387 research outputs found
Hydrogen Lyman emission through the solar cycle
We present observations and results of radiance and irradiance studies
completed by SoHO-SUMER during the past solar cycle. We find that the cycle
variation in Ly-alpha irradiance as observed by e.g. UARS-SOLSTICE can not be
explained by quiet sun radiance data, and conclude that the explanation must be
related to differences in the Ly-alpha radiance of various solar features and
changes in their fractional distribution over the solar cycle. Consequently, we
studied the emission of the hydrogen Ly-alpha line in various solar features -
for the first time observed by SUMER on disk in full resolution - to
investigate the imprint of the magnetic field on line profile and radiance
distribution. We also compare quasi-simultaneous Ly-alpha and Ly-beta line
profiles. Such high-resolution observations - not hampered by geocoronal
absorption - have never been completed before.Comment: 6 pages, 4 figures, to appear in "Understanding a peculiar Solar
Minimum", eds. S Cranmer, T.Hoeksema, J. Kohl, ASPC 201
Solar transition region in the quiet Sun and active regions
The solar transition region (TR), in which above the photosphere the tempera-
ture increases rapidly and the density drops dramatically, is believed to play
an important role in coronal heating and solar wind acceleration. Long-lasting
up-flows are present in the upper TR and interpreted as signatures of mass
supply to large coronal loops in the quiet Sun. Coronal bright points (BPs) are
local heating phenomena and we found a different Doppler-shift pattern at TR
and coronal temperatures in one BP, which might be related to the twisted loop
system. The dominant energy loss in the lower TR is the Ly-alpha emission. It
has been found that most Ly-alpha radiance profiles are stronger in the blue
peak, an asymmetry opposite to higher order Lyman lines. This asymmetry is
stronger when the downflow in the middle TR is stronger, indicating that the TR
flows play an important role in the line formation process. The peak separation
of Ly-alpha is found to be larger in coronal holes than in the quiet Sun,
reflecting the different magnetic structures and radiation fields between the
two regions. The Lyman line profiles are found to be not reversed in sunspot
plume and umbra regions, while they are obviously reversed in the surrounding
plage region. At TR temperatures, the densities of the sunspot plume and umbra
are a factor of 10 lower than of the plage, indicating that the sunspot plasma
emitting at TR temperatures is higher and possibly more extended above sunspots
than above the plage region.Comment: This paper has been withdrawn by the author because it's not a
referred pape
The SUMER Lyman-alpha line profile in quiescent prominences
Aims: Out of a novel observing technique, we publish for the first time,
SoHO-SUMER observations of the true spectral line profile of hydrogen
Lyman-alpha in quiescent prominences. With SoHO not being in Earth orbit, our
high-quality data set is free from geocoronal absorption. We study the line
profile and compare it with earlier observations of the higher Lyman lines and
recent model predictions. Methods: We applied the reduced-aperture observing
mode to two prominence targets and started a statistical analysis of the line
profiles in both data sets. In particular, we investigated the shape of the
profile, the radiance distribution and the line shape-to-radiance
interrelation. We also compare Ly-a data to co-temporal 1206 Si III data.
Results: We find that the average profile of Ly-a has a blue-peak dominance and
is more reversed, if the line-of-sight is perpendicular to the field lines. The
contrast of Ly-a prominence emission rasters is very low and the radiance
distribution differs from the log-normal distribution of the disk. Features
seen in the Si III line are not always co-spatial with Ly-a emission.
Conclusions: Our empirical results support recent multi-thread models, which
predict that asymmetries and depths of the self-reversal depend on the
orientation of the prominence axis relative to the line-of-sight.Comment: 4 pages, 7 figures; accepted for publication as A&A lette
Microflaring of a solar Bright point
A solar X-ray Bright point (BP) was observed with the SUMER-spectrograph of
the SOHO-observatory. The data consist of four far-UV spectral lines formed
between 2 10^4 - 6 10^5 K, with 2 arcsec spatial, 2.8 min temporal and 4 km/s
spectral resolution. A striking feature is the strong microflaring and
appearance of several short lived transients. Using simultaneous magnetic field
measurements the region observed seemed to lie above a cancelling flux region.
With respect to the filling factor and emission measure this particular BP was
similar to the average surface of a moderately active solar type star.Comment: 7 pages, 6 figures, in press Astronomy and Astrophysics; for Fig.3 it
is recommended to download separately the colour version h3653f3.pd
Jets or high velocity flows revealed in high-cadence spectrometer and imager co-observations?
We report on active region EUV dynamic events observed simultaneously at
high-cadence with SUMER/SoHO and TRACE. Although the features appear in the
TRACE Fe ix/x 171A images as jets seen in projection on the solar disk, the
SUMER spectral line profiles suggest that the plasma has been driven along a
curved large scale magnetic structure, a pre-existing loop. The SUMER
observations were carried out in spectral lines covering a large temperature
range from 10^4 K to 10^6 K. The spectral analysis revealed that a sudden
heating from an energy deposition is followed by a high velocity plasma flow.
The Doppler velocities were found to be in the range from 90 to 160 km/s. The
heating process has a duration which is below the SUMER exposure time of 25 s
while the lifetime of the events is from 5 to 15 min. The additional check on
soft X-ray Yohkoh images shows that the features most probably reach 3 MK
(X-ray) temperatures. The spectroscopic analysis showed no existence of cold
material during the events
Spectroscopic Diagnostics of Polar Coronal Plumes
Polar coronal plumes seen during solar eclipses can now be studied with
space-borne telescopes and spectrometers. We briefly discuss such observations
from space with a view to understanding their plasma characteristics. Using
these observations, especially from SUMER/SOHO, but also from EUVI/STEREO, we
deduce densities, temperatures, and abundance anomalies in plumes and
inter-plume regions, and discuss their implications for better understanding of
these structures in the Sun's atmosphere.Comment: 5 pages, 3 figures; To appear in "Magnetic Coupling between the
Interior and the Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten,
Astrophysics and Space Science Proceedings, Springer-Verlag, Heidelberg,
Berlin, 200
The redshifted network contrast of transition region emission
Aims: We study the VUV emission of the quiet Sun and the net redshift of
transition region lines in the SUMER spectral range. We aim at establishing a
link with atmospheric processes and interpreting the observed downflow as the
most evident part of the prevailing global coronal mass transport. Methods: We
rank and arrange all pixels of a monochromatic raster scan by radiance and
define equally-sized bins of bright, faint, and medium-bright pixels. Comparing
the bright pixels with the faint pixels, we determine the spectrally-resolved
network contrast for 19 emission lines. We then compare the contrast centroids
of these lines with the position of the line itself. We establish a
relationship between the observed redshift of the network contrast with the
line formation temperature. Results: We find that the network contrast is
offset in wavelength compared to the emission line itself. This offset, if
interpreted as redshift, peaks at middle transition region temperatures and is
10 times higher than the previously reported net redshift of transition region
emission lines. We demonstrate that the brighter pixels are more redshifted,
causing both a significant shift of the network contrast profile and the
well-known net redshift. We show that this effect can be reconstructed from the
radiance distribution. This result is compatible with loop models, which assume
downflows near both footpoints.Comment: 5 pages, 4 figure
Observations Supporting the Role of Magnetoconvection in Energy Supply to the Quiescent Solar Atmosphere
Identifying the two physical mechanisms behind the production and sustenance
of the quiescent solar corona and solar wind poses two of the outstanding
problems in solar physics today. We present analysis of spectroscopic
observations from the Solar and Heliospheric Observatory that are consistent
with a single physical mechanism being responsible for a significant portion of
the heat supplied to the lower solar corona and the initial acceleration of the
solar wind; the ubiquitous action of magnetoconvection-driven reprocessing and
exchange reconnection of the Sun's magnetic field on the supergranular scale.
We deduce that while the net magnetic flux on the scale of a supergranule
controls the injection rate of mass and energy into the transition region
plasma it is the global magnetic topology of the plasma that dictates whether
the released ejecta provides thermal input to the quiet solar corona or becomes
a tributary that feeds the solar wind.Comment: 34 pages, 13 figures - In press Astrophysical Journal (Jan 1 2007
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