143 research outputs found
Electron density in the quiet solar coronal transition region from SoHO/SUMER measurements of S VI line radiance and opacity
Context: The sharp temperature and density gradients in the coronal
transition region are a challenge for models and observations.
Aims: We set out to get linearly- and quadratically-weighted average electron
densities in the region emitting the S VI lines, using the observed opacity and
the emission measure of these lines.
Methods: We analyze SoHO/SUMER spectroscopic observations of the S VI lines,
using the center-to-limb variations and radiance ratios to derive the opacity.
We also use the Emission Measure derived from radiance at disk center.
Results: We get an opacity at S VI line center of the order of 0.05. The
resulting average electron density is 2.4 10^16 m^-3 at T = 2 10^5 K. This
value is higher than the values obtained from radiance measurements.
Conversely, taking a classical value for the density leads to a too high value
of the thickness of the emitting layer.
Conclusions: The pressure derived from the Emission Measure method compares
well with previous determinations and implies a low opacity of 5 10^-3 to
10^-2. The fact that a direct derivation leads to a much higher opacity remains
unexplained, despite tentative modeling of observational biases. Further
measurements need to be done, and more realistic models of the transition
region need to be used.Comment: 11 pages, 9 figure
SUMER: Solar Ultraviolet Measurements of Emitted Radiation
The SUMER (solar ultraviolet measurements of emitted radiation) experiment is described. It will study flows, turbulent motions, waves, temperatures and densities of the plasma in the upper atmosphere of the Sun. Structures and events associated with solar magnetic activity will be observed on various spatial and temporal scales. This will contribute to the understanding of coronal heating processes and the solar wind expansion. The instrument will take images of the Sun in EUV (extreme ultra violet) light with high resolution in space, wavelength and time. The spatial resolution and spectral resolving power of the instrument are described. Spectral shifts can be determined with subpixel accuracy. The wavelength range extends from 500 to 1600 angstroms. The integration time can be as short as one second. Line profiles, shifts and broadenings are studied. Ratios of temperature and density sensitive EUV emission lines are established
Explosive events - swirling transition region jets
In this paper, we extend our earlier work to provide additional evidence for
an alternative scenario to explain the nature of so-called `explosive events'.
The bi-directed, fast Doppler motion of explosive events observed
spectroscopically in the transition region emission is classically interpreted
as a pair of bidirectional jets moving upward and downward from a reconnection
site. We discuss the problems of such a model. In our previous work, we focused
basically on the discrepancy of fast Doppler motion without detectable motion
in the image plane. We now suggest an alternative scenario for the explosive
events, based on our observations of spectral line tilts and bifurcated
structure in some events. Both features are indicative of rotational motion in
narrow structures. We explain the bifurcation as the result of rotation of
hollow cylindrical structures and demonstrate that such a sheath model can also
be applied to explain the nature of the puzzling `explosive events'. We find
that the spectral tilt, the lack of apparent motion, the bifurcation, and a
rapidly growing number of direct observations support an alternative scenario
of linear, spicular-sized jets with a strong spinning motion.Comment: 9 pages, 3 figures, accepted for publication in Solar Physic
The emission line near 1319 A in solar and stellar spectra
An emission line near 1319 A is one of the strongest unidentified lines in
the ultraviolet spectra of cool dwarf stars. In most line lists it is
identified as a transition in N I, although its intensity would then be
anomalous and the observed wavelength does not fit precisely that expected for
N I. The line is also observed in cool giant stars. The measured wavelength of
the line in stellar spectra is 1318.94 (+,- 0.01) A. Observations of giant
stars provide further evidence that this line is not due to N I. It is proposed
that this line is a decay from a previously unknown level in S I, which lies
above the first ionization limit. This is identified with the 3d singlet D (odd
parity) term. The previous tentative assignment of this term to the S I line at
1309.3 A then needs to be revised. The 1309.3 A line has been identified here
for the first time in an astrophysical source. The singlet D (odd parity) level
could, in principle, be populated by collisions from nearby autoionizing levels
that have large number-densities, through population by di-electronic capture.
Spin-orbit interaction with the autoionizing triplet D (odd parity) term might
also lead to di-electronic capture into the singlet D (odd parity) level. A
line at 1309.87 A observed in cool giant stars is identified as a transition in
P II, pumped by the O I resonance lines.Comment: 9 pages, 3 figures, to be published in Monthly Notices of the Royal
Astronomical Societ
A statistical study of SUMER spectral images: events, turbulence, and intermittency
We analyze a series of full-Sun observations, which was performed with the
SoHO/SUMER instrument between March and October 1996. Some parameters
(radiance, shift and width) of the S VI 93.3 nm, S VI 94.4 nm, and Lyman
Epsilon line profiles were computed on board. Radiances and line-of-sight
velocities in a large central region of the Sun are studied statistically:
distributions of solar structures, field Fourier spectra and structure
functions are obtained. The structures have distributions with power-law tails,
the Fourier spectra of the radiance fields also display power laws, and the
normalized structure functions of the radiance and velocity fields increase at
small scales. These results support the idea of the existence of small scales,
created by turbulence, and of intermittency of the observed fields. These
properties may provide insight into the processes needed for heating the
transition region, or, if confirmed in the corona, the corona itself. The
difficulties encountered in this analysis, especially for the velocity data,
underline the needs for sensitive ultraviolet imaging spectrometers.Comment: 10 pages, 13 figures, Astronomy & Astrophysics, in pres
Signature of mass supply to quiet coronal loops
Aims. The physical implication of large blue shift of Ne viii in the quiet
Sun region is investigated in this paper. Methods. We compare the significant
Ne viii blue shifts, which are visible as large blue patches on the
Doppler-shift map of a middlelatitude quiet-Sun region observed by SUMER, with
the coronal magnetic-field structures as reconstructed from a simultaneous
photospheric magnetogram by means of a force-free-field extrapolation. Results.
We show for the first time that coronal funnels also exist in the quiet Sun.
The region studied contains several small 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 the Ne viii line are not generally
associated with funnels. A comparison between the projections of coronal loops
onto the solar x-y-plane and the Ne viii dopplergram indicates that there are
some loops that reveal large Ne viii blue shifts in both legs, and some loops
with upflow in one and downflow in the other leg. Conclusions. Our results
suggest that strong plasma outflow, which can be traced by large Ne viii blue
shift, is not necessarily associated with the solar wind originating in coronal
funnels but appears to be a signature of mass supply to coronal loops. Under
the assumption that the measured Doppler shift of the Ne viii line represents
the real outflow velocity of the neon ions being markers of the proton flow, we
estimate the mass supply rate to coronal loops to be about 10\^{34} s\^{-1}.Comment: 5 pages, 4 figure
The Structure and Dynamics of the Upper Chromosphere and Lower Transition Region as Revealed by the Subarcsecond VAULT Observations
The Very high Angular resolution ULtraviolet Telescope (VAULT) is a sounding
rocket payload built to study the crucial interface between the solar
chromosphere and the corona by observing the strongest line in the solar
spectrum, the Ly-a line at 1216 {\AA}. In two flights, VAULT succeeded in
obtaining the first ever sub-arcsecond (0.5") images of this region with high
sensitivity and cadence. Detailed analyses of those observations have
contributed significantly to new ideas about the nature of the transition
region. Here, we present a broad overview of the Ly-a atmosphere as revealed by
the VAULT observations, and bring together past results and new analyses from
the second VAULT flight to create a synthesis of our current knowledge of the
high-resolution Ly-a Sun. We hope that this work will serve as a good reference
for the design of upcoming Ly-a telescopes and observing plans.Comment: 28 pages, 11 figure
The Sunrise Mission
The first science flight of the balloon-borne \Sunrise telescope took place
in June 2009 from ESRANGE (near Kiruna/Sweden) to Somerset Island in northern
Canada. We describe the scientific aims and mission concept of the project and
give an overview and a description of the various hardware components: the 1-m
main telescope with its postfocus science instruments (the UV filter imager
SuFI and the imaging vector magnetograph IMaX) and support instruments (image
stabilizing and light distribution system ISLiD and correlating wavefront
sensor CWS), the optomechanical support structure and the instrument mounting
concept, the gondola structure and the power, pointing, and telemetry systems,
and the general electronics architecture. We also explain the optimization of
the structural and thermal design of the complete payload. The preparations for
the science flight are described, including AIV and ground calibration of the
instruments. The course of events during the science flight is outlined, up to
the recovery activities. Finally, the in-flight performance of the
instrumentation is briefly summarized.Comment: 35 pages, 17 figure
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
Hot Explosions in the Cool Atmosphere of the Sun
The solar atmosphere was traditionally represented with a simple
one-dimensional model. Over the past few decades, this paradigm shifted for the
chromosphere and corona that constitute the outer atmosphere, which is now
considered a dynamic structured envelope. Recent observations by IRIS
(Interface Region Imaging Spectrograph) reveal that it is difficult to
determine what is up and down even in the cool 6000-K photosphere just above
the solar surface: this region hosts pockets of hot plasma transiently heated
to almost 100,000 K. The energy to heat and accelerate the plasma requires a
considerable fraction of the energy from flares, the largest solar disruptions.
These IRIS observations not only confirm that the photosphere is more complex
than conventionally thought, but also provide insight into the energy
conversion in the process of magnetic reconnection.Comment: published in Science 346, 1255726 (2014), 30 pages, 13 figures; for
associated movie, see
http://www2.mps.mpg.de/data/outgoing/peter/papers/2014-iris-eb/fig1-movie.mo
- …