91 research outputs found
Dynamic Ly alpha jets
The solar chromosphere and transition region are highly structured and
complex regimes. A recent breakthrough has been the identification of dynamic
fibrils observed in H alpha as caused by field-aligned magnetoacoustic shocks.
We seek to find whether such dynamic fibrils are also observed in Ly alpha. We
used a brief sequence of four high-resolution Ly alpha images of the solar limb
taken by the Very high Angular resolution ULtraviolet Telescope (VAULT), which
displays many extending and retracting Ly alpha jets. We measured their top
trajectories and fitted parabolas to the 30 best-defined ones. Most jet tops
move supersonically. Half of them decelerate, sometimes superballistically, the
others accelerate. This bifurcation may arise from incomplete sampling of
recurrent jets. The similarities between dynamic Ly alpha jets and H alpha
fibrils suggest that the magnetoacoustic shocks causing dynamic H alpha fibrils
also affect dynamic Ly alpha jets.Comment: 5 pages, 7 figures; changed title and content; accepted in Astronomy
and Astrophysics; eps figures in full resolution are available at
http://www.astro.sk/~koza/publications/vault/figs
Observations of a solar flare and filament eruption in Lyman <span class='mathrm'>α</span> and X-rays
<p><b>Context</b>: Lα is a strong chromospheric emission line, which has been relatively rarely observed in flares. The Transition Region and Coronal Explorer (TRACE) has a broad “Lyman α” channel centered at 1216 Å used primarily at the beginning of the mission. A small number of flares were observed in this channel.</p>
<p><b>Aims</b>: We aim to characterise the appearance and behaviour of a flare and filament ejection which occurred on 8th September 1999 and was observed by TRACE in Lα, as well as by the Yohkoh Soft and Hard X-ray telescopes. We explore the flare energetics and its spatial and temporal evolution. We have in mind the fact that the Lα line is a target for the Extreme Ultraviolet Imaging telescope (EUI) which has been selected for the Solar Orbiter mission, as well as the LYOT telescope on the proposed SMESE mission.</p>
<p><b>Methods</b>: We use imaging data from the TRACE 1216 Å, 1600 Å and 171 Å channels, and the Yohkoh hard and soft X-ray telescopes. A correction is applied to the TRACE data to obtain a better estimate of the pure Lα signature. The Lα power is obtained from a knowledge of the TRACE response function, and the flare electron energy budget is estimated by interpreting Yohkoh/HXT emission in the context of the collisional thick target model.</p>
<p><b>Results</b>: We find that the Lα flare is characterised by strong, compact footpoints (smaller than the UV ribbons) which correlate well with HXR footpoints. The Lα power radiated by the flare footpoints can be estimated, and is found to be on the order of 1026 erg s-1 at the peak. This is less than 10% of the power inferred for the electrons which generate the co-spatial HXR emission, and can thus readily be provided by them. The early stages of the filament eruption that accompany the flare are also visible, and show a diffuse, roughly circular spreading sheet-like morphology, with embedded denser blobs.</p>
<p><b>Conclusions</b>: On the basis of this observation, we conclude that flare and filament observations in the Lα line with the planned EUI and LYOT telescopes will provide valuable insight into solar flare evolution and energetics, especially when accompanied by HXR imaging and spectroscopy.</p>
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
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
LEMUR: Large European Module for solar Ultraviolet Research. European contribution to JAXA's Solar-C mission
Understanding the solar outer atmosphere requires concerted, simultaneous
solar observations from the visible to the vacuum ultraviolet (VUV) and soft
X-rays, at high spatial resolution (between 0.1" and 0.3"), at high temporal
resolution (on the order of 10 s, i.e., the time scale of chromospheric
dynamics), with a wide temperature coverage (0.01 MK to 20 MK, from the
chromosphere to the flaring corona), and the capability of measuring magnetic
fields through spectropolarimetry at visible and near-infrared wavelengths.
Simultaneous spectroscopic measurements sampling the entire temperature range
are particularly important.
These requirements are fulfilled by the Japanese Solar-C mission (Plan B),
composed of a spacecraft in a geosynchronous orbit with a payload providing a
significant improvement of imaging and spectropolarimetric capabilities in the
UV, visible, and near-infrared with respect to what is available today and
foreseen in the near future.
The Large European Module for solar Ultraviolet Research (LEMUR), described
in this paper, is a large VUV telescope feeding a scientific payload of
high-resolution imaging spectrographs and cameras. LEMUR consists of two major
components: a VUV solar telescope with a 30 cm diameter mirror and a focal
length of 3.6 m, and a focal-plane package composed of VUV spectrometers
covering six carefully chosen wavelength ranges between 17 and 127 nm. The
LEMUR slit covers 280" on the Sun with 0.14" per pixel sampling. In addition,
LEMUR is capable of measuring mass flows velocities (line shifts) down to 2
km/s or better.
LEMUR has been proposed to ESA as the European contribution to the Solar C
mission.Comment: 35 pages, 14 figures. To appear on Experimental Astronom
The Solar-C_EUVST mission
Solar-C EUVST (EUV High-Throughput Spectroscopic Telescope) is a solar physics mission concept that was selected as a candidate for JAXA competitive M-class missions in July 2018. The onboard science instrument, EUVST, is an EUV spectrometer with slit-jaw imaging system that will simultaneously observe the solar atmosphere from the photosphere/chromosphere up to the corona with seamless temperature coverage, high spatial resolution, and high throughput for the first time. The mission is designed to provide a conclusive answer to the most fundamental questions in solar physics: how fundamental processes lead to the formation of the solar atmosphere and the solar wind, and how the solar atmosphere becomes unstable, releasing the energy that drives solar flares and eruptions. The entire instrument structure and the primary mirror assembly with scanning and tip-tilt fine pointing capability for the EUVST are being developed in Japan, with spectrograph and slit-jaw imaging hardware and science contributions from US and European countries. The mission will be launched and installed in a sun-synchronous polar orbit by a JAXA Epsilon vehicle in 2025. ISAS/JAXA coordinates the conceptual study activities during the current mission definition phase in collaboration with NAOJ and other universities. The team is currently working towards the JAXA final down-selection expected at the end of 2019, with strong support from US and European colleagues. The paper provides an overall description of the mission concept, key technologies, and the latest status
Solar low-lying cool loops and their contribution to the transition region EUV output
In the last 30 years, the existence of small and cool magnetic loops (height
< 8 Mm, T < 10^5 K) has been proposed and debated to explain the increase of
the DEM (differential emission measure) towards the chromosphere. We present
hydrodynamic simulations of low-lying cool loops to study their conditions of
existence and stability, and their contribution to the transition region EUV
output. We find that stable, quasi-static cool loops (with velocities < 1 km/s)
can be obtained under different and more realistic assumptions on the radiative
losses function with respect to previous works. A mixture of the DEMs of these
cool loops plus intermediate loops with temperatures between 10^5 and 10^6 K
can reproduce the observed emission of the lower transition region at the
critical turn-up temperature point (T ~ 2x10^5 K) and below T = 10^5 K.Comment: Accepted for publication in A&A on Nov 25th 201
Physics of Solar Prominences: II - Magnetic Structure and Dynamics
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.Comment: 75 pages, 31 pictures, review pape
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