Diagnostic techniques and instrument concepts for probing transition region and coronal magnetic fields

Abstract

Magnetic fields are the primary driver of the plasma thermodynamics in the upper solar atmosphere, especially in the transition region (TR) and the corona. These magnetic fields hold the key to long standing questions in solar physics, such as the problem of coronal heating, the acceleration source of the solar wind, and various eruptive events, thereby influencing the space weather dynamics. However, the lack of vector magnetic field measurements in the TR-corona, has limited our understanding of these physical processes. In this context, extreme-ultraviolet (EUV) and farultraviolet (FUV) spectropolarimetry is a potential measurement technique to quantify the TR and coronal magnetic field vector. This thesis is dedicated to finding new diagnostics of the TR and coronal magnetic field vector, exploring suitable instrument concepts and characterizing the instrument prototype in the laboratory as a proof of concept. Bommier et al. (1981) proposed a method (called the differential Hanle effect) to obtain the vector magnetic field information, which utilizes a minimum of two permitted lines with different Hanle sensitivity. This method has been successfully applied to derive the vector magnetic field in prominences, and was further extended to obtain empirical information on the TR fields. The same method can also be used to infer the coronal magnetic fields, prior to which it is necessary that combination of EUV and FUV lines exhibiting varied Hanle sensitivity to the coronal magnetic fields are identified so that any ambiguity can be removed from the derived magnetic field solution. Therefore, in the first study, we identified potential Hanle sensitive lines in the FUV and EUV spectral ranges which exhibit line-formation temperatures typical of the TR and corona. Several EUV lines, with the critical Hanle field ranging from a few milligauss to 200 gauss, were identified in the wavelength range of 100 to 1600 ˚A. EUV lines enable both off-limb and on-disk measurements, unlike at visible and IR wavelengths where only coronagraphic (off-limb) observations are possible due to the extremely bright solar disk. On-disk measurements are important to understand the magnetic field stratification in the solar atmosphere. However, there is no spectropolarimeter yet in these spectral ranges to infer the vector magnetic field in the upper atmospheric layers of the Sun. Since transmission optics is not feasible in these wavelengths, several configurations of EUV polarimeter based on highly reflective and polarizing coatings are explored in the thesis. On the contrary, it is essential to estimate the polarization signals and the signal-to-noise ratio (SNR) required to measure such signals in the wavelength range considered for the instrument design. Hence, the next project of the thesis involved modeling and synthesizing the polarization maps of one of the identified EUV lines, i.e., Ne viii 770 ˚A, by utilizing magnetohydrodynamic (MHD) simulation data cubes. Furthermore, our investigation of various polarimeter configurations revealed that a 3-mirror-based design, using barium fluoride coated mirrors, exhibits enhanced polarizing power and higher throughput in the wavelength range of 740 to 800 ˚A. As part of our efforts, an EUV spectropolarimeter has been designed using Zemax targeting the same wavelength range, with Ne viii 770 ˚A as the central wavelength. From the previous study of Ne viii 770 ˚A (one of the brightest EUV lines), the Stokes L/I (where L = p Q2 + U2) is estimated to be in the orders of 1 × 10−4 at the solar limb region. Therefore, detection of polarization sensitivity in these levels is an essential requirement which has driven the design of the instrument. Simultaneously, an in-house prototype of the 3-mirror polarimeter has been developed, and its preliminary testing has been conducted in the laboratory. Due to lack of facilities for fabrication and testing in the EUV, we conducted the initial characterization at a visible wavelength of 700 nm.