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The ambitious scientific goals of SKA require a matching capability forcalibration of instrumental and atmospheric propagation contributions asfunctions of time, frequency and position. The development of novel calibrationalgorithms to meet these requirements is an active field of research. In thiswork {we aim to characterize} these, focusing on the spatial and temporalstructure scales of the ionospheric effects; ultimately, these provide theguidelines for designing the optimum calibration strategy. We used empiricalionospheric measurements at the site where the SKA-Low will be built, using MWAPhase-2 Extended baseline observations and the station-based Low-frequencyExcision of Atmosphere in Parallel (LEAP) calibration algorithm. We have donethis via direct regression analysis of the ionospheric screens and by formingthe full and detrended structure functions. We found that 50% of the screensshow significant non-linear structures at scales >0.6km that dominate at >2km,and 1% show significant sub-minute temporal changes, providing that there issufficient sensitivity. Even at the moderate sensitivity and baseline lengthsof MWA, non-linear corrections are required at 88 MHz during moderate-weatherand at 154 MHz during poor weather, or for high SNR measurements. Therefore wepredict that improvements will come from correcting for higher-order defocusingeffects in observations with MWA Phase-2, and further with new developments inMWA Phase-3. Because of the giant leap in sensitivity, the correction forcomplex ionospheric structures will be mandatory on SKA-Low, for both imagingand tied-array beam formation.

Sub-kilometre scale ionospheric studies at the SKA-Low site, using MWA extended baselines