Design and development of chromospheric vector magnetograph for sunspot studies

Abstract

Solar atmosphere is permeated with magnetic fields of various spatial scales that exhibit dynamics in various temporal scales. Study of so called active regions and their magnetic field measurements are of paramount importance in understanding the solar atmospheric dynamics and energetic events. Although magnetic field maps of photosphere are produced with great regularity, the same cannot be said for chromosphere. Unique conditions offered by chromosphere such as being close to force-free state facilitate better modeling of solar magnetic fields with chromospheric magnetic field measurements. Magnetic field measurements at chromospheric heights are relatively challenging owing to factors such as (1) lower sensitivity of spectral lines to the magnetic fields, (2) difficulties in modeling the spectral lines, and (3) weaker magnetic field strength at these heights. In this thesis, we present instrumentation aspects of Solar Scanning Polarimeter that has been installed at Kodaikanal Tower-tunnel Telescope of Kodaikanal Solar Observatory to measure the active region magnetic fields at chromospheric level, using spectropolarimetry and Zeeman diagnostics of Ca II 8542 Å spectral line. In chapter 1, magnetic fields of active regions, their properties and their role in solar dynamics are discussed. Motivation for the thesis to measure chromospheric magnetic fields is explained. In chapter 2, design and development of Solar Scanning Polarimeter, and polarimetry strategy are discussed. Telescope instrumental polarization has been revisited and a possible way to reduce it has been proposed. Instrument controls and operating software are briefly described, along with testing of polarization optics. In chapter 3, polarimetric data acquisition, calibration of the instrument, observations and corrections for instrumental polarization are presented. Polarimetric accuracy and sensitivity are estimated to be few times 10􀀀2 and 10􀀀3 respectively. Line-of-sight magnetic field map that is synthesized from corrected Stokes profiles usingWeak Fields Approximation is presented. Main challenges such as low raster image resolution due to image motion and difficulty in computing transverse magnetic field due to low signal-to-noise ratio are addressed. In chapter 4, design, development and testing of Image Stabilization System are described. Its development is aimed to reduce image motion induced due to telescope system and seeing. Correlation tracking of sunspot is used to evaluate its performance, and with an acquisition and correction rate of 563 Hz, closed loop correction bandwidth of 110 Hz is achieved. In chapter 5, a pilot study of correlation between observed chromospheric magnetic field and modeled chromospheric magnetic field (obtained from potential extrapolation using photospheric magnetograms) in the active regions is discussed. In this context, probable formation height of chromospheric spectral line (Ca II 8542 Å) in the active regions is indicated. In chapter 6, thesis is summarized and future scope and needs are outlined.