Ionospheric contributions to the excess power in high-redshift 21-cm power-spectrum observations with LOFAR

TL;DR

This study investigates how ionospheric effects impact high-redshift 21-cm power spectrum observations with LOFAR, revealing that ionospheric errors are not the main source of excess power, and demonstrating mitigation techniques.

Contribution

First comprehensive analysis of ionospheric impact on LOFAR EoR data calibration, showing ionospheric errors can be mitigated and are not the dominant source of excess power.

Findings

Ionospheric effects cause baseline-dependent decorrelation in LOFAR data.

Gaussian process regression effectively removes ionospheric excess power.

Ionospheric errors are not the primary cause of excess power in EoR observations.

Abstract

The turbulent ionosphere causes phase shifts to incoming radio waves on a broad range of temporal and spatial scales. When an interferometer is not sufficiently calibrated for the direction-dependent ionospheric effects, the time-varying phase shifts can cause the signal to decorrelate. The ionosphere's influence over various spatiotemporal scales introduces a baseline-dependent effect on the interferometric array. We study the impact of baseline-dependent decorrelation on high-redshift observations with the Low Frequency Array (LOFAR). Datasets with a range of ionospheric corruptions are simulated using a thin-screen ionosphere model, and calibrated using the state-of-the-art LOFAR Epoch of Reionisation pipeline. For the first time ever, we show the ionospheric impact on various stages of the calibration process including an analysis of the transfer of gain errors from longer to shorter baselines using realistic end-to-end simulations. We find that direction-dependent calibration for source subtraction leaves excess power of up to two orders of magnitude above the thermal noise at the largest spectral scales in the cylindrically averaged auto-power spectrum under normal ionospheric conditions. However, we demonstrate that this excess power can be removed through Gaussian process regression, leaving no excess power above the ten per cent level for a $5~$km diffractive scale. We conclude that ionospheric errors, in the absence of interactions with other aggravating effects, do not constitute a dominant component in the excess power observed in LOFAR Epoch of Reionisation observations of the North Celestial Pole. Future work should therefore focus on less spectrally smooth effects, such as beam modelling errors.