1,087 research outputs found
Multi-strand coronal loop model and filter-ratio analysis
We model a coronal loop as a bundle of seven separate strands or filaments.
Each of the loop strands used in this model can independently be heated (near
their left footpoints) by Alfv\'en/ion-cyclotron waves via wave-particle
interactions. The Alfv\'en waves are assumed to penetrate the strands from
their footpoints, at which we consider different wave energy inputs. As a
result, the loop strands can have different heating profiles, and the
differential heating can lead to a varying cross-field temperature in the total
coronal loop. The simulation of TRACE observations by means of this loop model
implies two uniform temperatures along the loop length, one inferred from the
171:195 filter ratio and the other from the 171:284 ratio. The reproduced flat
temperature profiles are consistent with those inferred from the observed EUV
coronal loops. According to our model, the flat temperature profile is a
consequence of the coronal loop consisting of filaments, which have different
temperatures but almost similar emission measures in the cross-field direction.
Furthermore, when we assume certain errors in the simulated loop emissions
(e.g., due to photometric uncertainties in the TRACE filters) and use the
triple-filter analysis, our simulated loop conditions become consistent with
those of an isothermal plasma. This implies that the use of TRACE/EIT triple
filters for observation of a warm coronal loop may not help in determining
whether the cross-field isothermal assumption is satisfied or not
Multi-strand coronal loop model and filter-ratio analysis
We model a coronal loop as a bundle of seven separate strands or filaments.
Each of the loop strands used in this model can independently be heated (near
their left footpoints) by Alfv\'en/ion-cyclotron waves via wave-particle
interactions. The Alfv\'en waves are assumed to penetrate the strands from
their footpoints, at which we consider different wave energy inputs. As a
result, the loop strands can have different heating profiles, and the
differential heating can lead to a varying cross-field temperature in the total
coronal loop. The simulation of TRACE observations by means of this loop model
implies two uniform temperatures along the loop length, one inferred from the
171:195 filter ratio and the other from the 171:284 ratio. The reproduced flat
temperature profiles are consistent with those inferred from the observed EUV
coronal loops. According to our model, the flat temperature profile is a
consequence of the coronal loop consisting of filaments, which have different
temperatures but almost similar emission measures in the cross-field direction.
Furthermore, when we assume certain errors in the simulated loop emissions
(e.g., due to photometric uncertainties in the TRACE filters) and use the
triple-filter analysis, our simulated loop conditions become consistent with
those of an isothermal plasma. This implies that the use of TRACE/EIT triple
filters for observation of a warm coronal loop may not help in determining
whether the cross-field isothermal assumption is satisfied or not
Ray tracing of ion-cyclotron waves in a coronal funnel
Remote observations of coronal holes have strongly implicated the kinetic
interactions of ion-cyclotron waves with ions as a principal mechanism for
plasma heating and acceleration of the fast solar wind. In order to study these
waves, a linear perturbation analysis is used in the work frame of the
collisionless multi-fluid model. We consider a non-uniform background plasma
describing a funnel region and use the ray tracing equations to compute the ray
path of the waves as well as the spatial variation of their properties.Comment: 4 pages, 3 figures Modern Solar Facilities, Advanced Solar Science,
Universitatsverlag Goettingen 200
Solar wind and kinetic heliophysics
This paper reviews recent aspects of solar wind physics and elucidates the
role Alfvén waves play in solar wind acceleration and turbulence, which
prevail in the low corona and inner heliosphere. Our understanding of the
solar wind has made considerable progress based on remote sensing, in situ
measurements, kinetic simulation and fluid modeling. Further insights are
expected from such missions as the Parker Solar Probe and Solar Orbiter.
The sources of the solar wind have been identified in the chromospheric
network, transition region and corona of the Sun. Alfvén waves excited by
reconnection in the network contribute to the driving of turbulence and
plasma flows in funnels and coronal holes. The dynamic solar magnetic field
causes solar wind variations over the solar cycle. Fast and slow solar wind
streams, as well as transient coronal mass ejections, are generated by the
Sun's magnetic activity.
Magnetohydrodynamic turbulence originates at the Sun and evolves into
interplanetary space. The major Alfvén waves and minor magnetosonic waves,
with an admixture of pressure-balanced structures at various scales,
constitute heliophysical turbulence. Its spectra evolve radially and develop
anisotropies. Numerical simulations of turbulence spectra have reproduced key
observational features. Collisionless dissipation of fluctuations remains a
subject of intense research.
Detailed measurements of particle velocity distributions have revealed
non-Maxwellian electrons, strongly anisotropic protons and heavy ion beams.
Besides macroscopic forces in the heliosphere, local wave–particle
interactions shape the distribution functions. They can be described by the
Boltzmann–Vlasov equation including collisions and waves. Kinetic simulations
permit us to better understand the combined evolution of particles and waves
in the heliosphere.</p
Spectroscopic Observations of Propagating Disturbances in a Polar Coronal Hole: Evidence of Slow Magneto-acoustic Waves
We focus on detecting and studying quasi-periodic propagating features that
have been interpreted both in terms of slow magneto-acoustic waves and of high
speed upflows. We analyze long duration spectroscopic observations of the
on-disk part of the south polar coronal hole taken on 1997 February 25 by the
SUMER spectrometer aboard SOHO. We calibrated the velocity with respect to the
off-limb region and obtain time--distance maps in intensity, Doppler velocity
and line width. We also perform a cross correlation analysis on different time
series curves at different latitudes. We study average spectral line profiles
at the roots of propagating disturbances and along the propagating ridges, and
perform a red-blue asymmetry analysis. We find the clear presence of
propagating disturbances in intensity and Doppler velocity with a projected
propagation speed of about km s and a periodicity of
14.5 min. To our knowledge, this is the first simultaneous detection
of propagating disturbances in intensity as well as in Doppler velocity in a
coronal hole. During the propagation, an intensity enhancement is associated
with a blue-shifted Doppler velocity. These disturbances are clearly seen in
intensity also at higher latitudes (i.e. closer to the limb), while
disturbances in Doppler velocity becomes faint there. The spectral line
profiles averaged along the propagating ridges are found to be symmetric, to be
well fitted by a single Gaussian, and have no noticeable red-blue asymmetry.
Based on our analysis, we interpret these disturbances in terms of propagating
slow magneto-acoustic waves.Comment: accepted for publication by A&
Upflows in the upper transition region of the quiet Sun
We investigate the physical meaning of the prominent blue shifts of Ne VIII,
which is observed to be associated with quiet-Sun network junctions (boundary
intersections), through data analyses combining force-free-field extrapolations
with EUV spectroscopic observations. For a middle-latitude region, we
reconstruct the magnetic funnel structure in a sub-region showing faint
emission in EIT-Fe 195. This funnel appears to consist of several smaller
funnels that originate from network lanes, expand with height and finally merge
into a single wide open-field region. However, the large blue shifts of Ne VIII
are generally not associated with open fields, but seem to be associated with
the legs of closed magnetic loops. Moreover, in most cases significant upflows
are found in both of the funnel-shaped loop legs. These quasi-steady upflows
are regarded as signatures of mass supply to the coronal loops rather than the
solar wind. Our observational result also reveals that in many cases the
upflows in the upper transition region (TR) and the downflows in the middle TR
are not fully cospatial. Based on these new observational results, we suggest
different TR structures in coronal holes and in the quiet Sun.Comment: 4 pages, 4 figures, will appear in the Proceedings of the Solar wind
12 conferenc
Apparent temperature anisotropies due to wave activity in the solar wind
The fast solar wind is a collisionless plasma permeated by plasma waves on
many different scales. A plasma wave represents the natural interplay between
the periodic changes of the electromagnetic field and the associated coherent
motions of the plasma particles. In this paper, a model velocity distribution
function is derived for a plasma in a single, coherent, large-amplitude wave.
This model allows one to study the kinetic effects of wave motions on particle
distributions. They are by in-situ spacecraft measured by counting, over a
certain sampling time, the particles coming from various directions and having
different energies. We compare our results with the measurements by the Helios
spacecraft, and thus find that by assuming high wave activity we are able to
explain key observed features of the measured distributions within the
framework of our model. We also address the recent discussions on nonresonant
wave--particle interactions and apparent heating. The applied time-averaging
procedure leads to an apparent ion temperature anisotropy which is connected
but not identical to the intrinsic temperature of the underlying distribution
function.Comment: 9 pages, 4 figures, publisher version under
http://www.ann-geophys.net/29/909/2011/angeo-29-909-2011.htm
Links between magnetic fields and plasma flows in a coronal hole
We compare the small-scale features visible in the Ne viii Doppler-shift map
of an equatorial coronal hole (CH) as observed by SUMER with the small-scale
structures of the magnetic field as constructed from a simultaneous
photospheric magnetogram by a potential magnetic-field extrapolation. The
combined data set is analysed with respect to the small-scale flows of coronal
matter, which means that the Ne viii Doppler-shift used as tracer of the plasma
flow is investigated in close connection with the ambient magnetic field. Some
small closed-field regions in this largely open CH are also found in the
coronal volume considered. The Doppler-shift patterns are found to be clearly
linked with the field topology.Comment: 4 pages, 3 figure
Coronal ion-cyclotron beam instabilities within the multi-fluid description
Spectroscopic observations and theoretical models suggest resonant
wave-particle interactions, involving high-frequency ion-cyclotron waves, as
the principal mechanism for heating and accelerating ions in the open coronal
holes. However, the mechanism responsible for the generation of the
ion-cyclotron waves remains unclear. One possible scenario is that ion beams
originating from small-scale reconnection events can drive micro-instabilities
that constitute a possible source for the excitation of ion-cyclotron waves. In
order to study ion beam-driven electromagnetic instabilities, the multi-fluid
model in the low-beta coronal plasma is used. While neglecting the electron
inertia this model allows one to take into account ion-cyclotron wave effects
that are absent from the one-fluid MHD model. Realistic models of density and
temperature as well as a 2-D analytical magnetic field model are used to define
the background plasma in the open-field funnel region of a polar coronal hole.
Considering the WKB approximation, a Fourier plane-wave linear mode analysis is
employed in order to derive the dispersion relation. Ray-tracing theory is used
to compute the ray path of the unstable wave as well as the evolution of the
growth rate of the wave while propagating in the coronal funnel. We demonstrate
that, in typical coronal holes conditions and assuming realistic values of the
beam velocity, the free energy provided by the ion beam propagating parallel
the ambient field can drive micro-instabilities through resonant ion-cyclotron
excitation.Comment: 8 pages, 6 figures, submitted to A&
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