294 research outputs found
Quasi-Periodic Oscillation of a Coronal Bright Point
Coronal bright points (BPs) are small-scale luminous features seen in the
solar corona. Quasi-periodic brightenings are frequently observed in the BPs
and are generally linked with underneath magnetic flux changes. We study the
dynamics of a BP seen in the coronal hole using the Atmospheric Imaging
Assembly (AIA) images, the Helioseismic and Magnetic Imager (HMI) magnetogram
on board the Solar Dynamics Observatory (SDO) and spectroscopic data from the
newly launched Interface Region Imaging Spectrograph (IRIS). The detailed
analysis shows that the BP evolves throughout our observing period along with
changes in underlying photospheric magnetic flux and shows periodic
brightenings in different EUV and FUV images. With highest possible spectral
and spatial resolution of IRIS, we attempted to identify the sources of these
oscillations. IRIS sit and stare observation provided a unique opportunity to
study the time evolution of one foot point of the BP as the slit position
crossed it. We noticed enhanced line profile asymmetry, enhanced line width,
intensity enhancements and large deviation from the average Doppler shift in
the line profiles at specific instances which indicate the presence of sudden
flows along the line of sight direction. We propose that transition region
explosive events (EEs) originating from small scale reconnections and the
reconnection outflows are affecting the line profiles. The correlation between
all these parameters is consistent with the repetitive reconnection scenario
and could explain the quasi-periodic nature of the brightening.Comment: 5 Pages, 5 figures, Accepted for publication in Ap
Flows and Waves in Braided Solar Coronal Magnetic Structures
We study the high frequency dynamics in the braided magnetic structure of an
active region (AR 11520) moss as observed by High-Resolution Coronal Imager
(Hi-C). We detect quasi periodic flows and waves in these structures. We search
for high frequency dynamics while looking at power maps of the observed region.
We find that shorter periodicites (30 - 60 s) are associated with small spatial
scales which can be resolved by Hi-C only. We detect quasi periodic flows with
wide range of velocities from 13 - 185 km/s associated with braided regions.
This can be interpreted as plasma outflows from reconnection sites. We also
find presence of short period and large amplitude transverse oscillations
associated with braided magnetic region. Such oscillations could be triggered
by reconnection or such oscillation may trigger reconnection.Comment: 6 pages, 5 figures, Accepted for publication in ApJ Letter
Butterfly Diagram and Carrington Maps for Century-Long Ca II K Spectroheliograms from Kodaikanal Observatory
The century-long (1907-2007) Ca II K spectroheliograms from Kodaikanal Solar
Observatory (KSO) are calibrated, processed and analysed in the present study
to follow the evolution of bright on disc structures called plages, the
possible representatives of magnetic activity on the Sun. This has been the
longest dataset studied in Ca II K till date covering about 9.5 cycles of 11
year periods. Plages were segmented with area
using global thresholds for individual full disc images and subsequent
application of morphological closing operation. Plage index was calculated and
seen to have close positive correlation with fractional disc area covered by
plages. The newly generated plage area cycle (from KSO) was compared with the
same from Mount Wilson observatory (Correlation~) for the overlapping
years i.e. 1915-2000. Study illustrated time-latitude distribution of plage
centroids rendering butterfly diagram (as observed for sunspots). The 3D
visualisation of the diagram showed one to one mapping between plage location,
time and area. This work further delineated positional correlation between
magnetic patches and plage regions through comparison of synoptic maps derived
from both Kodaikanal Ca II K images and space based full disc LOS (line of
sight) magnetograms. Regular synoptic magnetograms from ground based
observatories are available only after 1970s. Thus the long term Ca II K data
from KSO can be used as a proxy for estimating magnetic activity locations and
their strengths at earlier times.Comment: 22 pages, 14 figures, accepted for publication in ApJ, additional
maps and movies are available online in the link-
ftp://ftp.iiap.res.in/subhamoy/cak_carrington_maps_kodaikanal
Variation of Supergranule Parameters With Solar Cycles: Results From Century-Long Kodaikanal Digitized Ca II K Data
The century-long (1907-2007) Ca II K spectroheliograms from Kodaikanal Solar
Observatory, India, has recently been digitised and calibrated. Applying a
fully-automated algorithm (which includes contrast enhancement and 'Watershed
method') on this data, we have identified the supergranules and calculated the
associated parameters, such as scale, circularity, fractal dimension. We have
segregated the quiet and active regions and obtained the supergranule
parameters separately for these two domains. In this way, we have isolated the
effect of large and small scale magnetic fields on such structures and find
significantly different behavior of the supergranule parameters over solar
cycles. Such differences indicate the intrinsic changes in the physical
mechanism behind generation and evolution of supergranules in presence of small
and large scale magnetic fields. This also highlights the need for further
studies using solar dynamo theory along with magneto-convection models.Comment: 24 pages, 10 figures, accepted for publication in Ap
Propagating Disturbances in The Solar Corona and Spicular Connection
Spicules are small hairy like structures seen at the solar limb mainly at
chromospheric and transition region lines. They generally live for 3-10
minutes. We observe these spicules in a south polar region of the Sun with a
coordinated observations using the Interface Region Imaging Spectrograph (IRIS)
and the Atmospheric Imaging Assembly (AIA) instruments on board the Solar
Dynamics Observatory. Propagating disturbances (PDs) are observed everywhere in
the polar off-limb regions of the Sun at coronal heights. From this
simultaneous observations we show that the spicules and the PDs may be
originated by a common process. From space-time maps we find that the start of
the trajectory of PDs is almost co-temporal with the time of the rise of the
spicular envelope as seen by IRIS slit-jaw images at 2796 {\deg}A and 1400
A{\deg} . During the return of spicular material, brightenings are seen in AIA
171 {\deg}A and 193 {\deg}A images. The quasi-periodic nature of the spicular
activity as revealed by the IRIS spectral image sequences and its relation to
coronal PDs as recorded by the coronal AIA channels suggest that they have a
common origin. We propose that reconnection like processes generate the
spicules and waves simultaneously. The waves escape while the cool spicular
material falls backComment: 6 pages, 4 figures, Accepted for publication in ApJ Letter
Precise fine-structure and hyperfine-structure measurements in Rb
We demonstrate a new technique for measuring the absolute frequencies of
atomic transitions. The technique uses a ring-cavity resonator whose length is
calibrated using a reference laser locked to the D_2 line in {87}Rb. The
frequency of this line is known to be 384 230 484.468(10) MHz. Using a second
laser locked to the D_1 line of Rb, we measure the frequencies of various
hyperfine transitions in the D_1 and D_2 lines with a precision of 30 kHz. We
obtain the following values: 120.687(17) MHz and 406.520(25) MHz for the
5P_{1/2} hyperfine constant A in {85}Rb and {87}Rb; 377 107 385.623(50) MHz and
377 107 463.209(50) MHz for the D_1 line in {85}Rb and {87}Rb; and 384 230
406.528(50) MHz for the D_2 line in {85}Rb. This yields the fine-structure
interval and the isotope shifts. The precision obtained is a significant
improvement over previous measurements.Comment: 4 pages, 4 figure
Association Of Supergranule Mean Scales with Solar Cycle Strengths and Total Solar Irradiance
We analyze the long-term behavior of supergranule scale parameter, in active
and quiet regions (AR, QR), using the Kodaikanal digitized data archive. This
database provides century-long daily full disc observations of the Sun in Ca-II
K wavelength. In this paper, we study the distributions of the supergranular
scales, over the whole data duration, which show identical shape in these two
regimes. We found that the AR mean scale values are always higher than that of
the QR for every solar cycle. The mean scale values are highly correlated with
the sunspot number cycle amplitude and also with total solar irradiance (TSI)
variations. Such correlation establishes the cycle-wise mean scale as a
potential calibrator for the historical data reconstructions. We also see an
upward trend in the mean scales, as already been reported in TSI. This may
provide new input for climate forcing models. These results also give us
insight into the different evolutionary scenarios of the supergranules in the
presence of strong (AR) and weak (QR) magnetic fields.Comment: Accepted for publications in Ap
Precise frequency measurements of the D-lines and fine-structure interval in K
We use a diode laser locked to a Rb transition as the frequency reference
along with a scanning Michelson interferometer to make precise measurements on
the and lines of potassium. The Rb reference frequency is known
with sub-MHz accuracy. We obtain the following values for the energy levels: 12
985.169 60(13)(15) cm for the state ( line); 13 042.875
95(14)(15) cm for the state ( line); and 57.706
35(19)(15) cm for the fine-structure interval. The
leading source of systematic error cancels in the determination of the
interval. The errors represent about an order of magnitude improvement over
tabulated values.Comment: 7 pages, 1 figur
Near-infrared properties of classical novae: a perspective gained from Mount Abu Infrared Observatory
We review the near-infrared properties of classical novae in the J, H and K
bands at wavelengths between 1.08 to 2.4 micron. A classification system exists
for the early post-outburst optical spectra of novae on the basis of the
strength of group of non-hydrogen emission lines. A similar scheme for the
near-infrared regime, which is not available at present, is presented here. In
the optical system there are two principal classes, namely, "Fe II" and "He/N"
for novae with either prominent Fe II lines or prominent "He/N" lines. There is
also a small subset of the hybrid Fe IIb type. From spectroscopic observations
we show the differences and similarities between these classes of novae in the
near-infrared. The spectral lines common to the two principal classes arise
from H, He, N and O. However, the near-IR features that separate these two
classes are the numerous, and often strong, Carbon lines which are seen only in
the spectra of the Fe II class of novae. The dust formation process in novae is
discussed based on broad-band observations. The first-overtone carbon monoxide
(CO) detections in novae are analyzed to understand the formation and evolution
of this molecule in the nova ejecta and to discuss the observed 12C/13C ratio.Comment: Review article in Bulletin of the Astron. Soc. of India, Volume
dedicated to Novae Research, 2012, 40, 243-26
Latitude Distribution of Sunspots: Analysis Using Sunspot Data and A Dynamo Model
In this paper, we explore the evolution of sunspot latitude distribution and
explore its relations with the cycle strength. With the progress of the solar
cycle, the distributions in two hemispheres from mid-latitudes propagate toward
the equator and then (before the usual solar minimum) these two distributions
touch each other. By visualizing the evolution of the distributions in two
hemispheres, we separate the solar cycles by excluding this hemispheric
overlap. From these isolated solar cycles in two hemispheres, we generate
latitude distributions for each cycle, starting from cycle 8 to cycle 23. We
find that the parameters of these distributions, namely, the central latitude
(), width () and height () evolve with the cycle number and they
show some hemispheric asymmetries. Although the asymmetries in these parameters
persist for a few successive cycles, they get corrected within a few cycles and
the new asymmetries appear again. In agreement with the previous study, we find
that distribution parameters are correlated with the strengths of the cycles,
although these correlations are significantly different in two hemispheres. The
general trend that stronger cycles begin sunspot eruptions at relatively higher
latitudes and have wider bands of sunspot emergence latitudes are confirmed
when combining the data from two hemispheres. We explore these features using a
flux transport dynamo model with stochastic fluctuations. We find that these
features are correctly reproduced in this model. The solar cycle evolution of
the distribution center is also in good agreement with observations. Possible
explanations of the observed features based on this dynamo model are presented.Comment: Accepted for publication in Ap
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