Assessment of weather conditions for ALMA observations of the Sun

University essay from Luleå tekniska universitet/Institutionen för teknikvetenskap och matematik

Abstract: The closest star to Earth is the Sun. The difference between our Sun and other stars is that the dynamics are observable with telescopes. A complex and strange part of our Sun is the chromosphere which is a part of the solar atmosphere. The chromosphere is located between the corona and photosphere where the temperature increases very rapidly within a short distance. A wavelength suited for observing this region is millimetre wavelength. A millimetre observing radio telescope suited to be pointed at the Sun is Atacama Large millimetres/submillimeter Array (ALMA). ALMA is using smaller telescopes to synthesis a larger aperture by correlating the difference between antenna pairs. This technique is called interferometry. When sampling a larger telescope, the projected distance between the smaller telescopes determines the sampling points in Fourier space. When the distance between the antennas increases, they will experience different noise due to Earth's atmosphere. This difference in noise is because the signal travels a different path through the Earth's atmosphere. Different Precipitable Water Vapour (PWV) levels in this path play a major role in this disturbance[1]. To acquire further knowledge of how different seeing effects affect ALMA it is important to enhance the understanding of what could be expected from actual observation. Realistic simulated observations can be a useful tool to extend this knowledge and is investigated in this report. This is done with the CASA (Common Astronomy Software Applications) package and simalma and the simulator tool. The simulator tool gives the possibility to include phase noise from Earth's atmosphere. This noise is calculated with atmospheric transmission at microwaves model and the aatm library [2]. This phase noise is simulated as a fluctuating PWV screen over that array that blows at specific wind speed[3].  Traditionally only thermal noise has been implemented when simulating an observation with the CASA task simalma and simobserve. Initial results indicate a big difference between traditional thermal noise and phase noise. Phase noises generate a greatly increased error as a function of radius compared to a noise free simulated observation. This effect is enhanced for higher PWV levels. This behaviour is due to that the antennas are more sensitive in the centre. This tool shows great potential since more realistic simulations give the possibility to investigate different phenomena in a controlled environment. One could optimize the reconstruction algorithm for noisy observations and investigate how physical phenomena could be affected by different seeing effects. There are a large variety of cases where this type of simulation could be used. Optimization of PWV fluctuating for specific cadences should be done first. This is important because the atmosphere is expected to behave differently at a different cadence because of different movement and averaging. However, optimization and comparison for 1.1 s cadence is doable with data generated from cosmological observations with ALMA[1].

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