221 research outputs found
Coronal hole boundaries at small scales: III. EIS and SUMER views
We report on the plasma properties of small-scale transient events identified
in the quiet Sun, coronal holes and their boundaries.
We use spectroscopic co-observations from SUMER/SoHO and EIS/Hinode combined
with high cadence imaging data from XRT/Hinode. We measure Doppler shifts using
single and multiple Gauss fits of transition region and coronal lines as well
as electron densities and temperatures. We combine co-temporal imaging and
spectroscopy to separate brightening expansions from plasma flows. The
transient brightening events in coronal holes and their boundaries were found
to be very dynamical producing high density outflows at large speeds. Most of
these events represent X-ray jets from pre-existing or newly emerging coronal
bright points at X-ray temperatures. The average electron density of the jets
is logNe ~ 8.76 cm^-3 while in the flaring site it is logNe ~ 9.51 cm^-3. The
jet temperatures reach a maximum of 2.5 MK but in the majority of the cases the
temperatures do not exceed 1.6 MK. The footpoints of jets have temperatures of
a maximum of 2.5 MK though in a single event scanned a minute after the flaring
the measured temperature was 12 MK. The jets are produced by multiple
microflaring in the transition region and corona. Chromospheric emission was
only detected in their footpoints and was only associated with downflows. The
Doppler shift measurements in the quiet Sun transient brightenings confirmed
that these events do not produce jet-like phenomena. The plasma flows in these
phenomena remain trapped in closed loops.Comment: 16 pages, accepted for publication in A&
Coronal hole boundaries evolution at small scales: I. EIT 195 A and TRACE 171 A view
We aim at studying the small-scale evolution at the boundaries of an
equatorial coronal hole connected with a channel of open magnetic flux with the
polar region and an `isolated' one in the extreme-ultraviolet spectral range.
We intend to determine the spatial and temporal scale of these changes. Imager
data from TRACE in the Fe IX/X 171 A passband and EIT on-board Solar and
Heliospheric Observatory in the Fe XII 195 A passband were analysed. We found
that small-scale loops known as bright points play an essential role in coronal
holes boundaries evolution at small scales. Their emergence and disappearance
continuously expand or contract coronal holes. The changes appear to be random
on a time scale comparable with the lifetime of the loops seen at these
temperatures. No signature was found for a major energy release during the
evolution of the loops. Although coronal holes seem to maintain their general
shape during a few solar rotations, a closer look at their day-by-day and even
hour-by-hour evolution demonstrates a significant dynamics. The small-scale
loops (10" - 40" and smaller) which are abundant along coronal hole boundaries
have a contribution to the small-scale evolution of coronal holes. Continuous
magnetic reconnection of the open magnetic field lines of the coronal hole and
the closed field lines of the loops in the quiet Sun is more likely to take
place.Comment: 7 pages, 6 figures. in press in A&
Chromospheric Magnetic Reconnection caused by Photospheric Flux Emergence: Implications for Jet-like Events Formation
Magnetic reconnection in the low atmosphere, e.g. chromosphere, is
investigated in various physical environments. Its implications for the
origination of explosive events (small--scale jets) are discussed. A
2.5-dimensional resistive magnetohydrodynamic (MHD) model in Cartesian
coordinates is used. It is found that the temperature and velocity of the
outflow jets as a result of magnetic reconnection are strongly dependent on the
physical environments, e.g. the magnitude of the magnetic field strength and
the plasma density. If the magnetic field strength is weak and the density is
high, the temperature of the jets is very low (~10,000 K) as well as its
velocity (~40 km/s). However, if environments with stronger magnetic field
strength (20 G) and smaller density (electron density Ne=2x10^{10} cm^{-3}) are
considered, the outflow jets reach higher temperatures of up to 600,000 K and a
line-of-sight velocity of up to 130 km/s which is comparable with the
observational values of jet-like events.Comment: 9 pages, 8 figures, 1 table, submitted to A&
What is the true nature of blinkers?
Aims.
The aim of this work is to identify the true nature of the transient EUV brightenings, called blinkers.
Methods.
Co-spatial and co-temporal multi-instrument data, including imaging (EUVI/STEREO, XRT and SOT/Hinode), spectroscopic (CDS/SoHO and EIS/Hinode) and magnetogram (SOT/Hinode) data, of an isolated equatorial coronal hole were used. An automatic program for identifying transient brightenings in CDS O v 629 Ã…, EUVI 171 Ã… and XRT was applied.
Results.
We identified 28 blinker groups in the CDS O v 629 Ã… raster images. All CDS O v 629 Ã… blinkers showed counterparts in EUVI 171 Ã… and 304 Ã… images. We classified these blinkers into two categories, one associated with coronal counterparts and other with no coronal counterparts as seen in XRT images and EIS Fe xii 195.12 Ã… raster images. Around two-thirds of the blinkers show coronal counterparts and correspond to various events like EUV/X-ray jets, brightenings in coronal bright points or foot-point brightenings of larger loops. These brightenings occur repetitively and have a lifetime of around 40 min at transition region temperatures. The remaining blinker groups with no coronal counterpart in XRT and EIS Fe xii 195.12 Ã… appear as point-like brightenings and have chromospheric/transition region origin. They take place only once and have a lifetime of around 20 min. In general, lifetimes of blinkers are different at different wavelengths, i.e. different temperatures, decreasing from the chromosphere to the corona.
Conclusions.
This work shows that the term blinker covers a range of phenomena. Blinkers are the EUV response of various transient events originating at coronal, transition region and chromospheric heights. Hence, events associated with blinkers contribute to the formation and maintenance of the temperature gradient in the transition region and the corona
Can coronal hole spicules reach coronal temperatures?
We aim with the present study to provide observational evidences on whether
coronal hole spicules reach coronal temperatures. We combine multi-instrument
co-observations obtained with the SUMER/SoHO and with the EIS/SOT/XRT/Hinode.
The analysed three large spicules were found to be comprised of numerous thin
spicules which rise, rotate and descend simultaneously forming a bush-like
feature. Their rotation resembles the untwisting of a large flux rope. They
show velocities ranging from 50 to 250 km/s. We clearly associated the red- and
blue-shifted emissions in transition region lines with rotating but also with
rising and descending plasmas, respectively. Our main result is that these
spicules although very large and dynamic, show no presence in spectral lines
formed at temperatures above 300 000 K. The present paper brings out the
analysis of three Ca II H large spicules which are composed of numerous dynamic
thin spicules but appear as macrospicules in EUV lower resolution images. We
found no coronal counterpart of these and smaller spicules. We believe that the
identification of phenomena which have very different origins as macrospicules
is due to the interpretation of the transition region emission, and especially
the He II emission, wherein both chromospheric large spicules and coronal X-ray
jets are present. We suggest that the recent observation of spicules in the
coronal AIA/SDO 171 A and 211 A channels is probably due to the existence of
transition region emission there.Comment: 4 pages, 4 figures, accepted for publication in A&
Cool transition region loops observed by the Interface Region Imaging Spectrograph
We report on the first Interface Region Imaging Spectrograph (IRIS) study of
cool transition region loops. This class of loops has received little attention
in the literature. A cluster of such loops was observed on the solar disk in
active region NOAA11934, in the Si IV 1402.8 \AA\ spectral raster and 1400 \AA\
slit-jaw (SJ) images. We divide the loops into three groups and study their
dynamics and interaction. The first group comprises relatively stable loops,
with 382--626\,km cross-sections. Observed Doppler velocities are suggestive of
siphon flows, gradually changing from -10 km/s at one end to 20 km/s at the
other end of the loops. Nonthermal velocities from 15 to 25 km/s were
determined. These physical properties suggest that these loops are impulsively
heated by magnetic reconnection occurring at the blue-shifted footpoints where
magnetic cancellation with a rate of Mx/s is found. The released
magnetic energy is redistributed by the siphon flows. The second group
corresponds to two footpoints rooted in mixed-magnetic-polarity regions, where
magnetic cancellation occurred at a rate of Mx/s and line profiles
with enhanced wings of up to 200 km/s were observed. These are suggestive of
explosive-like events. The Doppler velocities combined with the SJ images
suggest possible anti-parallel flows in finer loop strands. In the third group,
interaction between two cool loop systems is observed. Evidence for magnetic
reconnection between the two loop systems is reflected in the line profiles of
explosive events, and a magnetic cancellation rate of Mx/s
observed in the corresponding area. The IRIS observations have thus opened a
new window of opportunity for in-depth investigations of cool transition region
loops. Further numerical experiments are crucial for understanding their
physics and their role in the coronal heating processes.Comment: Accepted for publication in Ap
A coronal wave and an asymmetric eruptive filament in SUMER, CDS, EIT, and TRACE co-observations
The objectives of the present study is to provide a better physical
understanding of the complex inter-relation and evolution of several solar
coronal features comprising a double-peak flare, a coronal dimming caused by a
CME, a CME-driven compression, and a fast-mode wave. For the first time, the
evolution of an asymmetric eruptive filament is analysed in simultaneous SUMER
spectroscopic and TRACE and EIT imaging data. We use imaging observations from
EIT and TRACE in the 195A channel and spectroscopic observations from the CDS
in a rastering and SUMER in a sit-and-stare observing mode. The SUMER spectra
cover spectral lines with formation temperatures from logT(K) ~ 4.0 to 6.1.
Although the event was already analysed in two previous studies, our analysis
brings a wealth of new information on the dynamics and physical properties of
the observed phenomena. We found that the dynamic event is related to a complex
flare with two distinct impulsive peaks, one according to the GOES
classification as C1.1 and the second - C1.9. The first energy release triggers
a fast-mode wave and a CME with a clear CME driven compression ahead of it.
This activity is related to, or possibly caused, by an asymmetric filament
eruption. The filament is observed to rise with its leading edge moving at a
speed of ~300 km/s detected both in the SUMER and CDS data. The rest of the
filament body moves at only ~150 km/s while untwisting. No signature is found
of the fast-mode wave in the SUMER data, suggesting that the plasma disturbed
by the wave had temperatures above 600 000 K. The erupting filament material is
found to emit only in spectral lines at transition region temperatures. Earlier
identification of a coronal response detected in the Mg X 609.79 A line is
found to be caused by a blend from the O IV 609.83 A line.Comment: 10 pages, 8 figures, A&A, in pres
Short-term evolution of coronal hole boundaries
The interaction of open and closed field lines at coronal hole boundaries is
widely accepted to be due to interchange magnetic reconnection. To date, it is
unclear how the boundaries vary on short timescales and at what velocity this
occurs. Here, we describe an automated boundary tracking method used to
determine coronal hole boundary displacements on short timescales. The bound-
ary displacements were found to be isotropic and to have typical
expansion/contraction speeds of \leq2 km s^-1, which indicate magnetic
reconnection rates of \leq 3 \times 10^-3. The observed displacements were used
in conjunction with the interchange reconnection model to derive typical
diffusion coeffi- cients of \leq 3 \times 10^13 cm^2 s^-1. These results are
consistent with an interchange reconnection process in the low corona driven by
the random granular motion of open and closed fields in the photosphere
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
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