Quantifying Ionospheric Effects on Global 21-cm Observations

TL;DR

This paper models Earth's ionosphere to understand its impact on global 21-cm observations, revealing significant distortions and amplitude weakening of the signal caused by ionospheric effects, which must be corrected for accurate measurements.

Contribution

It introduces a simplified spatial-temporal model of the ionosphere to quantify its chromatic effects on 21-cm signals, highlighting the need for time-dependent calibration.

Findings

Ionospheric disruptions can exceed the expected 21-cm signal magnitude.

The shape of the 21-cm signal is distorted after ionospheric propagation.

Ionospheric effects do not cancel out over time, requiring ongoing calibration.

Abstract

We modelled the two major layer of Earth's ionosphere, the F-layer and the D-layer, by a simplified spatial model with temporal variance to study the chromatic ionospheric effects on global 21-cm observations. From the analyses, we found that the magnitude of the ionospheric disruptions due to ionospheric refraction and absorption can be greater than the expected global 21-cm signal, and the variation of its magnitude can differ, depending on the ionospheric conditions. Within the parameter space adopted in the model, the shape of the global 21-cm signal is distorted after propagating through the ionosphere, while its amplitude is weakened. It is observed that the ionospheric effects do not cancel out over time, and thus should be accounted for in the foreground calibration at each timestep to account for the chromaticity introduced by the ionosphere.