202 research outputs found
Iris si iv line profiles: An indication for the plasmoid instability during small-scale magnetic reconnection on the sun
Our understanding of the process of fast reconnection has undergone a
dramatic change in the last 10 years driven, in part, by the availability of
high-resolution numerical simulations that have consistently demonstrated the
break-up of current sheets into magnetic islands, with reconnection rates that
become independent of Lundquist number, challenging the belief that fast
magnetic reconnection in flares proceeds via the Petschek mechanism that
invokes pairs of slow-mode shocks connected to a compact diffusion region. The
reconnection sites are too small to be resolved with images but these
reconnection mechanisms, Petschek and the plasmoid instability, have
reconnection sites with very different density and velocity structures and so
can be distinguished by high-resolution line-profiles observations. Using IRIS
spectroscopic observations we obtain a survey of typical line profiles produced
by small-scale events thought to be reconnection sites on the Sun. Slit-jaw
images are used to investigate the plasma heating and re-configuration at the
sites. A sample of 15 events from two active regions is presented. The line
profiles are complex with bright cores and broad wings extending to over 300
km/s. The profiles can be reproduced with the multiple magnetic islands and
acceleration sites that characterise the plasmoid instability but not by
bi-directional jets that characterise the Petschek mechanism. This result
suggests that if these small-scale events are reconnection sites, then fast
reconnection proceeds via the plasmoid instability, rather than the Petschek
mechanism during small-scale reconnection on the Sun.Comment: 10 pages, 18 Figures, to be published in Ap
Observations of supra-arcade fans: instabilities at the head of reconnection jets
Supra-arcade fans are bright, irregular regions of emission that develop
during eruptive flares, above flare arcades. The underlying flare arcades are
thought to be a consequence of magnetic reconnection along a current sheet in
the corona. At the same time, theory predicts plasma jets from the reconnection
site which would be extremely difficult to observe directly because of their
low density. It has been suggested that the dark supra-arcade downflows (SADs)
seen falling through supra-arcade fans may be low density jet plasma. The head
of a low density jet directed towards higher density plasma would be
Rayleigh-Taylor unstable, and lead to the development of rapidly growing low
and high density fingers along the interface. Using SDO/AIA 131A images, we
show details of SADs seen from three different orientations with respect to the
flare arcade and current sheet, and highlight features that have been
previously unexplained, such as the splitting of SADs at their heads, but are a
natural consequence of instabilities above the arcade. Comparison with 3-D
magnetohydrodynamic simulations suggests that supra-arcade downflows are the
result of secondary instabilities of the Rayleigh-Taylor type in the exhaust of
reconnection jets.Comment: 10 pages, 7 figures To be published in ApJ, 796, 27 (2014
Ecological Balance of Oral Microbiota is Required to Maintain Oral Mesenchymal Stem Cell Homeostasis
Oral microbiome is essential for maintenance of oral cavity health. Imbalanced oral microbiome causes periodontal and other diseases. It is unknown whether oral microbiome affect oral stem cells function. In this study, we used a common clinical anti-biotic treatment approach to alter oral microbiome ecology and examine whether oral mesenchymal stem cells (MSCs) are affected. We found that altered oral microbiome resulted gingival MSCs deficiency, leading to a delayed wound healing in male mice. Mechanistically, oral microbiome release LPS that stimulates the expression of microRNA-21 (miR-21) and then impair the normal function of gingival MSCs and wound healing process through miR-21/Sp1/TERT pathway. This is the first study indicate that interplay between oral microbiome and MSCs homeostasis in male mice
The Fas/Fap-1/Cav-1 Complex Regulates IL-1RA Secretion in Mesenchymal Stem Cells to Accelerate Wound Healing
Mesenchymal stem cells (MSCs) are capable of secreting exosomes, extracellular vesicles, and cytokines to regulate cell and tissue homeostasis. However, it is unknown whether MSCs use a specific exocytotic fusion mechanism to secrete exosomes and cytokines. We show that Fas binds with Fas-associated phosphatase–1 (Fap-1) and caveolin-1 (Cav-1) to activate a common soluble N-ethylmaleimide–sensitive factor (NSF) attachment protein receptor (SNARE)–mediated membrane fusion mechanism to release small extracellular vesicles (sEVs) in MSCs. Moreover, we reveal that MSCs produce and secrete interleukin-1 receptor antagonist (IL-1RA) associated with sEVs to maintain rapid wound healing in the gingiva via the Fas/Fap-1/Cav-1 cascade. Tumor necrosis factor–α (TNF-α) serves as an activator to up-regulate Fas and Fap-1 expression via the nuclear factor κB pathway to promote IL-1RA release. This study identifies a previously unknown Fas/Fap-1/Cav-1 axis that regulates SNARE-mediated sEV and IL-1RA secretion in stem cells, which contributes to accelerated wound healing
Solar Flare Termination Shock and the Synthetic Fe XXI 1354.08 Line
Solar flares are one of the most energetic phenomena occurred in the solar system. In the standard solar flare model, a fast mode shock, which is often referred to as the flare termination shock (TS), can exist above the loop-top source of hard X-ray emissions. The existence of the termination shock has been recently related to spectral hardening of flare hard X-ray spectrum at energies > 300 keV. Observations of the Fe XXI 1354.08 line during solar flares by the IRIS spacecraft have found significant redshift with >100 km/s, which is consistent with a reconnection downflow. The ability to identify such a redshift by IRIS is made possible by IRIS's high time resolution, high spatial resolution, high sensitivity and cadence spectral observations. The ability to identify such a redshift by IRIS suggests that one may be able to use IRIS observations to identify flare termination shocks. Using a MHD simulation to model magnetic reconnection of a solar flare and assuming the existence of a TS in the downflow of the reconnection plasma, we model the synthetic emission of the Fe XXI 1354.08 line in this work. We show that the existence of the TS in the solar flare may manifest itself from the Fe XXI 1354.08 line.Plain Language SummaryA solar flare is a brief eruption of intense high-energy radiation from the sun's surface, associated with sunspots and causing electromagnetic disturbances on the earth, as with radio frequency communications and power line transmissions. The standard solar flare model proposes that magnetic reconnection drives solar flares and fast mode shock, which is often referred to as the flare termination shock (TS), can exist at the reconnection outflows. The existence of the termination shock has been recently related to spectral hardening of flare hard X-ray spectrum
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