Probing Equatorial Ionospheric TEC at Sub-GHz Frequencies with Wide-Band (B4) uGMRT Interferometric Data

Abstract

Phase stability at low radio frequencies is severely impacted by ionospheric propagation delays. Radio interferometers such as the Giant Metrewave Radio Telescope (GMRT) are capable of detecting changes in the ionosphere's total electron content (TEC) over larger spatial scales and with greater sensitivity compared to conventional tools like the Global Navigation Satellite System (GNSS). Thanks to its unique design featuring both a dense central array and long outer arms-and its strategic location, the GMRT is particularly well-suited for studying the sensitive ionospheric region located between the northern peak of the Equatorial Ionization Anomaly (EIA) and the magnetic equator. In this study, we observe the bright flux calibrator 3C48 for ten hours to characterize and study the low-latitude ionosphere with the upgraded GMRT (uGMRT). We outline the methods used for wideband data reduction and processing to accurately measure differential TEC (dTEC) between antenna pairs, achieving a precision of less than 1 mTECU for the central square antennas and approximately 1 mTECU for the arm antennas. The measured dTEC values are used to estimate the TEC gradient across the GMRT arm antennas. We measure the ionospheric phase structure function and find a power-law slope of $\beta =1.72$, indicating deviations from pure Kolmogorov turbulence. The inferred diffractive scale the spatial separation over which the phase variance reaches $1{\text{rad}}^2$ is 6.66 km. A small diffractive scale implies high phase variability across the field of view and reduced temporal coherence, which poses challenges for calibration and imaging.

25 pages, 13 figures, Accepted to Universe

References in corpus (7)

• CASA, the Common Astronomy Software Applications for Radio Astronomy
• Revisiting the radio interferometer measurement equation. I. A full-sky Jones formalism
• Characterisation of the ionosphere above the Murchison Radio Observatory using the Murchison Widefield Array
• Atmospheric Refractive Electromagnetic Wave Bending and Propagation Delay
• Ionospheric Modelling using GPS to Calibrate the MWA. II: Regional ionospheric modelling using GPS and GLONASS to estimate ionospheric gradients
• Study of the equatorial ionosphere using the Giant Metrewave Radio Telescope (GMRT) at sub-GHz frequencies
• Spectral Analysis of ionospheric density variations measured with the large radio telescope in the Low-Latitude region