Ionospheric effects not detrimental to EoR detection from ground
30 October 2015
The Epoch of Reionisation (EoR) is the time in the early Universe when the first stars and galaxies formed and re-ionised the neutral hydrogen. Indirect information about the EoR has been obtained from the Cosmic Microwave Background and spectra of the distant quasars. However, the bulk of information about the physical parameters of the EoR is encoded in the 21cm line (1420 MHz) from neutral hydrogen redshifted into the low radio frequency range 200 – 50 MHz, for redshifts of 6 < z < 30.
The observational approaches range from large interferometer arrays to single antenna experiments. The latter, so-called global EoR experiments, spatially average the signal from the entire visible sky and try to identify the tiny signature of the EoR (of order 100 milliKelvin, which is a few orders of magnitude smaller than the Galactic foregrounds) in the sky-averaged spectrum. This extremely challenging precision requires very long observations (hundreds of hours) to achieve a sufficiently high signal-to-noise ratio. Moreover, ground-based global EoR experiments are affected by frequency-dependent effects (i.e. absorption and refraction) due to the propagation of radio-waves in the Earth's ionosphere. The amplitude of these effects changes in time. There has therefore been an ongoing discussion in the literature on the importance of ionospheric effects and whether the global EoR signature can feasibly be detected from the ground.
The team of CAASTRO researches at Curtin University, led by Dr Marcin Sokolowski, used three months’ worth of 2014/2015 data collected with the BIGHORNS system with a conical log-spiral antenna deployed at the Murchison Radio-astronomy Observatory to study the impact of the ionosphere on its capability to detect the global EoR signal. Comparison of data collected on different days at the same sidereal time enabled the researchers to infer some properties of the ionosphere, such as electron temperature (Te≈470 K at night-time) and amplitude and variability of ionospheric absorption of radio waves. Furthermore, the data sample shows that the sky-averaged spectrum indeed varies in time due to fluctuations of these ionospheric properties. Nevertheless, the data analysis indicates that averaging over very long observations (several days or even several weeks) suppresses the noise and leads to an improved signal-to-noise ratio. Therefore, the ionospheric effects and fluctuations are not fundamental impediments that prevent ground-based instruments, such as BIGHORNS, from integrating down to the precision required for global EoR experiments, provided that the ionospheric contribution is properly accounted for in the data analysis.