269 research outputs found
Solar off-limb line widths: Alfven waves, ion-cyclotron waves, and preferential heating
Alfven waves and ion-cyclotron absorption of high-frequency waves are
frequently brought into models devoted to coronal heating and fast solar-wind
acceleration. Signatures of ion-cyclotron resonance have already been observed
in situ in the solar wind (HELIOS spacecrafts) and, recently, in the upper
corona (UVCS/SOHO remote-sensing results). We propose a method to constrain
both the Alfven wave amplitude and the preferential heating induced by
ion-cyclotron resonance, above a partially developed polar coronal hole
observed with the SUMER/SOHO spectrometer. The instrumental stray light
contribution is first substracted from the spectra. By supposing that the
non-thermal velocity is related to the Alfven wave amplitude, it is constrained
through a density diagnostic and the gradient of the width of the Mg X 625 A
line. The temperatures of several coronal ions, as functions of the distance
above the limb, are then determined by substracting the non-thermal component
to the observed line widths. The effect of stray light explains the apparent
decrease with height in the width of several spectral lines, this decrease
usually starting about 0.1-0.2 Rs above the limb. This result rules out any
direct evidence of damping of the Alfven waves, often suggested by other
authors. We also find that the ions with the smallest charge-to-mass ratios are
the hottest ones at a fixed altitude and that they are subject to a stronger
heating, as compared to the others, between 57" and 102" above the limb. This
constitutes a serious clue to ion-cyclotron preferential heating.Comment: 15 pages, 12 figures, 3 table
The Spectroscopic Footprint of the Fast Solar Wind
We analyze a large, complex equatorial coronal hole (ECH) and its immediate
surroundings with a focus on the roots of the fast solar wind. We start by
demonstrating that our ECH is indeed a source of the fast solar wind at 1AU by
examining in situ plasma measurements in conjunction with recently developed
measures of magnetic conditions of the photosphere, inner heliosphere and the
mapping of the solar wind source region. We focus the bulk of our analysis on
interpreting the thermal and spatial dependence of the non-thermal line widths
in the ECH as measured by SOHO/SUMER by placing the measurements in context
with recent studies of ubiquitous Alfven waves in the solar atmosphere and line
profile asymmetries (indicative of episodic heating and mass loading of the
coronal plasma) that originate in the strong, unipolar magnetic flux
concentrations that comprise the supergranular network. The results presented
in this paper are consistent with a picture where a significant portion of the
energy responsible for the transport of heated mass into the fast solar wind is
provided by episodically occurring small-scale events (likely driven by
magnetic reconnection) in the upper chromosphere and transition region of the
strong magnetic flux regions that comprise the supergranular network.Comment: 25 pages, accepted to appear in the Astrophysical Journal. Supporting
movies can be found in http://download.hao.ucar.edu/pub/mscott/papers/ECH
Sizes of transition-region structures in coronal holes and in the quiet Sun
We study the height variations of the sizes of chromospheric and
transition-region features in a small coronal hole and the adjacent quiet Sun,
considering images of the intensity, Doppler shift, and non-thermal motion of
ultraviolet emission lines as measured by SUMER, together with the magnetic
field as obtained by extrapolation from photospheric magnetograms. In order to
estimate the characteristic sizes of the different features present in the
chromosphere and transition region, we have calculated the autocorrelation
function for the images as well as the corresponding extrapolated magnetic
field at different heights. The HWHM of the autocorrelation function is
considered to be the characteristic size of the feature shown in the
corresponding image. Our results indicate that, in both the coronal hole and
quiet Sun, the HWHM of the intensity image is larger than that of the images of
Doppler-shift and non-thermal width at any given altitude. The HWHM of the
intensity image is smaller in the chromosphere than in the TR, where the sizes
of intensity features of lines at different temperatures are almost the same.
But in the upper part of the transition region, the intensity size increases
more strongly with temperature in the coronal hole than in the quiet Sun. We
also studied the height variations of the HWHM of the magnetic field magnitude
B and its component |Bz|, and found they are equal to each other at a certain
height below 40 Mm in the coronal hole. The height variations of the HWHM of
|Bz/B| seem to be consistent with the temperature variations of the intensity
size. Our results suggest that coronal loops are much lower, and magnetic
structures expand through the upper TR and lower corona much more strongly with
height in the coronal hole than in the quiet Sun.Comment: 6 pages, 4 figure
New views on the emission and structure of the solar transition region
The Sun is the only star that we can spatially resolve and it can be regarded
as a fundamental plasma laboratory of astrophysics. The solar transition region
(TR), the layer between the solar chromosphere and corona, plays an important
role in solar wind origin and coronal heating. Recent high-resolution
observations made by SOHO, TRACE, and Hinode indicate that the TR is highly
nonuniform and magnetically structured. Through a combination of spectroscopic
observations and magnetic field extrapolations, the TR magnetic structures and
plasma properties have been found to be different in coronal holes and in the
quiet Sun. In active regions, the TR density and temperature structures also
differ in sunspots and the surrounding plage regions. Although the TR is
believed to be a dynamic layer, quasi-steady flows lasting from several hours
to several days are often present in the quiet Sun, coronal holes, and active
regions, indicating some kind of plasma circulation/convection in the TR and
corona. The emission of hydrogen Lyman lines, which originates from the lower
TR, has also been intensively investigated in the recent past. Observations
show clearly that the flows and dynamics in the middle and upper TR can greatly
modify the Lyman line profiles.Comment: This paper has been withdrawn by the authors. This is a repetition of
another record in ADS: New Astronomy Reviews, Volume 54, Issue 1-2, p. 13-3
The quiet Sun average Doppler shift of coronal lines up to 2 MK
The average Doppler shift shown by spectral lines formed from the
chromosphere to the corona reveals important information on the mass and energy
balance of the solar atmosphere, providing an important observational
constraint to any models of the solar corona. Previous spectroscopic
observations of vacuum ultra-violet (VUV) lines have revealed a persistent
average wavelength shift of lines formed at temperatures up to 1 MK. At higher
temperatures, the behaviour is still essentially unknown. Here we analyse
combined SUMER/SoHO and EIS/Hinode observations of the quiet Sun around disk
centre to determine, for the first time, the average Doppler shift of several
spectral lines formed between 1 and 2 MK, where the largest part of the quiet
coronal emission is formed. The measurements are based on a novel technique
applied to EIS spectra to measure the difference in Doppler shift between lines
formed at different temperatures. Simultaneous wavelength-calibrated SUMER
spectra allow establishing the absolute value at the reference temperature of 1
MK. The average line shifts at 1 MK < T < 1.8 MK are modestly, but clearly
bluer than those observed at 1 MK. By accepting an average blue shift of about
(-1.8+/-0.6) km/s at 1 MK (as provided by SUMER measurements), this translates
into a maximum Doppler shift of (-4.4+/-2.2) km/s around 1.8 MK. The measured
value appears to decrease to about (-1.3+/-2.6) km/s at the Fe XV formation
temperature of 2.1 MK. The measured average Doppler shift between 0.01 and 2.1
MK, for which we provide a parametrisation, appears to be qualitatively and
roughly quantitatively consistent with what foreseen by 3-D coronal models
where heating is produced by dissipation of currents induced by photospheric
motions and by reconnection with emerging magnetic flux.Comment: 9 pages, 10 figures. Astronomy and Astrophysics (in press
A search for edge-on galaxy lenses in the CFHT Legacy Survey
[ABRIDGED] The new generation of wide field optical imaging like the Canada
France Hawaii Telescope Legacy Survey (CFHTLS) enables discoveries of all types
of gravitational lenses present in the sky. The Strong Lensing Legacy Survey
(SL2S) project has started an inventory, respectively for clusters or groups of
galaxies lenses, and for Einstein rings around distant massive ellipticals.
Here we attempt to extend this inventory by finding lensing events produced by
massive edge-on disk galaxies which remains a poorly documented class of
lenses. We implement and test an automated search procedure of edge-on galaxy
lenses in the CFHTLS Wide fields with magnitude 18Comment: several major edits, 8 pages, A&A accepte
Horizontal supergranule-scale motions inferred from TRACE ultraviolet observations of the chromosphere
We study horizontal supergranule-scale motions revealed by TRACE observation
of the chromospheric emission, and investigate the coupling between the
chromosphere and the underlying photosphere. A highly efficient
feature-tracking technique called balltracking has been applied for the first
time to the image sequences obtained by TRACE (Transition Region and Coronal
Explorer) in the passband of white light and the three ultraviolet passbands
centered at 1700 {\AA}, 1600 {\AA}, and 1550 {\AA}. The resulting velocity
fields have been spatially smoothed and temporally averaged in order to reveal
horizontal supergranule-scale motions that may exist at the emission heights of
these passbands. We find indeed a high correlation between the horizontal
velocities derived in the white-light and ultraviolet passbands. The horizontal
velocities derived from the chromospheric and photospheric emission are
comparable in magnitude. The horizontal motions derived in the UV passbands
might indicate the existence of a supergranule-scale magnetoconvection in the
chromosphere, which may shed new light on the study of mass and energy supply
to the corona and solar wind at the height of the chromosphere. However, it is
also possible that the apparent motions reflect the chromospheric brightness
evolution as produced by acoustic shocks which might be modulated by the
photospheric granular motions in their excitation process, or advected partly
by the supergranule-scale flow towards the network while propagating upward
from the photosphere. To reach a firm conclusion, it is necessary to
investigate the role of granular motions in the excitation of shocks through
numerical modeling, and future high-cadence chromospheric magnetograms must be
scrutinized.Comment: 5 figures, accepted by Astronomy & Astrophysic
Modeling the (upper) solar atmosphere including the magnetic field
The atmosphere of the Sun is highly structured and dynamic in nature. From
the photosphere and chromosphere into the transition region and the corona
plasma- changes from above to below one, i.e. while in the lower
atmosphere the energy density of the plasma dominates, in the upper atmosphere
the magnetic field plays the governing role -- one might speak of a ``magnetic
transition''. Therefore the dynamics of the overshooting convection in the
photosphere, the granulation, is shuffling the magnetic field around in the
photosphere. This leads not only to a (re-)structuring of the magnetic field in
the upper atmosphere, but induces also the dynamic reaction of the coronal
plasma e.g. due to reconnection events. Therefore the (complex) structure and
the interaction of various magnetic patches is crucial to understand the
structure, dynamics and heating of coronal plasma as well as its acceleration
into the solar wind.
The present article will emphasize the need for three-dimensional modeling
accounting for the complexity of the solar atmosphere to understand these
processes. Some advances on 3D modeling of the upper solar atmosphere in
magnetically closed as well as open regions will be presented together with
diagnostic tools to compare these models to observations. This highlights the
recent success of these models which in many respects closely match the
observations.Comment: 14 pages, 7 figures, accepted for publication in Adv. Space Re
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