682 research outputs found
Test-particle acceleration in a hierarchical three-dimensional turbulence model
The acceleration of charged particles is relevant to the solar corona over a
broad range of scales and energies. High-energy particles are usually detected
in concomitance with large energy release events like solar eruptions and
flares, nevertheless acceleration can occur at smaller scales, characterized by
dynamical activity near current sheets. To gain insight into the complex
scenario of coronal charged particle acceleration, we investigate the
properties of acceleration with a test-particle approach using
three-dimensional magnetohydrodynamic (MHD) models. These are obtained from
direct solutions of the reduced MHD equations, well suited for a plasma
embedded in a strong axial magnetic field, relevant to the inner heliosphere. A
multi-box, multi-scale technique is used to solve the equations of motion for
protons. This method allows us to resolve an extended range of scales present
in the system, namely from the ion inertial scale of the order of a meter up to
macroscopic scales of the order of km (th of the outer scale of
the system). This new technique is useful to identify the mechanisms that,
acting at different scales, are responsible for acceleration to high energies
of a small fraction of the particles in the coronal plasma. We report results
that describe acceleration at different stages over a broad range of time,
length and energy scales.Comment: 12 pages, 8 figures, ApJ (in press
Kinetic dissipation and anisotropic heating in a turbulent collisionless plasma
The kinetic evolution of the Orszag-Tang vortex is studied using
collisionless hybrid simulations. In the magnetohydrodynamic regime this vortex
leads rapidly to broadband turbulence. Significant differences from MHD arise
at small scales, where the fluid scale energy dissipates into heat almost
exclusively through the magnetic field because the protons are decoupled from
the magnetic field. Although cyclotron resonance is absent, the protons heat
preferentially in the plane perpendicular to the mean field, as in the corona
and solar wind. Effective transport coefficients are calculated.Comment: 4 pages, 4 figures. Submitted to PR
Solar wind turbulent heating by interstellar pickup protons: 2-component model
We apply a recently developed 2-component phenomenology to the turbulent heating of the core solar wind protons as seen at the Voyager 2 spacecraft. We find that this new description improves the model predictions of core temperature and correlation scale of the fluctuations, yielding excellent agreement with the Voyager measurements. However, the model fluctuation intensity substantially exceeds the Voyager measurements in the outer heliosphere, indicating that this picture needs further refinement
Magnetic field reversals and long-time memory in conducting flows
Employing a simple ideal magnetohydrodynamic model in spherical geometry,we
show that the presence of either rotation or finite magnetic helicity is
sufficient to induce dynamical reversals of the magnetic dipole moment. The
statistical character of the model is similar to that of terrestrial magnetic
field reversals, with the similarity being stronger when rotation is
present.The connection between long time correlations, noise, and
statistics of reversals is supported, consistent with earlier suggestions.Comment: accepted in Physical Review
Empirical Constraints on Proton and Electron Heating in the Fast Solar Wind
We analyze measured proton and electron temperatures in the high-speed solar
wind in order to calculate the separate rates of heat deposition for protons
and electrons. When comparing with other regions of the heliosphere, the fast
solar wind has the lowest density and the least frequent Coulomb collisions.
This makes the fast wind an optimal testing ground for studies of collisionless
kinetic processes associated with the dissipation of plasma turbulence. Data
from the Helios and Ulysses plasma instruments were collected to determine mean
radial trends in the temperatures and the electron heat conduction flux between
0.29 and 5.4 AU. The derived heating rates apply specifically for these mean
plasma properties and not for the full range of measured values around the
mean. We found that the protons receive about 60% of the total plasma heating
in the inner heliosphere, and that this fraction increases to approximately 80%
by the orbit of Jupiter. A major factor affecting the uncertainty in this
fraction is the uncertainty in the measured radial gradient of the electron
heat conduction flux. The empirically derived partitioning of heat between
protons and electrons is in rough agreement with theoretical predictions from a
model of linear Vlasov wave damping. For a modeled power spectrum consisting
only of Alfvenic fluctuations, the best agreement was found for a distribution
of wavenumber vectors that evolves toward isotropy as distance increases.Comment: 11 pages (emulateapj style), 5 figures, ApJ, in pres
A turbulence-driven model for heating and acceleration of the fast wind in coronal holes
A model is presented for generation of fast solar wind in coronal holes,
relying on heating that is dominated by turbulent dissipation of MHD
fluctuations transported upwards in the solar atmosphere. Scale-separated
transport equations include large-scale fields, transverse Alfvenic
fluctuations, and a small compressive dissipation due to parallel shears near
the transition region. The model accounts for proton temperature, density, wind
speed, and fluctuation amplitude as observed in remote sensing and in situ
satellite data.Comment: accepted for publication in ApJ
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