337 research outputs found
Evolution of microflares associated with bright points in coronal holes and in quiet regions
We aim to find similarities and differences between microflares at coronal
bright points found in quiet regions and coronal holes, and to study their
relationship with large scale flares. Coronal bright points in quiet regions
and in coronal holes were observed with Hinode/EIS using the same sequence.
Microflares associated with bright points are identified from the X-ray
lightcurve. The temporal variation of physical properties was traced in the
course of microflares. The lightcurves of microflares indicated an impulsive
peak at hot emission followed by an enhancement at cool emission, which is
compatible with the cooling model of flare loops. The density was found to
increase at the rise of the impulsive peak, supporting chromospheric
evaporation models. A notable difference is found in the surroundings of
microflares; diffuse coronal jets are produced above microflares in coronal
holes while coronal dimmings are formed in quiet regions. The microflares
associated with bright points share common characteristics to active region
flares. The difference in the surroundings of microflares are caused by open
and closed configurations of the pre-existing magnetic field.Comment: 9 pages, 11 figures, accepted for publication in A&
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
2D and 3D Polar Plume Analysis from the Three Vantage Positions of STEREO/EUVI A, B, and SOHO/EIT
Polar plumes are seen as elongated objects starting at the solar polar
regions. Here, we analyze these objects from a sequence of images taken
simultaneously by the three spacecraft telescopes STEREO/EUVI A and B, and
SOHO/EIT. We establish a method capable of automatically identifying plumes in
solar EUV images close to the limb at 1.01 - 1.39 R in order to study their
temporal evolution. This plume-identification method is based on a multiscale
Hough-wavelet analysis. Then two methods to determined their 3D localization
and structure are discussed: First, tomography using the filtered
back-projection and including the differential rotation of the Sun and,
secondly, conventional stereoscopic triangulation. We show that tomography and
stereoscopy are complementary to study polar plumes. We also show that this
systematic 2D identification and the proposed methods of 3D reconstruction are
well suited, on one hand, to identify plumes individually and on the other
hand, to analyze the distribution of plumes and inter-plume regions. Finally,
the results are discussed focusing on the plume position with their
cross-section area.Comment: 22 pages, 10 figures, Solar Physics articl
Horizontal supergranule-scale motions inferred from TRACE ultraviolet observations of the chromosphere
We study horizontal supergranule-scale motions revealed by TRACE observation
of the chromospheric emission, and investigate the coupling between the
chromosphere and the underlying photosphere. A highly efficient
feature-tracking technique called balltracking has been applied for the first
time to the image sequences obtained by TRACE (Transition Region and Coronal
Explorer) in the passband of white light and the three ultraviolet passbands
centered at 1700 {\AA}, 1600 {\AA}, and 1550 {\AA}. The resulting velocity
fields have been spatially smoothed and temporally averaged in order to reveal
horizontal supergranule-scale motions that may exist at the emission heights of
these passbands. We find indeed a high correlation between the horizontal
velocities derived in the white-light and ultraviolet passbands. The horizontal
velocities derived from the chromospheric and photospheric emission are
comparable in magnitude. The horizontal motions derived in the UV passbands
might indicate the existence of a supergranule-scale magnetoconvection in the
chromosphere, which may shed new light on the study of mass and energy supply
to the corona and solar wind at the height of the chromosphere. However, it is
also possible that the apparent motions reflect the chromospheric brightness
evolution as produced by acoustic shocks which might be modulated by the
photospheric granular motions in their excitation process, or advected partly
by the supergranule-scale flow towards the network while propagating upward
from the photosphere. To reach a firm conclusion, it is necessary to
investigate the role of granular motions in the excitation of shocks through
numerical modeling, and future high-cadence chromospheric magnetograms must be
scrutinized.Comment: 5 figures, accepted by Astronomy & Astrophysic
On-disk coronal rain
Small and elongated, cool and dense blob-like structures are being reported
with high resolution telescopes in physically different regions throughout the
solar atmosphere. Their detection and the understanding of their formation,
morphology and thermodynamical characteristics can provide important
information on their hosting environment, especially concerning the magnetic
field, whose understanding constitutes a major problem in solar physics. An
example of such blobs is coronal rain, a phenomenon of thermal non- equilibrium
observed in active region loops, which consists of cool and dense chromospheric
blobs falling along loop-like paths from coronal heights. So far, only off-limb
coronal rain has been observed and few reports on the phenomenon exist. In the
present work, several datasets of on-disk H{\alpha} observations with the CRisp
Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar Telescope (SST) are
analyzed. A special family of on-disk blobs is selected for each dataset and a
statistical analysis is carried out on their dynamics, morphology and
temperatures. All characteristics present distributions which are very similar
to reported coronal rain statistics. We discuss possible interpretations
considering other similar blob-like structures reported so far and show that a
coronal rain interpretation is the most likely one. Their chromospheric nature
and the projection effects (which eliminate all direct possibility of height
estimation) on one side, and their small sizes, fast dynamics, and especially,
their faint character (offering low contrast with the background intensity) on
the other side, are found as the main causes for the absence until now of the
detection of this on-disk coronal rain counterpart.Comment: 18 pages, 10 figures. Accepted for Solar Physic
Characteristics of EUV coronal jets observed with STEREO/SECCHI
In this paper we present the first comprehensive statistical study of EUV
coronal jets observed with the SECCHI imaging suites of the two STEREO
spacecraft. A catalogue of 79 polar jets is presented, identified from
simultaneous EUV and white-light coronagraph observations, taken during the
time period March 2007 to April 2008. The appearances of the coronal jets were
always correlated with underlying small-scale chromospheric bright points. A
basic characterisation of the morphology and identification of the presence of
helical structure were established with respect to recently proposed models for
their origin and temporal evolution. A classification of the events with
respect to previous jet studies shows that amongst the 79 events there were 37
Eiffel tower-type jet events commonly interpreted as a small-scale (about 35
arcsec) magnetic bipole reconnecting with the ambient unipolar open coronal
magnetic fields at its looptops, and 12 lambda-type jet events commonly
interpreted as reconnection with the ambient field happening at the bipoles
footpoints. Five events were termed micro-CME type jet events because they
resembled the classical coronal mass ejections (CMEs) but on much smaller
scales. A few jets are also found in equatorial coronal holes. The typical
lifetimes in the SECCHI/EUVI (Extreme UltraViolet Imager) field of view between
1.0 to 1.7 solar radius and in SECCHI/COR1 field of view between 1.4 to 4 solar
radius are obtained, and the derived speed are roughly estimated. In summary,
the observations support the assumption of continuous small-scale reconnection
as an intrinsic feature of the solar corona, with its role for the heating of
the corona, particle acceleration, structuring and acceleration of the solar
wind remaining to be explored in more details in further studies.Comment: 20 pages, 7 tables of figures, 2 tables of plots, an appendix with
list event
Cyclic AMP Responsive Element Binding Proteins Are Involved in ‘Emergency’ Granulopoiesis through the Upregulation of CCAAT/Enhancer Binding Protein β
In contrast to the definitive role of the transcription factor, CCAAT/Enhancer binding protein α (C/EBPα), in steady-state granulopoiesis, previous findings have suggested that granulopoiesis during emergency situations, such as infection, is dependent on C/EBPβ. In this study, a novel lentivirus-based reporter system was developed to elucidate the molecular switch required for C/EBPβ-dependency. The results demonstrated that two cyclic AMP responsive elements (CREs) in the proximal promoter region of C/EBPβ were involved in the positive regulation of C/EBPβ transcription during granulocyte-macrophage colony-stimulating factor (GM-CSF)–induced differentiation of bone marrow cells. In addition, the transcripts of CRE binding (CREB) family proteins were readily detected in hematopoietic stem/progenitor cells. CREB was upregulated, phosphorylated and bound to the CREs in response to GM-CSF stimulation. Retroviral transduction of a dominant negative CREB mutant reduced C/EBPβ mRNA levels and significantly impaired the proliferation/differentiation of granulocyte precursors, while a constitutively active form of CREB facilitated C/EBPβ transcription. These data suggest that CREB proteins are involved in the regulation of granulopoiesis via C/EBPβ upregulation
Molecular Basis of Increased Serum Resistance among Pulmonary Isolates of Non-typeable Haemophilus influenzae
Non-typeable Haemophilus influenzae (NTHi), a common commensal of the human pharynx, is also an opportunistic pathogen if it becomes established in the lower respiratory tract (LRT). In comparison to colonizing isolates from the upper airway, LRT isolates, especially those associated with exacerbations of chronic obstructive pulmonary disease, have increased resistance to the complement- and antibody-dependent, bactericidal effect of serum. To define the molecular basis of this resistance, mutants constructed in a serum resistant strain using the mariner transposon were screened for loss of survival in normal human serum. The loci required for serum resistance contribute to the structure of the exposed surface of the bacterial outer membrane. These included loci involved in biosynthesis of the oligosaccharide component of lipooligosaccharide (LOS), and vacJ, which functions with an ABC transporter encoded by yrb genes in retrograde trafficking of phospholipids from the outer to inner leaflet of the cell envelope. Mutations in vacJ and yrb genes reduced the stability of the outer membrane and were associated with increased cell surface hyrophobicity and phospholipid content. Loss of serum resistance in vacJ and yrb mutants correlated with increased binding of natural immunoglobulin M in serum as well as anti-oligosaccharide mAbs. Expression of vacJ and the yrb genes was positively correlated with serum resistance among clinical isolates. Our findings suggest that NTHi adapts to inflammation encountered during infection of the LRT by modulation of its outer leaflet through increased expression of vacJ and yrb genes to minimize recognition by bactericidal anti-oligosaccharide antibodies
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