Spectral analysis of ionospheric TEC gradients observed with the Giant Metrewave Radio Telescope at sub-GHz frequencies

Abstract:

Radio-frequency arrays are designed to observe astrophysical sources in the universe. These observations require a detailed calibration procedure to mitigate the effects of the ionosphere; also, the same calibration data can be used to study the ionosphere at low frequencies. An observation of a cosmic radio source(3C68.2) with the Giant Metrewave Radio Telescope (GMRT) through the nighttime till noon is used to study the ionosphere. GMRT array configuration and geophysical location are well suited to study geophysically sensitive regions between the magnetic equator and the northern crest of the Equatorial Ionization Anomaly(EIA). In one of our recent analysis, we have shown that the GMRT is capable of detecting irregularities in total electron content (TEC) with amplitudes ≤ 10^-3 TECU and can measure TEC gradients with a precision of about 7 × 10^-4 TECU km^-1. In continuation with such an analysis, we introduce two methods to do spectral analysis on these measured TEC gradient. The first is to track individual waves with wavelengths of approximately half the size of the array, which aids to detect and characterize travelling ionospheric disturbances (TIDs), especially during the nighttime along with smaller-scale structures associated with E_s layers. The second method allows the detection and gives a statistical description of the characteristics properties of a group of waves propagating in the same direction, which corresponds to small wavelengths(few Km).