Studying Ionospheric Phase Structure Functions Using Wide-Band uGMRT (Band-4) Interferometric Data

Interferometric observations of the low-frequency radio sky (< 1 GHz) are largely limited by systematic effects introduced by the ionosphere. Here, we analyse a ten-hour nighttime uGMRT Band-4 observation of 3C48 to characterise ionospheric phase fluctuations across baselines up to 25 km. We compute spatial phase structure functions across three sub-bands (575-725~MHz), revealing power-law behaviour consistent with turbulence and a diffractive scale r_diff ~ 6.7 - 8.3 km useful for assessing calibration requirements. The turbulence exhibits anisotropy with smallest scales perpendicular to Earth's magnetic field - consistent with wave-like structures such as MSTIDs rather than field-aligned irregularities. These findings from a single case study demonstrate uGMRT's sensitivity for ionospheric characterisation at low-latitudes (~ 19 deg N) and inform direction-dependent calibration strategies for similar conditions.