14 research outputs found
Can the Solar Wind be Driven by Magnetic Reconnection in the Sun's Magnetic Carpet?
The physical processes that heat the solar corona and accelerate the solar
wind remain unknown after many years of study. Some have suggested that the
wind is driven by waves and turbulence in open magnetic flux tubes, and others
have suggested that plasma is injected into the open tubes by magnetic
reconnection with closed loops. In order to test the latter idea, we developed
Monte Carlo simulations of the photospheric "magnetic carpet" and extrapolated
the time-varying coronal field. These models were constructed for a range of
different magnetic flux imbalance ratios. Completely balanced models represent
quiet regions on the Sun and source regions of slow solar wind streams. Highly
imbalanced models represent coronal holes and source regions of fast wind
streams. The models agree with observed emergence rates, surface flux
densities, and number distributions of magnetic elements. Despite having no
imposed supergranular motions, a realistic network of magnetic "funnels"
appeared spontaneously. We computed the rate at which closed field lines open
up (i.e., recycling times for open flux), and we estimated the energy flux
released in reconnection events involving the opening up of closed flux tubes.
For quiet regions and mixed-polarity coronal holes, these energy fluxes were
found to be much lower than required to accelerate the solar wind. For the most
imbalanced coronal holes, the energy fluxes may be large enough to power the
solar wind, but the recycling times are far longer than the time it takes the
solar wind to accelerate into the low corona. Thus, it is unlikely that either
the slow or fast solar wind is driven by reconnection and loop-opening
processes in the magnetic carpet.Comment: 25 pages (emulateapj style), 13 figures, ApJ, in pres
The Effect of Proton Temperature Anisotropy on the Solar Minimum Corona and Wind
A semi-empirical, axisymmetric model of the solar minimum corona is developed
by solving the equations for conservation of mass and momentum with prescribed
anisotropic temperature distributions. In the high-latitude regions, the proton
temperature anisotropy is strong and the associated mirror force plays an
important role in driving the fast solar wind; the critical point where the
outflow velocity equals the parallel sound speed is reached already at 1.5 Rsun
from Sun center. The slow wind arises from a region with open field lines and
weak anisotropy surrounding the equatorial streamer belt. The model parameters
were chosen to reproduce the observed latitudinal extent of the equatorial
streamer in the corona and at large distance from the Sun. We find that the
magnetic cusp of the closed-field streamer core lies at about 1.95 Rsun. The
transition from fast to slow wind is due to a decrease in temperature
anisotropy combined with the non-monotonic behavior of the non-radial expansion
factor in flow tubes that pass near the streamer cusp. In the slow wind, the
plasma beta is of order unity and the critical point lies at about 5 Rsun, well
beyond the magnetic cusp. The predicted outflow velocities are consistent with
OVI Doppler dimming measurements from UVCS/SOHO. We also find good agreement
with polarized brightness (pB) measurements from LASCO/SOHO and HI Ly-alpha
images from UVCS/SOHO.Comment: 36 pages, 13 figures. AAS LaTeX Macros v5.0. To appear in The
Astrophysical Journal, Vol. 598, No. 2, Issue December 1, 200
Proton, Electron, and Ion Heating in the Fast Solar Wind from Nonlinear Coupling Between Alfvenic and Fast-Mode Turbulence
In the parts of the solar corona and solar wind that experience the fewest
Coulomb collisions, the component proton, electron, and heavy ion populations
are not in thermal equilibrium with one another. Observed differences in
temperatures, outflow speeds, and velocity distribution anisotropies are useful
constraints on proposed explanations for how the plasma is heated and
accelerated. This paper presents new predictions of the rates of collisionless
heating for each particle species, in which the energy input is assumed to come
from magnetohydrodynamic (MHD) turbulence. We first created an empirical
description of the radial evolution of Alfven, fast-mode, and slow-mode MHD
waves. This model provides the total wave power in each mode as a function of
distance along an expanding flux tube in the high-speed solar wind. Next we
solved a set of cascade advection-diffusion equations that give the time-steady
wavenumber spectra at each distance. An approximate term for nonlinear coupling
between the Alfven and fast-mode fluctuations is included. For reasonable
choices of the parameters, our model contains enough energy transfer from the
fast mode to the Alfven mode to excite the high-frequency ion cyclotron
resonance. This resonance is efficient at heating protons and other ions in the
direction perpendicular to the background magnetic field, and our model
predicts heating rates for these species that agree well with both
spectroscopic and in situ measurements. Nonetheless, the high-frequency waves
comprise only a small part of the total Alfvenic fluctuation spectrum, which
remains highly two-dimensional as is observed in interplanetary space.Comment: Accepted for publication in the Astrophysical Journal. 30 pages
(emulateapj style), 18 figure
Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence
We present a series of models for the plasma properties along open magnetic
flux tubes rooted in solar coronal holes, streamers, and active regions. These
models represent the first self-consistent solutions that combine: (1)
chromospheric heating driven by an empirically guided acoustic wave spectrum,
(2) coronal heating from Alfven waves that have been partially reflected, then
damped by anisotropic turbulent cascade, and (3) solar wind acceleration from
gradients of gas pressure, acoustic wave pressure, and Alfven wave pressure.
The only input parameters are the photospheric lower boundary conditions for
the waves and the radial dependence of the background magnetic field along the
flux tube. For a single choice for the photospheric wave properties, our models
produce a realistic range of slow and fast solar wind conditions by varying
only the coronal magnetic field. Specifically, a 2D model of coronal holes and
streamers at solar minimum reproduces the latitudinal bifurcation of slow and
fast streams seen by Ulysses. The radial gradient of the Alfven speed affects
where the waves are reflected and damped, and thus whether energy is deposited
below or above the Parker critical point. As predicted by earlier studies, a
larger coronal ``expansion factor'' gives rise to a slower and denser wind,
higher temperature at the coronal base, less intense Alfven waves at 1 AU, and
correlative trends for commonly measured ratios of ion charge states and
FIP-sensitive abundances that are in general agreement with observations. These
models offer supporting evidence for the idea that coronal heating and solar
wind acceleration (in open magnetic flux tubes) can occur as a result of wave
dissipation and turbulent cascade. (abridged abstract)Comment: 32 pages (emulateapj style), 18 figures, ApJ Supplement, in press (v.
171, August 2007