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

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

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 $\geq 1\:\textrm {arcmin}^2$ 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~$95.6\%$) 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

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 $D_1$ and $D_2$ 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$^{-1}$ for the $4P_{1/2}$ state ($D_1$ line); 13 042.875 95(14)(15) cm$^{-1}$ for the $4P_{3/2}$ state ($D_2$ line); and 57.706 35(19)(15) cm$^{-1}$ for the $4P_{3/2}-4P_{1/2}$ 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 ($C$), width ($\delta$) and height ($H$) 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