Study of the equatorial ionosphere using the Giant Metrewave Radio Telescope (GMRT) at sub-GHz frequencies

TL;DR

This paper demonstrates the use of the Giant Metrewave Radio Telescope (GMRT) to study the Earth's ionosphere by measuring differential TEC and its gradients, revealing higher activity near the magnetic equator during night-time.

Contribution

It introduces novel data processing techniques for TEC measurement with GMRT and validates its capability to detect ionospheric activities, especially at the magnetic equator.

Findings

GMRT can measure differential TEC with high accuracy.

The ionosphere near the magnetic equator shows increased activity.

GMRT's ionospheric observations are more active than those from other arrays.

Abstract

Radio interferometers, which are designed to observe astrophysical objects in the universe, can also be used to study the Earth's ionosphere. Radio interferometers like the Giant Metrewave Radio Telescope (GMRT) detect variations in ionospheric total electron content (TEC) on a much wider spatial scale at a relatively higher sensitivity than traditional ionospheric probes like the Global Navigation Satellite System (GNSS). The hybrid configuration of the GMRT (compact core and extended arms) and its geographical location make this interferometer an excellent candidate to explore the sensitive regions between the northern crest of the Equatorial Ionization Anomaly (EIA) and the magnetic equator. For this work, a bright radio source, 3C68.2, is observed from post-midnight to post-sunrise ($\sim$\,9 hours) to study the ionospheric activities at solar-minima. This study presents data reduction and processing techniques to measure differential TEC ($\delta\rm{TEC}$) between the set of antennas with an accuracy of $1\times10^{-3}$ TECU. Furthermore, using these $\delta\rm{TEC}$ measurements, we have demonstrated techniques to compute the TEC gradient over the full array and micro-scale variation in two-dimensional TEC gradient surface. These variations are well equipped to probe ionospheric plasma, especially during the night-time. Our study, for the first time, reports the capability of the GMRT to detect ionospheric activities. Our result validates, compared to previous studies with VLA, LOFAR and MWA, the ionosphere over the GMRT is more active, which is expected due to its location near the magnetic equator.