Employing AARTFAAC as an ionospheric instrument
AARTFAAC, the Amsterdam-ASTRON Radio Transient Facility And Analysis Centre, is an all-sky monitor based on the low band antennas of LOFAR, thus operating between 30 and 80 MHz. AARTFAAC constantly monitors the full visible radio sky, creating 1 second snapshot images by combining the data from 6 of the 12 LOFAR core stations. The high time-resolution snapshot-imaging of AARTFAAC is extremely useful to investigate the behaviour of the ionosphere on short time scales, it being a very turbulent and highly dynamic medium. In addition, the sky coverage of AARTFAAC allows to have a complete view of the ionosphere above LOFAR.
The image showing the radio sky above LOFAR on 2018-11-07 at 19:55:09.3 was calibrated using a model of CasA ad CygA, as well as the clean components and the diffuse emission found in a self-calibration iteration by WSClean. To get such a clear view of the sky, the members of the A-team, both being at zenith at the time of observation, were subtracted too.
By comparing the position of the sources in the image with their true celestial coordinates, we are able to identify position shifts and reconstruct an ionospheric wave. Around 1050 radio sources were simulated from catalog data with phase errors induced by a typical ionospheric wave (mTID) over the LOFAR core for all individual antennas in all directions, including a realistic noise level. We were able to reconstruct the apparent position shifts induced by these phase errors for half of the sources in a 2-minute snapshot image, clearly demonstrating the capability to detect such an ionospheric wave with AARTFAAC.
The ability to monitor the ionosphere in real time is particularly useful for telecommunication and satellite applications, such as GPS and GNSS, and if implemented successfully would allow AARTFAAC to be used as an ionospheric instrument.