13,174 research outputs found
Studying the Imaging Characteristics of Ultra Violet Imaging Telescope (UVIT) through Numerical Simulations
Ultra-Violet Imaging Telescope (UVIT) is one of the five payloads aboard the
Indian Space Research Organization (ISRO)'s ASTROSAT space mission. The science
objectives of UVIT are broad, extending from individual hot stars, star-forming
regions to active galactic nuclei. Imaging performance of UVIT would depend on
several factors in addition to the optics, e.g. resolution of the detectors,
Satellite Drift and Jitter, image frame acquisition rate, sky background,
source intensity etc. The use of intensified CMOS-imager based photon counting
detectors in UVIT put their own complexity over reconstruction of the images.
All these factors could lead to several systematic effects in the reconstructed
images. A study has been done through numerical simulations with artificial
point sources and archival image of a galaxy from GALEX data archive, to
explore the effects of all the above mentioned parameters on the reconstructed
images. In particular the issues of angular resolution, photometric accuracy
and photometric-nonlinearity associated with the intensified CMOS-imager based
photon counting detectors have been investigated. The photon events in image
frames are detected by three different centroid algorithms with some energy
thresholds. Our results show that in presence of bright sources, reconstructed
images from UVIT would suffer from photometric distortion in a complex way and
the presence of overlapping photon events could lead to complex patterns near
the bright sources. Further the angular resolution, photometric accuracy and
distortion would depend on the values of various thresholds chosen to detect
photon events.Comment: Submitted to PASP, 16 Pages, 9 figure
Spectroscopic Observations and Modelling of Impulsive Alfv\'en Waves Along a Polar Coronal Jet
Using the Hinode/EIS 2 spectroscopic observations, we study the intensity,
velocity, and FWHM variations of the strongest Fe XII 195.12 \AA\ line along
the jet to find the signature of Alfv\'en waves. We simulate numerically the
impulsively generated Alfv\'en waves within the vertical Harris current-sheet,
forming the jet plasma flows, and mimicking their observational signatures.
Using the FLASH code and the atmospheric model with embedded weakly expanding
magnetic field configuration within a vertical Harris current-sheet, we solve
the two and half-dimensional (2.5-D) ideal magnetohydrodynamic (MHD) equations
to study the evolution of Alfv\'en waves and vertical flows forming the plasma
jet. At a height of from the base of the jet, the
red-shifted velocity component of Fe XII 195.12 \AA\ line attains its maximum
() which converts into a blue-shifted one between the
altitude of . The spectral intensity continously increases up
to , while FWHM still exhibits the low values with almost
constant trend. This indicates that the reconnection point within the jet's
magnetic field topology lies in the corona from its
footpoint anchored in the Sun's surface. Beyond this height, FWHM shows a
growing trend. This may be the signature of Alfv\'en waves that impulsively
evolve due to reconnection and propagate along the jet. From our numerical
data, we evaluate space- and time- averaged Alfv\'en waves velocity amplitudes
at different heights in the jet's current-sheet, which contribute to the
non-thermal motions and spectral line broadening. The synthetic width of Fe XII
line exhibits similar trend of increment as in the
observational data, possibly proving the existence of impulsively generated (by
reconnection) Alfv\'en waves which propagate along the jet
On Thermal-Pulse-Driven Plasma Flows in Coronal Funnels as Observed by Hinode/EUV Imaging Spectrometer (EIS)
Using one-arcsecond-slit scan observations from the Hinode/EUV Imaging
Spectrometer (EIS) on 05 February 2007, we find the plasma outflows in the open
and expanding coronal funnels at the eastern boundary of AR 10940. The Doppler
velocity map of Fe XII 195.120 A shows that the diffuse close-loop system to be
mostly red-shifted. The open arches (funnels) at the eastern boundary of AR
exhibit blue-shifts with a maximum speed of about 10-15 km/s. This implies
outflowing plasma through these magnetic structures. In support of these
observations, we perform a 2D numerical simulation of the expanding coronal
funnels by solving the set of ideal MHD equations in appropriate VAL-III C
initial temperature conditions using the FLASH code. We implement a rarefied
and hotter region at the footpoint of the model funnel, which results in the
evolution of slow plasma perturbations propagating outward in the form of
plasma flows. We conclude that the heating, which may result from magnetic
reconnection, can trigger the observed plasma outflows in such coronal funnels.
This can transport mass into the higher corona, giving rise to the formation of
the nascent solar wind.Comment: 17 Pages; 7 Figure
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