High precision full stokes spectropolarimetry of the sun as a star instrument design aspects

Abstract

The sun is very interesting particularly because it is very dynamic. The magnetic field plays a major role in governing dynamics of the sun. Many interesting features like sunspots, flares, prominences, and Coronal Mass Ejections (CMEs) occur on the surface of the sun due to the dynamics associated with magnetic fields. Therefore it is quintessential to measure and predict them as accurately as possible. Spectropolarimetry is a powerful diagnostic tool to infer the magnetic field from the incident radiation. The magnetic field on the sun varies from the smallest scale, such as flux tubes to the spatial scale as large as the sun itself. Construction of large aperture telescopes are required to obtain small scale field information and the contribution of these fields to the overall activity or the dynamics of the sun is an important aspect of research nowadays. High-resolution spectropolarimetry is one of the techniques which are being performed all over the world with many large solar telescopes to infer the small scale magnetic field information. However, it is also important to study how the vector magnetic field (strength and the direction) of the entire sun as a whole manifests itself with time or the solar cycle. Till date the global magnetic field of the sun has been studied but mostly with the Line Of Sight (LOS) component. However, vector magnetic field studies with sufficient accuracy and precision are found to be lacking mainly due to instrumental polarization. The main motivation of the project is to perform high-precision (high polarimetric accuracy) spectropolarimetry of sun-as-a-star and completely avoid instrumental polarization by modulating the light before it enters the telescope. The expected polarization signal level is quite weak. This is because most of the flux gets canceled when we consider both the hemispheres of the sun except the very weak global field which mostly resembles a magnetic dipole. Our aim of this project is to image the sun as a star and measure the magnetic dipole field on daily basis. This thesis explores various design aspects of the instrument to carryout such measurements and the challenges that are faced if one is bestowed with a ground-based observing facility. The effect of the atmosphere is significant if the instrument is not based on Adaptive Optics (AO). AO is more important however, in the disk-resolved or rather high-resolution imaging. For low resolution measurements such as ours, the effect of image aberrations is insignificant compared to the scintillation. The turbulence or the randomness of the atmosphere introduces crosstalk or spurious polarizations in the measurements to a significant degree and therefore it is absolutely essential to minimize them. One approach to overcome the effects of crosstalk (due to image motion and scintillation) is to modulate the signals at high frequencies with the idea to make the measurements before the atmosphere changes (typically within 1 ms). We describe this approach in detail and also show that how the effect of the spurious polarization varies with temporal cadence and what can be the acceptable level of crosstalk to perform such measurements. Besides, we also compare the atmospheric effects on a high-resolution image and a point image. Instrumental polarization can also be a possible source of crosstalk which introduces spurious polarization in the measurements. This work highlights the technique of modulating the light before it enters the telescope so as to prevent instrumental polarization. Therefore the size of the telescope aperture is limited by the size of the modulators that are available. To get an idea of the amplitude of global polarization signal and also the minimum signal that one should expect from such weakly polarized radiation (precision), we attempted to perform sun-as-a-star spectropolarimetric measurements from the existing Tunnel Telescope of the Kodaikanal Solar Observatory of the Indian Institute of Astrophysics. Because of the large integration time (> 5 hours) required to achieve the required precision level (< 10-5), we didn’t succeed in detecting any signal. Mostly the signal remains buried under the noise. Further, we show from the data sets obtained from SOLIS-VSM of the NSO and SDO-HMI of the NASA that roughly the signal amplitude expected from the full disk integrated Stokes spectra is of the order of 10-5 and the required precision level is of the order of 10-7. Performing fast polarimetry can definitely lead to a precision of this level, as the desired Signal-to-Noise- Ratio (SNR) can be achieved much faster. Keeping in mind these requirements, we explore several optical design aspects in the form of polarization modulators, spectrometers and the detectors which are available, along with their applicability. Each of the optical components described has certain advantages and disadvantages and we discuss each of them in the context of performing fast solar vector polarimetry. It is to be noted here that fast modulation, also implies equally fast demodulation technique, therefore, detectors are also to be chosen by keeping this aspect in mind. In the discussion that follows in chapter 4, we intend to bring out the possible concepts that can be followed to perform high precision sun-as-a-star spectropolarimetry.