8,503 research outputs found
Frequency-dependent damping in propagating slow magneto-acoustic waves
Propagating slow magneto-acoustic waves are often observed in polar plumes
and active region fan loops. The observed periodicities of these waves range
from a few minutes to few tens of minutes and their amplitudes were found to
decay rapidly as they travel along the supporting structure. Previously,
thermal conduction, compressive viscosity, radiation, density stratification,
and area divergence, were identified to be some of the causes for change in the
slow wave amplitude. Our recent studies indicate that the observed damping in
these waves is frequency dependent. We used imaging data from SDO/AIA, to study
this dependence in detail and for the first time from observations we attempted
to deduce a quantitative relation between damping length and frequency of these
oscillations. We developed a new analysis method to obtain this relation. The
observed frequency dependence does not seem to agree with the current linear
wave theory and it was found that the waves observed in the polar regions show
a different dependence from those observed in the on-disk loop structures
despite the similarity in their properties.Comment: accepted for publication in Ap
Propagating Disturbances along fan-like coronal loops in an active region
Propagating disturbances are often observed in active region fan-like coronal
loops. They were thought to be due to slow mode MHD waves based on some of the
observed properties. But the recent studies involving spectroscopy indicate
that they could be due to high speed quasi-periodic upflows which are difficult
to distinguish from upward propagating slow waves. In this context, we have
studied a fan loop structure in the active region AR 11465 using simultaneous
spectroscopic and imaging observations from Extreme-ultraviolet Imaging
Spectrometer (EIS) on board Hinode and Atmospheric Imaging Assembly (AIA) on
board SDO. Analysis of the data shows significant oscillations at different
locations. We explore the variations in different line parameters to determine
whether the waves or flows could cause these oscillations to improve the
current understanding on the nature of these disturbances.Comment: 12 pages, 6 figures. Accepted for publication in RA
Omnipresent long-period intensity oscillations in open coronal structures
Quasi-periodic propagating disturbances in coronal structures have been
interpreted as slow magneto-acoustic waves and/or periodic upflows. Here we aim
to understand their nature from the observed properties using a three-hour
imaging sequence from AIA/SDO in two different temperature channels. We also
compare the characteristics with a simple wave model. We searched for
propagating disturbances in open-loop structures at three different locations;
a fan loop-structure off-limb, an on-disk plume-like structure and the
plume/interplume regions in the north pole of the sun. In each of the subfield
regions chosen to cover these structures, the time series at each pixel
location was subjected to wavelet analysis to find the different periodicities.
We then constructed powermaps in three different period ranges. We also
constructed space-time maps for the on-disk plume structure to estimate the
propagation speeds in different channels. We find propagating disturbances in
all three structures. Powermaps indicate that the power in the long-period
range is significant up to comparatively longer distances along the loop than
that in the shorter periods. This nature is observed in all three structures. A
detailed analysis on the on-disk plume structure gives consistently higher
propagation speeds in the 193 \AA channel and also reveals spatial damping
along the loop. The amplitude and the damping length values are lower in hotter
channels, indicating their acoustic dependence. These properties can be
explained very well with a propagating slow-wave model. We suggest that these
disturbances are more likely to be caused by propagating slow magneto-acoustic
waves than by high-speed quasi-periodic upflows. We find that intensity
oscillations in longer periods are omnipresent at larger heights even in active
regions.Comment: accepted for publication in A &
The effects of rotational flow, viscosity, thickness, and shape on transonic flutter dip phenomena
The transonic flutter dip phenomena on thin airfoils, which are employed for propfan blades, is investigated using an integrated Euler/Navier-Stokes code and a two degrees of freedom typical section structural model. As a part of the code validation, the flutter characteristics of the NACA 64A010 airfoil are also investigated. In addition, the effects of artificial dissipation models, rotational flow, initial conditions, mean angle of attack, viscosity, airfoil thickness and shape on flutter are investigated. The results obtained with a Euler code for the NACA 64A010 airfoil are in reasonable agreement with published results obtained by using transonic small disturbance and Euler codes. The two artificial dissipation models, one based on the local pressure gradient scaled by a common factor and the other based on the local pressure gradient scaled by a spectral radius, predicted the same flutter speeds except in the recovery region for the case studied. The effects of rotational flow, initial conditions, mean angle of attack, and viscosity for the Reynold's number studied seem to be negligible or small on the minima of the flutter dip
A complete description of the magnetic ground state in spinel vanadates
Capturing the non-collinear magnetic ground state of the spinel vanadates
AVO (A= Mn, Fe and Co) remains an outstanding challenge for
state-of-the-art ab-initio methods. We demonstrate that both the non-collinear
spin texture, as well as the magnitude of local moments, are captured by a
single value of the on-site Hubbard of 2.7~eV in conjunction with the local
spin density approximation (LSDA+), provided the source term (i.e., magnetic
monopole term) is removed from the exchange-correlation magnetic field . We further demonstrate that the magnetic monopole structure in is highly sensitive to the value of , to the extent that the
interplay between on-site localization and local moment magnitude is
qualitatively different depending on whether the source term is removed or not.
This suggests that in treating strongly correlated magnetic materials within
the LSDA+ formalism, subtraction of the unphysical magnetic monopole term
from the exchange-correlation magnetic field is essential to correctly treat
the magnetic ground state.Comment: 4 pages, 3 figure
Soft tissue neoplasms: a clinicopathological study
Background: The current WHO classification has categorized soft tissue tumours into benign, malignant and so-called intermediate neoplasms. Soft tissue sarcoma comprises <1% of adult cancers. The aim of the study was to clinically correlate soft tissue neoplasms and study the histomorphological features of various malignant soft tissue tumors.Methods: This was a retrospective study conducted in a tertiary care hospital in Mangalore, India from January 2019 to June 2020. Clinical details of all cases of soft tissue neoplasms retrieved from the medical records of our institution. Data collected included age, gender, presenting symptoms, site and size of soft tissue neoplasms and clinical diagnosis. Pathological diagnosis of these tumours was made and details recorded.Results: A total of 113 cases of soft tissue neoplasms were collected. 94.4% benign and 18.6 %. malignant tumours were present. 77% cases presented with swelling whereas 23% presented with pain. Majority of benign soft tissue tumours were located in the trunk (36.9%) and the most common type was lipomas (66.38%). Malignant soft tissue tumours showed male to female ratio of 1.33:1. Most predilection was noted for the extremities (42.8%) and leiomyosarcomas were the most common type (38%).Conclusions: The incidence of malignant soft tissue tumours is rare. Majority of the cases were noted in the extremities. Leiomyosarcoma was the most common type, in our study. Lipomas were the most common benign soft tissue tumours, and majority of the benign tumours were located in the trunk.
Nanoscale quantum dot infrared sensors with photonic crystal cavity
We report high performance infrared sensors that are based on intersubband transitions in nanoscale self-assembled quantum dots combined with a microcavity resonator made with a high-index-contrast two-dimensional photonic crystal. The addition of the photonic crystal cavity increases the photocurrent, conversion efficiency, and the signal to noise ratio (represented by the specific detectivity D*) by more than an order of magnitude. The conversion efficiency of the detector at Vb=–2.6 V increased from 7.5% for the control sample to 95% in the PhC detector. In principle, these photonic crystal resonators are technology agnostic and can be directly integrated into the manufacturing of present day infrared sensors using existing lithographic tools in the fabrication facility
The effects of transients on photospheric and chromospheric power distributions
We have observed a quiet Sun region with the Swedish 1-meter Solar Telescope
(SST) equipped with CRISP Imaging SpectroPolarimeter. High-resolution,
high-cadence, H line scanning images were taken to observe different
layers of the solar atmosphere from the photosphere to upper chromosphere. We
study the distribution of power in different period-bands at different heights.
Power maps of the upper photosphere and the lower chromosphere show suppressed
power surrounding the magnetic-network elements, known as "magnetic shadows".
These also show enhanced power close to the photosphere, traditionally referred
to as "power halos". The interaction between acoustic waves and inclined
magnetic fields is generally believed to be responsible for these two effects.
In this study we explore if small-scale transients can influence the
distribution of power at different heights. We show that the presence of
transients, like mottles, Rapid Blueshifted Excursions (RBEs) and Rapid
Redshifted Excursions (RREs), can strongly influence the power-maps. The short
and finite lifetime of these events strongly affects all powermaps, potentially
influencing the observed power distribution. We show that Doppler-shifted
transients like RBEs and RREs that occur ubiquitously, can have a dominant
effect on the formation of the power halos in the quiet Sun. For magnetic
shadows, transients like mottles do not seem to have a significant effect in
the power suppression around 3 minutes and wave interaction may play a key role
here. Our high cadence observations reveal that flows, waves and shocks
manifest in presence of magnetic fields to form a non-linear
magnetohydrodynamic system.Comment: 11 pages, 11 Figures, 4 movies (will be available online in ApJ). ApJ
(accepted
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