First demonstration of the combination of observations and datasets from diverse instruments (in a coordinated experiment using KAIRA, EISCAT, GNSS) that enables the extraction of new information about the ionosphere

Abstract. The plasma of the Earth’s ionosphere introduces effects on radio waves that traverse it. Propagation effects reflect the presence of the bulk ionisation along the ray path as well as the presence of inhomogeneities (or irregularities) in the spatial distribution of the plasma (or electron) density. Ionospheric propagation effects are visible on radio signals received from artificial satellites (with radio wave frequencies approximately between the VHF and the C band), on terrestrial HF propagation, and on radio waves (between VHF and L/C bands) typically utilised in radio astronomy.

The presence of propagation effects can be utilised to deduce properties of the ionosphere: for example, through the dispersion of radio signals from satellite navigation satellites it is possible to appreciate the spatial and temporal evolution of the bulk of the ionisation in the ionosphere all over the Earth. On the other hand, properties of ionospheric irregularities (spatial and temporal gradients in the background ionisation) can be deduced from the presence of phase fluctuations and scintillation (whose magnitude decreases with frequency). However, the capability of detecting the presence of plasma structures depend upon the sensitivity of the instruments utilised.

Here, an experiment (the first of this kind) was conducted in the European auroral and polar sectors to demonstrate how the combination of diverse instruments and datasets enables the extraction of new information about the ionosphere and its spatio-temporal evolution, based on the combination of observations from a smaller radio telescope (KAIRA), incoherent scatter radars (EISCAT/ESR), satellite radio receivers (GNSS), and an ionosonde.

In particular, KAIRA (an instrument built using LOFAR radio telescope technology) was utilised to simultaneously collect all-sky images and beam-formed VHF scintillation on radio signals received from the source Cassiopeia A (Cas A). At the same time, the EISCAT incoherent scatter radar was utilised to measure profiles of electron density along lines of sight to Cas A closely matching those used in KAIRA observations. Finally, temporal fluctuations in the ionospheric Total Electron Content (rate of change of TEC, ) from Global Navigation Satellite Systems (GNSS) signals received at ground stations in the surrounding of KAIRA and EISCAT, were utilised to infer large-scale conditions in the ionosphere at the time of the observations as well as the type of ionisation gradients originating specific propagation effects.

The results of this experiment demonstrate for the first time that stronger ionisation gradients (|ROT| ≳ 0.2 TECU min^-1) that are extended over a wider range of altitudes in the ionosphere can be detected through EISCAT electron density profiles as well as through propagation effects detectable through GNSS (phase fluctuations) and KAIRA (VHF scintillation). The experiment’s results also indicate that weaker ionisation gradients (|ROT| ≲ 0.2 TECU min^-1) can induce VHF scintillation but may not be detectable through GNSS and EISCAT (their sensitivity and resolution is limited). The position and shape of astronomical sources appears to be affected by two concurring aspects: slower trends (due to changes in the ionisation with gradients probably having |ROT| ≪ 0.2 TECU min^-1 as a consequence of horizontal plasma drift and/or local ionospheric mechanisms) and degradations in the estimates of source position and shape (due to scintillation, when noise and other parts of the sky around a given source have higher contributions – an effect similar to optical blurring).

By combining the evidence from diverse instruments and datasets through a novel methodology, the experiment demonstrates that ionisation gradients in the ionosphere can occur over multiple spatial scales (both horizontally and in altitude). Whilst observations from modern radio telescopes (e.g., LOFAR, MWA, SKA-Low) provide a new way to detect and characterise the spatial and temporal evolution of plasma gradients in the ionosphere (due to a higher sensitivity), it is their combination with observations from traditional ionospheric instruments like GNSS, rather than their use in isolation, that enables novel understanding of ionospheric physics through a more accurate reconstruction of the ionospheric state over multiple spatial scales in the presence of different space weather conditions. This study provides a methodology to extract new information on ionospheric structures across multiple spatial scales based on the combination of observations from radio telescopes, GNSS, and incoherent scatter radars, which can be applied to other current and future instruments.