Probing the Ionosphere with the LWA by Rapid Cycling of Celestial Radio Emitters

Using knowledge of the low-frequency radio sky and the simulated observational capabilities of the Long Wavelength Array (LWA) we present a feasible method for the LWA to probe the overall structure of the ionosphere above the LWA. Such measurements could serve as the initial step in calibrating the ionospheric phases that distort astronomical images, thereby allowing the LWA to reach its full astrophysical potential. At the same time, it could generate a dynamic, all-sky model model of the ionosphere rich in phenomenology. This method consists of rapidly cycling through and imaging the several hundred celestial radio emitters (sources) that we predict will be isolated enough to be imaged with standard calibration techniques. Such sources can therefore function as ionospheric calibrators, because calibrating to them determines the differences in ionospheric column density (Total Electron Content differences, or ∆TEC) along the lines of sight from each LWA station towards each calibrator source. Our analysis, based on VLSS fluxes and LWA antenna simulations, show that, at 74 MHz, roughly 100 calibrator sources can be observed at any single time, which is far greater than the number of instantaneously available GPS or similar satellite beacons. With 52 stations, the LWA will be capable of probing at least 5,000 different lines of sight through the ionosphere. And with the effective collecting area of the LWA, we find that all available calibrator sources can be observed in well under 10 seconds. Implementation of this scheme requires that the LWA monitor and control system be capable of switching between sources on a 5-10 millisecond time scale.