313 research outputs found
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
The Extreme Ultraviolet Imager (EUI) onboard the SOLAR ORBITER mission
peer reviewedSolar Orbiter will for the first time study the Sun with a full suite of in-situ and remote sensing instruments from inside 0.25 AU and will provide imaging and spectral observations of the Sun’s polar regions, from out of the ecliptic. This proximity to the Sun will also have the significant advantage that the spacecraft will fly in near synchronization with the Sun’s rotation, allowing observations of the solar surface and heliosphere to be studied from a near co-rotating vantage point for almost a complete solar rotation. The mission’s ambitious characteristics draw severe constraints on the design of these instruments. The scientific objectives of Solar Orbiter rely ubiquitously on the Extreme EUV Imager suite (EUI). The EUI instrument suite on board of Solar Orbiter is composed of two high resolution imagers (HRI), one at Lyman α and one dual band at the two 174 and 335 EUV passbands in the extreme UV, and one dual band full-sun imager (FSI) working alternatively at the two 174 and 304 EUV passbands. In all the units, the image is produced by a mirror-telescope, working in nearly normal incidence. The EUV reflectivity of the optical surfaces is obtained with specific EUV multilayered coatings, providing the spectral selection of the EUV units (1HRI and 1 FSI). The spectral selection is complemented with very thin filters rejecting the visible and IR radiation. Due to its orbit, EUI / Solar Orbiter will see 20 solar constants and an entrance baffle to limit the solar heat input into EUI is needed. The paper presents the scientific objectives of EUI and also covers the EUI instrument development plan which will require some trade-off between existing and promising technologies
The Ly-alpha profile and center-to-limb variation of the quiet Sun
We study the emission of the hydrogen Lyman-a line in the quiet Sun, its
center-to-limb variation (CLV), and its radiance distribution. We also compare
quasi-simultaneous Ly-a and Ly-b line profiles. We used the high spectral and
spatial resolution of the SUMER spectrometer and completed raster scans at
various locations along the disk. For the first time, we used a method to
reduce the incoming photon flux to a 20%-level by partly closing the aperture
door. We also performed a quasi-simultaneous observation of both Ly-a and Ly-b
at Sun center in sit-and-stare mode. We infer the flow characteristic in the
Ly-a map from variations in the calibrated 1206 Si III line centroids. We
present the average profile of Ly-a, its radiance distribution, its CLV
behaviour, and the signature of flows on the line profiles. Little CLV and no
limb brightening are observed in the profiles of the Ly-a line. In contrast to
all other lines of the Lyman series, which have a stronger red-horn, Ly-a has a
dominating blue-horn asymmetry. There appears to be a brightness-to-asymmetry
relationship. A similar and even clearer trend is observed in the
downflow-to-asymmetry relationship. This important result is consistent with
predictions from models that include flows. However, the absence of a clear CLV
in the profiles may be more indicative of an isotropic field than a radial
flow. It appears that the ubiquitous hydrogen behaves similar to a filter that
dampens all signatures of the line formation by processes in the chromosphere
and transition region.Comment: 4 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
Extreme-ultraviolet brightenings in the quiet Sun: Signatures in spectral and imaging data from the Interface Region Imaging Spectrograph
CONTEXT: Localised transient EUV brightenings, sometimes named ‘campfires’, occur throughout the quiet Sun. However, there are still many open questions about these events, in particular regarding their temperature range and dynamics. AIM: We aim to determine whether any transition region response can be detected for small-scale extreme-ultraviolet (EUV) brightenings and, if so, to identify whether the measured spectra correspond to any previously reported bursts in the transition region, such as explosive events (EEs). METHODS: EUV brightenings were detected in a ∼29.4 min dataset sampled by the Solar Orbiter Extreme Ultraviolet Imager (EUI) on 8 March 2022 using an automated detection algorithm. Any potential transition region response was inferred through analysis of imaging and spectral data sampled through coordinated observations conducted by the Interface Region Imaging Spectrograph (IRIS). RESULTS: EUV brightenings display a range of responses in IRIS slit-jaw imager (SJI) data. Some events have clear signatures in the Mg II and Si IV SJI filters, whilst others have no discernible counterpart. Both extended and more complex EUV brightenings are sometimes found to have responses in IRIS SJI data. Examples of EUI intensities peaking before, during, and after their IRIS counterparts were found in light curves constructed co-spatial to EUV brightenings. Importantly, therefore, it is likely that not all EUV brightenings are driven in the same way, with some events seemingly being magnetic reconnection driven and others not. A single EUV brightening occurred co-spatial to the IRIS slit, with the returned spectra matching the properties of EEs. CONCLUSIONS: EUV brightening is a term used to describe a range of small-scale events in the solar corona. The physics behind all EUV brightenings is likely not the same. More research is therefore required to assess their importance for global questions in the field, such as coronal heating
Search for photospheric footpoints of quiet Sun transition region loops
CONTEXT:The footpoints of quiet Sun Transition Region (TR) loops do not seem
to coincide with the photospheric magnetic structures appearing in traditional
low-sensitivity magnetograms.
AIMS: To look for the so-far unidentified photospheric footpoints of TR loops
using G-band bright points (BPs) as proxies for photospheric magnetic field
concentrations.
METHODS: Comparison of TR measurements with SoHO/SUMER and photospheric
magnetic field observations obtained with the Dutch Open Telescope.
RESULTS: Photospheric BPs are associated with bright TR structures, but they
seem to avoid the brightest parts of the structure. BPs appear in regions that
are globally redshifted, but they avoid extreme velocities. TR explosive events
are not clearly associated with BPs.
CONCLUSIONS: The observations are not inconsistent with the BPs being
footpoints of TR loops, although we have not succeeded to uniquely identify
particular BPs with specific TR loops.Comment: Accepted for publication in A&A. 10 pages, 10 figures. Due to size
limitations, the quality of fig3 is not goo
EUV brightenings in the quiet-Sun: Signatures in spectral and imaging data from the Interface Region Imaging Spectrograph
Localised transient EUV brightenings, sometimes named `campfires', occur
throughout the quiet-Sun. However, there are still many open questions about
such events, in particular regarding their temperature range and dynamics. In
this article, we aim to determine whether any transition region response can be
detected for small-scale EUV brightenings and, if so, to identify whether the
measured spectra correspond to any previously reported bursts in the transition
region, such as Explosive Events (EEs). EUV brightenings were detected in a
~29.4 minute dataset sampled by Solar Orbiter's Extreme Ultraviolet Imager on 8
March 2022 using an automated detection algorithm. Any potential transition
region response was inferred through analysis of imaging and spectral data
sampled through coordinated observations conducted by the Interface Region
Imaging Spectrograph (IRIS). EUV brightenings display a range of responses in
IRIS slit-jaw imager (SJI) data. Some events have clear signatures in the Mg II
and Si IV SJI filters, whilst others have no discernible counterpart. Both
extended and more complex EUV brightenings are found to, sometimes, have
responses in IRIS SJI data. Examples of EUI intensities peaking before, during,
and after their IRIS counterparts were found in lightcurves constructed
co-spatial to EUV brightenings. Importantly, therefore, it is likely that not
all EUV brightenings are driven in the same way, with some seemingly being
magnetic reconnection driven and others not. A single EUV brightening occurred
co-spatial to the IRIS slit, with its spectra matching the properties of EEs.
EUV brightenings is a term used to describe a range of small-scale event in the
solar corona. The physics responsible for all EUV brightenings is likely not
the same and, therefore, more research is required to assess their importance
towards global questions in the field, such as coronal heating.Comment: Accepted to A&A, 9 figure
The SWAP EUV Imaging Telescope Part I: Instrument Overview and Pre-Flight Testing
The Sun Watcher with Active Pixels and Image Processing (SWAP) is an EUV
solar telescope on board ESA's Project for Onboard Autonomy 2 (PROBA2) mission
launched on 2 November 2009. SWAP has a spectral bandpass centered on 17.4 nm
and provides images of the low solar corona over a 54x54 arcmin field-of-view
with 3.2 arcsec pixels and an imaging cadence of about two minutes. SWAP is
designed to monitor all space-weather-relevant events and features in the low
solar corona. Given the limited resources of the PROBA2 microsatellite, the
SWAP telescope is designed with various innovative technologies, including an
off-axis optical design and a CMOS-APS detector. This article provides
reference documentation for users of the SWAP image data.Comment: 26 pages, 9 figures, 1 movi
In-flight performance of the solar UV radiometer LYRA/PROBA-2
LYRA is a solar radiometer, part of the PROBA-2 micro-satellite payload (Fig. 1). The PROBA-2 [1] mission has been launched on 02 November 2009 with a Rockot launcher to a Sun-synchronous orbit at an altitude of 725 km. Its nominal operation duration is two years with possible extension of 2 years. PROBA-2 is a small satellite developed under an ESA General Support Technology Program (GSTP) contract to perform an in-flight demonstration of new space technologies and support a scientific mission for a set of selected instruments [2]. PROBA-2 host 17 technological demonstrators and 4 scientific instruments. The mission is tracked by the ESA Redu Mission Operation Center. One of the four scientific instruments is LYRA that monitors the solar irradiance at a high cadence (> 20 Hz) in four soft X-Ray to VUV large passbands: the “Lyman-Alpha” channel, the “Herzberg” continuum range, the “Aluminium” and “Zirconium” filter channels. The radiometric calibration is traceable to synchrotron source standards [3]. LYRA benefits from wide bandgap detectors based on diamond. It is the first space assessment of these revolutionary UV detectors for astrophysics. Diamond sensors make the instruments radiation-hard and solar-blind (insensitive to the strong solar visible light) and, therefore, visible light blocking filters become superfluous. To correlate the data of this new detector technology, silicon detectors with well known characteristics are also embarked. Due to the strict allocated mass and power budget (5 kg, 5W), and poor priority to the payload needs on such platform, an optimization and a robustness of the instrument was necessary. The first switch-on occured on 16 November 2009. Since then the instrument performances have been monitored and analyzed during the commissioning period. This paper presents the first-light and preliminary performance analysis
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