115 research outputs found
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&
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&
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&
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
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 Ă…, EUVI 171 Ă… and XRT was applied.
Results.
We identified 28 blinker groups in the CDS O v 629 Ă… raster images. All CDS O v 629 Ă… blinkers showed counterparts in EUVI 171 Ă… and 304 Ă… 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 Ă… 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 Ă… 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
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
Coronal hole boundaries at small scales: IV. SOT view Magnetic field properties of small-scale transient brightenings in coronal holes
We study the magnetic properties of small-scale transients in coronal hole.
We found all brightening events are associated with bipolar regions and caused
by magnetic flux emergence followed by cancellation with the pre-existing and
newly emerging magnetic flux. In the coronal hole, 19 of 22 events have a
single stable polarity which does not change its position in time. In eleven
cases this is the dominant polarity. The dominant flux of the coronal hole form
the largest concentration of magnetic flux in terms of size while the opposite
polarity is distributed in small concentrations. In the coronal hole the number
of magnetic elements of the dominant polarity is four times higher than the
non-dominant one. The supergranulation configuration appears to preserve its
general shape during approximately nine hours of observations although the
large concentrations in the network did evolve and were slightly displaced, and
their strength either increased or decreased. The emission fluctuations seen in
the X-ray bright points are associated with reoccurring magnetic cancellation
in the footpoints. Unique observations of an X-ray jet reveal similar magnetic
behaviour in the footpoints, i.e. cancellation of the opposite polarity
magnetic flux. We found that the magnetic flux cancellation rate during the jet
is much higher than in bright points. Not all magnetic cancellations result in
an X-ray enhancement, suggesting that there is a threshold of the amount of
magnetic flux involved in a cancellation above which brightening would occur at
X-ray temperatures. Our study demonstrates that the magnetic flux in coronal
holes is continuously recycled through magnetic reconnection which is
responsible for the formation of numerous small-scale transient events. The
open magnetic flux forming the coronal-hole phenomenon is largely involved in
these transient features.Comment: 19 pages, 18 figures, A&A in pres
Diagnosing transient ionization in dynamic events
The present study aims to provide a diagnostic line ratio that will enable
the observer to determine whether a plasma is in a state of transient
ionization. We use the Atomic Data and Analysis Structure (ADAS) to calculate
line contribution functions for two lines, Si IV 1394 A and O IV 1401 A, formed
in the solar transition region. The generalized collisional-radiative theory is
used. It includes all radiative and electron collisional processes, except for
photon-induced processes. State-resolved direct ionization and recombination to
and from the next ionization stage are also taken into account. For dynamic
bursts with a decay time of a few seconds, the Si IV 1394 A line can be
enhanced by a factor of 2-4 in the first fraction of a second with the peak in
the line contribution function occurring initially at a higher electron
temperature due to transient ionization compared to ionization equilibrium
conditions. On the other hand, the O IV 1401 A does not show such any
enhancement. Thus the ratio of these two lines, which can be observed with the
Interface Region Imaging Spectrograph, can be used as a diagnostic of transient
ionization. We show that simultaneous high-cadence observations of two lines
formed in the solar transition region may be used as a direct diagnostic of
whether the observed plasma is in transient ionization. The ratio of these two
lines can change by a factor of four in a few seconds owing to transient
ionization alone.Comment: 3 pages, in press A&
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