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AARTFAAC and the Ionosphere

Submitter: Peeyush Prasad
Description: The AARTFAAC project will create an image domain All-Sky Radio transient monitor. Its precise calibration is challenging due to the temporally and spatially varying ionosphere, which can introduce source scintillation and position wander. When this affects the model sources in the sky which are used for calibration, both the sensitivity and dynamic range of the instrument can be severely affected.

These plots show the flux of the familiar A-team as estimated using a Maximum Likelihood estimator on the visibilities. The observations were carried out during the night of 11 July, 2012, and show varying levels of scintillation during part of the daily life cycle of the ionosphere. The short spatial extent of the AARTFAAC (350m baselines) 'should' have made close-by lines of sight from a pair of antennas resilient to differential ionospheric delays, but the scintillations seem to point toward the presence of cells of turbulent plasma with linear sizes comparable to the baseline lengths. The ~1sec calibration and imaging capability of the AARTFAAC allows the sampling of such phenomena, which is just one of the challenges faced by the instrument.

At about 10 PM (top), CasA is seen to be rapidly scintillating (compared to CygA) due to its lower elevation, and thus the longer path traversed through the relatively benign ionosphere. This is also seen in its attenuated amplitude compared to the lower luminosity CygA. At about 2 AM (middle), the ionosphere is recombining, creating smaller cells of turbulence. These result in rapid scintillation of both CasA (which has risen in the meantime) and CygA. During dawn (bottom), the ionosphere is relatively quiet, although low level scintillation can still be seen. We see the Sun in green beginning to rise during this time, along with TauA skimming the field of view. The breaks in this plot are due to the sparse sampling of the ~1 Hr dawn duration. All plots are to the same scale.
Copyright: AARTFAAC Project
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