DSpace Collection:
http://hdl.handle.net/2248/93
Fri, 15 Feb 2019 04:35:49 GMT2019-02-15T04:35:49ZWaves in solar atmosphere and their role in dynamics of the corona
http://hdl.handle.net/2248/7392
Title: Waves in solar atmosphere and their role in dynamics of the corona
Authors: Ajanta, Datta
Abstract: Solar atmosphere is stratified and exhibits dynamic structures with confined hot plasmas. The corona consists of dynamic magnetic structures,in the form of closed and open loops with an enhanced plasma density . Moreover , these elastic and compressible structures support different types of waves , playing the role of the wave guide. The waves can be described using magnetohydrodynamics (MHD).
Description: Thesis Supervisor Prof. Dipankar BanerjeeMon, 01 May 2017 00:00:00 GMThttp://hdl.handle.net/2248/73922017-05-01T00:00:00ZThe Search for the Electric Dipole Moment of the Electron (eEDM) in Mercury Halides Using the Relativistic Coupled Cluster Theory
http://hdl.handle.net/2248/7314
Title: The Search for the Electric Dipole Moment of the Electron (eEDM) in Mercury Halides Using the Relativistic Coupled Cluster Theory
Authors: Srinivasa Prasannaa, V
Abstract: The electric dipole moment of the electron (eEDM) could arise due to the simultaneous
violations of parity (P) and time reversal (T) symmetries. If observed, it would be an in-
trinsic property of the electron, like its mass and spin. If the CPT theorem holds, then T
violation would imply CP violation. The Standard Model of particle physics (SM) predicts
an extremely small value for the eEDM; less than 1038 e cm. Extensions of SM, like sev-
eral variants of supersymmetry, however, predict much larger ranges of eEDM. Therefore,
upper bounds on eEDMs from experiments are useful in constraining theories beyond the
SM. Since one of the necessary conditions for the matter-antimatter asymmetry is CP vi-
olation, and since CP violation is also necessary for the eEDM, the latter can throw light
into the baryon asymmetry problem.
Since an electron accelerates away in the presence of an external electric field, therefore
making accurate measurements impossible, neutral systems like atoms or molecules, typ-
ically with one unpaired electron, are used to measure eEDM. Heavy polar diatomics are
preferred over atoms, due to their experimental sensitivity being much larger. A shift in
the energy of a diatomic in some state, due to the outer electron having an eEDM, is equal
to the product of the negative of the eEDM and an effective electric field that the unpaired
electron experiences, due to the other electrons and nuclei. This effective electric field,
Eeff , cannot be measured, and has to be calculated using relativistic quantum many-body
theory. A molecule with a large Eeff is preferred, for better sensitivity in experiment,
among other factors. It is also important that the molecule itself has a reasonably large
molecular electric dipole moment (PDM).
In this thesis, mercury monohalides (HgF, HgCl, HgBr, and HgI) have been considered, and
identified as promising candidates for future eEDM search experiments. A fully relativistic
coupled cluster theory was employed, in the singles and doubles approximation (CCSD),
to calculate the effective fields and the PDMs of these molecules. None of the occupied or-
bitals were frozen, in our calculations. Only the linear terms in the expansion of the CCSD
wave function were included, in the expectation value expressions for the properties. The
effective electric fields are about 100 GV/cm, for all of these molecules, which is one and a half times that in ThO, the molecule that provides the current best limit on eEDM. In our
work, we had also identified that the mercury monohalides are experimentally very attractive,
and among the HgX, HgBr may hold a lot of promise.
The alkaline earth monofluorides (AEMs), a class of molecules that are useful to probe
several aspects of fundamental physics, were also studied, to try understand the interplay
between relativistic and correlation effects. The PDMs were calculated, at the double,
triple, and the quadruple zeta levels of basis. The accuracy of our calculations were also
established in this work, with the calculated PDMs (at the quadruple zeta level) agreeing
very well with experiment. It was observed that the correlation effects increase as the
molecule becomes progressively heavier. We also performed an in-depth analysis of the
terms that make the PDM.
In subsequent works, the effective electric fields and the PDMs of HgX were also analyzed.
The expressions for both the properties, at the Dirac-Fock (DF) as well as the correlation
levels, have been broken down into its constituent terms, to identify where the leading contributions
come from, and thereby attempt to explain why some terms are larger than the
others. The PDMs in HgX follow an unusual trend; the effect of correlations is to reduce
the PDM. In fact, in HgI, the DF value is reduced by more than half, because of the correlation
effects!
It was observed from our analysis of the effective electric fields of HgX that at the DF level,
the mixing between the s and p1=2 orbitals, of the heavier atom contribute the most, at the
DF level. The mixing from the lighter atom is almost always negligible, even in the case of
HgI!We then choose HgF as a representative candidate, and look at the various correlation
terms, to get insights into what type of matrix elements contribute, and also to understand
which terms cancel each other out, and to what extent. Finally, we compared our work with
earlier ones (on HgF), in a detailed manner, and show how our approach is very accurate,
as compared to earlier works.
The thesis chapters are structured as follows: Chapter one gives an introduction to the
eEDM. It briefly discusses why P and T need to be simultaneously violated for a non-zero
eEDM. It also discusses how the eEDM can be measured, and illustrates that a combination
of theory and experiment is necessary, to determine the eEDM. The chapter also discusses
briefly about higher order electric and magnetic moments. Chapter two is a review of
many-electron theory. It starts with the Born-Oppenheimer approximation, and proceeds
to introduce the use of second quantization in many-body theory. The Hartree-Fock equations,
their relativistic version, and the matrix formulation of the theory are discussed. The
chapter then discusses electron correlation, and theories that include this effect, like configuration
interaction and many-body perturbation theory. The third chapter is a detailed discussion on the relativistic coupled cluster method. The chapter, towards the end, also
covers details of how we tested the accuracy of our coupled cluster method in the SrF
molecule. It then explains the interplay between the relativistic and correlation effects in
the alkaline earth monofluoride systems. Chapter four and five are the core chapters of the
thesis. In chapter four, mercury monohalides are identified as promising eEDM search candidates.
We do this by calculating the effective electric fields of the molecules, and show
that they are extremely large. We also discuss the experimental advantages that they offer,
briefly. Chapter five elaborates on the previous one, where we perform an in-depth analysis
of the effective fields. Chapter six discusses the PDMs of HgX, since the property also
plays a role in determining the sensitivity of the experiment. Chapter seven mentions the
future work, and includes basis set dependence, the finite field method and our preliminary
results using this approach, as well as the mercury alkalis as possible candidates for eEDM
search experiments.
Description: Thesis Supervisor Prof. Bhanu Pratap Das and Co-supervisor Prof. Vishnu Mayya BannurThu, 01 Jun 2017 00:00:00 GMThttp://hdl.handle.net/2248/73142017-06-01T00:00:00ZGeometrical and topological properties of CMB polarization fields
http://hdl.handle.net/2248/7313
Title: Geometrical and topological properties of CMB polarization fields
Authors: Vidhya, G
Abstract: Cosmic Microwave Background (CMB) is a relic from the early Universe. It was generated due to the physical processes in the early Universe during an epoch known as the recombination or decoupling epoch. The CMB has highly uniform temperature over the entire sky but with small variations in different directions. Due to the Thomson scattering between photons and electrons and also because of the quadrupole anisotropies induced in the cosmic plasma during the decoupling epoch, the CMB is linearly polarized. The CMB radiation in each line of sight is associated with temperature (T) and polarization. The polarization can be decomposed into Stokes parameters Q=U, or E mode (E) and B mode (B) fields. Here, Q=U fields transform as spin _2 objects under rotation transformation while the E=B fields remain invariant. The fluctuations observed in CMB is due to the quantum fluctuations generated during the inflationary phase, which is a period of exponential expansion moments after the Big Bang in the early Universe. The CMB fluctuations will have statistical properties similar to this primordial fluctuations only for the linear evolution of fluctuations. Statistical observable can be used to capture the morphological properties of the CMB fluctuations. Then these morphological properties can be studied in relation to the parameters describing the physical mechanisms of the inflationary phase. In this research work, we use the geometrical and topological observables to study the CMB polarization fields and we also introduce a novel statistical observable for the analysis of CMB fields.
The models about the inflationary phase predict that the Probability Distribution Function (PDF) of primordial fluctuations are close to the Gaussian distribution but with small deviation. The information about the exact form of deviation in the PDF of primordial fluctuation will be encapsulated in the CMB fields. We investigate the local type non-Gaussian features in the CMB polarization fields, which is parametrized by fNL. We derive the analytic expression for the PDF of any general local type non-Gaussian field such as the T and E fields of CMB, and also for the local type non-Gaussian polarization intensity (IP ) constructed from local type non-Gaussian Q and U fields. We use the analytic expression and simulations of local type non-Gaussian CMB fields, namely T, E and IP , and study the deviation in their PDF relative to the Gaussian PDF. We found from the analytic expression that the non-Gaussian deviation in the PDF of the T and E fields are proportional to (fNL 𝛿) 2 while that of IP field is proportional to (fNL 𝛿)2. The numerical calculations show that the non-Gaussian deviation in the PDF of E field is similar to that of the T field. While the non-Gaussian deviation corresponding to the IP field has smaller amplitude and large error bars in comparison to that of T field. This analysis was repeated using the geometrical and topological observables, namely Scalar Minkowski Functionals (SMFs) and Betti numbers of fields. These observables capture different morphological features of a given field. The results obtained using these observablesare similar to those from the PDF of the fields. Hence from the theoretical point of view, these results imply that the E field can provide an independent constraint on fNL similar to the T field. Further, the results also show that when the IP field is used independently for such analysis, it cannot provide any statistically significant information. In the realistic scenario, the observational data contains instrumental systematics which will lead to the reduction in the statistical significance of the above results.
The CMB polarization is usually analyzed using the E=B fields as they are scalar fields. We investigate the theoretical aspects of using the Q=U fields as a complementary analysis of CMB polarization. We show that the variance of Q=U and its gradient fields are invariant under rotation transformation, and hence the invariance of the SMFs of a Gaussian Q=U fields. However, this statement breaks down for incomplete sky. Then we studied the non-Gaussian deviation in Q=U fields constructed from the simulations of local type non-Gaussian E field. These simulations use the same xy coordinates along each line of sight. We found that its amplitude is about an order of magnitude lower than that of T field and has different shape. This finding will be useful for distinguishing different non-Gaussian signals in the observational data from future experiments. Further, we studied the effect of the presence of primordial tensor perturbation, which is parametrized by r, on the SMFs of Q=U and IP Fields, and the number density of singularities in IP field. Here, a singularity is a point on the CMB field where IP = 0. We found that the amplitude of SMFs of these fields are sensitive to the presence of primordial tensor perturbation and it decreases with r. We also show that the number density of singularities in IP field decreases with r. This finding will be useful for the searches of primordial tensor perturbation in the future experiments. The instrumental systematic in the observational data will decrease the statistical significance of the above results.
We introduce Tensor Minkowski Functionals (TMFs), which are tensor generalization of Minkowski Functionals, as a new statistical observable for the analysis of CMB data. Since these are tensor quantities, they are capable of capturing more morphological properties in a given field than their scalar counterparts. We have developed a code, referred as TMF Code, to compute the TMFs for any general field on an Euclidean plane. In order to apply the TMFs, specifically W1;1 2 which is a tensor of rank 2, to CMB fields which lies on a 2-d spherical surface, we map each point on the sphere with a point on a plane using stereographic projection. The code computes W1;1 2 , and then the net anisotropy (β) and net orientation (α) of the structures are estimated. We investigated the numerical error in this computation due to pixelization. We found the error in β increases with the increasing curvature of the boundaries of the structure. The error in β is negligible when the structures are completely unoriented with each other and it increases as the structures become more and more aligned with each other. We present the numerical calculation of the systematic variation of β and α with the threshold value for the simulated Gaussian and isotropic CMB T and E fields. We found that the value of β shows a characteristic variation with the threshold value while α is at. We show that according to the standard model, β = 0:62 for T and β = 0:63 for E, where the values are corrected for pixelization. The value of α is one for both the fields, which is as expected for an isotropic field.
We applied W1;12 for the analysis of PLANCK data as an illustration of its application. The instrumental systematics and the gravitational lensing due to large scale structure affects the morphological features of the CMB fields. We study the effect of these factors on the value of α and β using the simulations of CMB frequency bands, namely 44GHz and 70GHz provided in PLANCK data, which contains the respective instrumental characteristics. We found that the percentage difference in α and β due to these factors are less than 2% and it significantly increases the size of their error bars. We use the CMB simulations corresponding to the frequency band 44GHz as the basis for testing the consistency of different PLANCK data sets with theoretical expectations. We estimated the deviation in α and β for the foreground cleaned CMB maps namely SMICA, COMMANDER, SEVEM and NILC corresponding to full mission, half mission 1, half mission 2, half ring 1 and half ring 2 provided in the PLANCK data. These calculations showed that _ is consistent with the standard model within 2- δ for all data sets, except the T map of NILC half mission 2 which has slightly higher deviation. We found the values of α for T map of different data sets to be in excellent agreement with the standard model within 1:2- δ. The deviation in α of E map of all data sets are higher than 3- δ except the SMICA full mission data. Further, α for E map corresponding to the half mission 1 of all data sets showed consistently higher deviation of 5 - δ. These results imply that the structures in the E map has an extent of alignment with each other. This alignment could be cosmological or due to instrumental systematics. Since we are comparing the PLANCK maps which are obtained by co-adding all frequency bands with that of the simulations with the instrumental characteristics of a specific frequency band, namely 44GHz, the instrumental systematics is more probable reason for the alignment measured in E map.
Description: Open Access; Thesis Supervisor Dr. Pravabati ChingangbamSat, 01 Jul 2017 00:00:00 GMThttp://hdl.handle.net/2248/73132017-07-01T00:00:00ZSolar Radio Observations At Low Frequencies With High Spectral And Temporal Resolution
http://hdl.handle.net/2248/7312
Title: Solar Radio Observations At Low Frequencies With High Spectral And Temporal Resolution
Authors: Hariharan, K
Abstract: Ground-based radio observations of the solar corona at low frequencies (< 100 MHz) provide useful information in the height range ≈ 1−2 R☉ above the photosphere which is presently difficult to observe at other regions in the electromagnetic spectrum. Observations in the above height range are crucial since it is widely believed that some of the transient Sun-induced disturbances in the near-Earth space have their origin there. The other advantage is that the radio emission associated with transient solar activities are primarily non-thermal in nature, and hence more intense at low frequencies. Radio spectral observations in the corresponding frequency range are more worthwhile in this regard since they provide information over the entire range of heliocentric distance. Note that the radio emission at different frequencies originate at different distances from the Sun due to the inherent decrease in the electron density with increasing distance and the characteristics of radio wave propagation in an ionized medium like the solar atmosphere. So radio emission associated with any propagating disturbance in the solar atmosphere can be observed as a distinct feature in a time-frequency waterfall plot of the dynamic spectrum. This thesis describes the attempts made in this regard to carry out spectral observations at low frequencies down to the local ionospheric cut-off with high spectral and temporal resolutions at the Gauribidanur Radio Observatory under the auspices of the Indian Institute of Astrophysics. A low-frequency single antenna spectrograph system for observing solar transient radio emissions was configured to operate over the frequency range 15−85 MHz with the commercially available analog spectrum analyzer as back-end. A detailed description on the design of low-frequency antennas and filters is presented. The details of various experimental tests and inferences of the analog front-end components is also described. Using the new low-frequency antenna set-up and the existing instruments at the GRO we reported the radio imaging, spectral, and polarimeter observations of a type II radio burst associated with the solar coronal mass ejection (CMEs). Simultaneous ground-based, low-frequency « 100 MHz) radio imaging, spectral, and polarization observations of type II bursts are quite rare. From the radio spectral and imaging observations in tandem with the coronagraph observations we reinforce the close spatiotemporal association of CMEs and type II radio bursts which has been one of the long standing debates in solar radiophysics. We also had the opportunity to estimate coronal magnetic field strength ahead of and behind a CME, particularly from the dcp of the associated harmonic type II radio burst, with proper justifications. Though observations of circularly polarized harmonic type II bursts have been reported earlier, estimates of the coronal magnetic field from the observed circular polarization are not available. In this respect this was a unique set of observation with a combination of ground-based radio « 100 MHz) and space-based coronagraph observations. Another rare variant of the type II solar radio emission is the multiple type II radio burst phenomenon in which two or more type II bursts occur in quick succession. The occurrence of two successive Type-II bursts (referred to as Type-II doublets) in particular have been attributed to either two successive flares or two successive CMEs or a flare and a CME. We have reported radio spectral and polarimeter observations of two successive split-band Type-II bursts within a time interval of one minute at low frequencies ( < 100 MHz). Interestingly, both bursts exhibit Fundamental Harmonic as well as split-band structure. Type-II doublets with the above combined characteristics have rarely been reported. Our analysis of the data indicates that the first and the second Type-II bursts in the present case were likely due to MHD shocks generated by the near-simultaneous interaction of two different regions of the aforementioned CME with a preceding CME and a pre-existing coronal streamer. In the case of a conventional analog spectrum analyzer, at any give time data is obtained only at a single frequency since it is a swee-tuned instrument. The temporal resolution of such an instrument is determined by the sweep-time and the frequncy range of operation. The number of frequency channels over the operating frequency range is often fixed and hence results in poor frequency resolution for larger bandwidths. Note that an improvement in the temporal resolution could help us to understand the quasi-periodicity in the solar radio bursts and the related estimates of the coronal magnetic field strength. Higher spectral resolution can be used to identify and remove the RFI in frequency space while an improvement in the amplitude resolution leads to an enhancement of dynamic range in the measurements. In view of the above, we commissioned a new digital spectro-corrlator instrument using commercially available digitizer, for regular solar obsevations over the frequency range 15 - 30 MHz. The details of the instrument and the observational results obtained with it are presented in detail. From the encouraging results obtained with the above narrow-band spectrograph system, we configured a wide-band full digital back-end for the new spectrograph array that was recently commissioned at the GRO. An high-speed ADC-FPGA based data recorder was tested and characterised to be used as digital back-end receiver for the spectrograph array. Preliminary observations were carried out with both the digital and analog back-end for the spectrograph array to compare and contrast the results. The characterization of the ADC and the details of the FPGA board are presented in detail. The details on the data acquisition and pre-processing of the spectral data is also described. Quasi-continuum radio emissions of duration 10 - 60 min that occur in close association with CMEs in the solar atmosphere are termed as moving type IV bursts. The latter is found to accompany only about 5% of the CMEs. This provides a unique opportunity to study and infer the characteristics of CMEs viz. electron density, magnetic field strength etc. by simultaneously observing both the phenomena. We have reported the near-Sun (r ≤ 2 R☉) radio heliograph, radio spectrograph, and radio polarimeter observations of a type IVm burst that was co-spatial with the leading edge (LE) of a white light CME. To our knowledge, concurrent observations of the above type have rarely been reported. A complete analysis was performed to infer the electron density of the CME and the associated coronal magnetic field strength.
Description: Open Access; Thesis Supervisors Prof. R. RameshSat, 01 Apr 2017 00:00:00 GMThttp://hdl.handle.net/2248/73122017-04-01T00:00:00Z