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Radio polarimetric imaging of the solar corona at low frequencies a thesis submitted for the degree of doctor of philosophy (Technology) in the department of applied optics and photonics, University of Calcutta, Kolkata Anshu Kumari [Thesis]

By: Contributor(s): Material type: TextTextPublication details: Bangalore Indian Institute of Astrophysics 2019Description: xxx, 128pDissertation note: Doctor of Philosophy University of Calcutta, Kolkata 2019 Summary: The coronal magnetic field (B) plays an important role in the formation, evolution, and dynamics of the small, as well as large scale structures in the solar corona. Measurements of the coronal magnetic field strengths, particularly in the radial distance range ≈ 1.1 - 3.0 R (R is the radius of photospheric Sun) is presently difficult because of practical reasons. Polarization observations, by measuring the Stokes-V parameter of the received radio signal, are generally used as a tool to measure the magnetic field strength associated with the radio emission; the latter is one of the widely pursued areas of research in the solar coronal physics, in addition to the currently available but limited methods of estimating the magnetic field strength using simultaneous radio imaging and spectral observations. Ground based radio observations provide an excellent tool to observe and study the corona in the aforesaid height range. Therefore, the primary objectives of this thesis are to : 1. design, develop and characterize spectro-polarimeter that can receive the polarized radio emission (at low frequencies) from the Sun with high temporal, spectral resolution, etc.; 2. compare the data obtained using the new instruments with the existing instruments in order to improve the observing capabilities of new instruments; 3. estimate the magnetic field strengths from the observed polarized radio emission associated with various forms of solar coronal activities observed at other wavelength parts of the electromagnetic spectrum. As for the first objective is concerned, two spectro-polarimeters were developed : i Near ionospheric cut-off frequency spectro-polarimeter that can operate over 15 - 85 MHz frequency range. ii Wide-band spectro-polarimeter that can operate over 50 - 500 MHz frequency range. For the first spectro-polarimeter system, a Log-Periodic Dipole Antenna (LPDA) that works in the 15 - 85 MHz range was designed and characterized. The VSWR (Voltage Standing Wave Ratio) of the antenna is ≤ 3.0 throughout the band. It has a directional gain of 6 dBi and has an effective collecting area of ≈ 0.3λ 2 . Two identical antennas were fabricated and kept in mutually orthogonal orientations to arrange the frontend. Since the signal strength received by the antennas from the background sky were found to vary drastically over the observing bandwidth it was difficult to handle the entire frequency band with a single digital backend system. Therefore, the signal was split into two sub-bands (15 - 25 MHz & 25 - 85 MHz) using appropriate filters designed as part of the thesis work. The signals were then amplified using Low Noise Amplifier (LNA) and were fed to a digital backened receiver.
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Ph.D. Thesis, Pondicherry University, Puducherry

Doctor of Philosophy University of Calcutta, Kolkata 2019

The coronal magnetic field (B) plays an important role in the formation, evolution, and dynamics of the small, as well as large scale structures in the solar corona. Measurements of the coronal magnetic field strengths, particularly in the radial distance range ≈ 1.1 - 3.0 R (R is the radius of photospheric Sun) is presently difficult because of practical reasons. Polarization observations, by measuring the Stokes-V parameter of the received radio signal, are generally used as a tool to measure the magnetic field strength associated with the radio emission; the latter is one of the widely pursued areas of research in the solar coronal physics, in addition to the currently available but limited methods of estimating the magnetic field strength using simultaneous radio imaging and spectral observations. Ground based radio observations provide an excellent tool to observe and study the corona in the aforesaid height range. Therefore, the primary objectives of this thesis are to : 1. design, develop and characterize spectro-polarimeter that can receive the polarized radio emission (at low frequencies) from the Sun with high temporal, spectral resolution, etc.; 2. compare the data obtained using the new instruments with the existing instruments in order to improve the observing capabilities of new instruments; 3. estimate the magnetic field strengths from the observed polarized radio emission associated with various forms of solar coronal activities observed at other wavelength parts of the electromagnetic spectrum. As for the first objective is concerned, two spectro-polarimeters were developed : i Near ionospheric cut-off frequency spectro-polarimeter that can operate over 15 - 85 MHz frequency range. ii Wide-band spectro-polarimeter that can operate over 50 - 500 MHz frequency range. For the first spectro-polarimeter system, a Log-Periodic Dipole Antenna (LPDA) that works in the 15 - 85 MHz range was designed and characterized. The VSWR (Voltage Standing Wave Ratio) of the antenna is ≤ 3.0 throughout the band. It has a directional gain of 6 dBi and has an effective collecting area of ≈ 0.3λ 2 . Two identical antennas were fabricated and kept in mutually orthogonal orientations to arrange the frontend. Since the signal strength received by the antennas from the background sky were found to vary drastically over the observing bandwidth it was difficult to handle the entire frequency band with a single digital backend system. Therefore, the signal was split into two sub-bands (15 - 25 MHz & 25 - 85 MHz) using appropriate filters designed as part of the thesis work. The signals were then amplified using Low Noise Amplifier (LNA) and were fed to a digital backened receiver.

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