic field. These predictions can be tested by future
observations
Spectral features of solar wind turbulent plasma
Spectral properties of a fully compressible solar wind Hall
Magnetohydrodynamic plasma are investigated by means of time dependent three
dimensional Hall MHD simulations. Our simulations, in agreement with spacecraft
data, identify a spectral break in turbulence spectra at characteristic
length-scales associated with electromagnetic fluctuations that are smaller
than the ion gyroradius. In this regime, our 3D simulations show that turbulent
spectral cascades in the presence of a mean magnetic field follow an
omnidirectional anisotropic inertial range spectrum close to . The
onset of the spectral break in our simulations can be ascribed to the presence
of nonlinear Hall interactions that modify the spectral cascades. Our
simulations further show that the underlying charachteristic turbulent
fluctuations are spectrally anisotropic, the extent of which depends critically
on the local wavenumber. The fluctuations associated with length scales smaller
than the ion gyroradius are highly compressible and tend to exhibit a near
equipartition in the velocity and magnetic fields. Finally, we find that the
orientation of velocity and magnetic field fluctuations critically determine
the character of nonlinear interactions that predominantly govern a Hall MHD
plasma, like the solar wind.Comment: This paper is accepted for publication in Monthly Notices of the
Royal Astronomical Society Main Journa
A Model of Turbulence in Magnetized Plasmas: Implications for the Dissipation Range in the Solar Wind
This paper studies the turbulent cascade of magnetic energy in weakly
collisional magnetized plasmas. A cascade model is presented, based on the
assumptions of local nonlinear energy transfer in wavenumber space, critical
balance between linear propagation and nonlinear interaction times, and the
applicability of linear dissipation rates for the nonlinearly turbulent plasma.
The model follows the nonlinear cascade of energy from the driving scale in the
MHD regime, through the transition at the ion Larmor radius into the kinetic
Alfven wave regime, in which the turbulence is dissipated by kinetic processes.
The turbulent fluctuations remain at frequencies below the ion cyclotron
frequency due to the strong anisotropy of the turbulent fluctuations,
k_parallel << k_perp (implied by critical balance). In this limit, the
turbulence is optimally described by gyrokinetics; it is shown that the
gyrokinetic approximation is well satisfied for typical slow solar wind
parameters. Wave phase velocity measurements are consistent with a kinetic
Alfven wave cascade and not the onset of ion cyclotron damping. The conditions
under which the gyrokinetic cascade reaches the ion cyclotron frequency are
established. Cascade model solutions imply that collisionless damping provides
a natural explanation for the observed range of spectral indices in the
dissipation range of the solar wind. The dissipation range spectrum is
predicted to be an exponential fall off; the power-law behavior apparent in
observations may be an artifact of limited instrumental sensitivity. The
cascade model is motivated by a programme of gyrokinetic simulations of
turbulence and particle heating in the solar wind.Comment: 29 pages, 14 figure
Radial evolution of solar wind intermittency in the inner heliosphere
We analyzed intermittency in the solar wind, as observed on the ecliptic
plane, looking at magnetic field and velocity fluctuations between 0.3 and 1
AU, for both fast and slow wind and for compressive and directional
fluctuations. Our analysis focused on the property that probability
distribution functions of a fluctuating field affected by intermittency become
more and more peaked at smaller and smaller scales. Since the peakedness of a
distribution is measured by its flatness factor we studied the behavior of this
parameter for different scales to estimate the degree of intermittency of our
time series. We confirmed that both magnetic field and velocity fluctuations
are rather intermittent and that compressive magnetic fluctuations are
generally more intermittent than the corresponding velocity fluctuations. In
addition, we observed that compressive fluctuations are always more
intermittent than directional fluctuations and that while slow wind
intermittency does not depend on the radial distance from the sun, fast wind
intermittency of both magnetic field and velocity fluctuations clearly
increases with the heliocentric distance....Comment: 17 pages, 7 figure
No evidence for the localized heating of solar wind protons at intense velocity shear zones
Using measurements from the Wind spacecraft at 1âAU, the heating of protons in the solar wind at locations of intense velocity shear is examined. The 4321 sites of intense shear in fast coronal hole origin plasma are analyzed. The proton temperature, the proton specific entropy, and the proton number density at the locations of the shears are compared with the same quantities in the plasmas adjacent to the shears. A very slight but statistically significant enhancement of the proton temperature is seen at the sites of the shears, but it is accompanied by a larger enhancement of the proton number density at the sites of the shears. Consequently, there is no enhancement of the proton specific entropy at the shear sites, indicating no production of entropy; hence, no evidence for plasma heating is found at the sites of the velocity shears. Since the shearing velocities have appreciable Mach numbers, the authors suggest that there can be a slight adiabatic compression of the plasma at the shear zones. Key Points No proton heating is observed at the sites of intense velocity shear Temperatureâdensity signatures are consistent with adiabatic compressions The compressions could be associated with the large Mach numbers of the shearsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106821/1/jgra50896.pd
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