Characterization of the ionosphere above the Murchison Radio Observatory using the Murchison Widefield Array

Abstract

We detail new techniques for analysing ionospheric activity, using Epoch of Reionisation (EoR) datasets obtained with the Murchison Widefield Array (MWA), calibrated by the 'Real-Time System' (RTS). Using the high spatial-and temporal-resolution information of the ionosphere provided by the RTS calibration solutions over 19 nights of observing, we find four distinct types of ionospheric activity, and have developed a metric to provide an 'at a glance' value for data quality under differing ionospheric conditions. For each ionospheric type, we analyse variations of this metric as we reduce the number of pierce points, revealing that a modest number of pierce points is required to identify the intensity of ionospheric activity; it is possible to calibrate in real-time, providing continuous information of the phase screen. We also analyse temporal correlations, determine diffractive scales, examine the relative fractions of time occupied by various types of ionospheric activity, and detail a method to reconstruct the total electron content responsible for the ionospheric data we observe. These techniques have been developed to be instrument agnostic, useful for application on LOFAR and SKA-Low.

Deep multiredshift limits on Epoch of Reionization 21 cm power spectra from four seasons of Murchison Widefield Array observations

“…This latter point has some support from the results of using different amounts of data for the EoR0 high-band ( Figure 11), and the lack of correlation found between 1D power spectrum limits and ionospheric activity metric (plot not shown). Although previous work has demonstrated that moderate-strong ionospheric activity can leave residual power in the power spectrum (Jordan et al 2017;Trott et al 2017), the data in this work are selected to be ionospherically quiet, and sorting based on that metric as the primary dimension may not be the optimal approach.…”

“…(ii) Ionospheric Activity: we use the ionospheric activity metric developed by Jordan et al (2017), which uses the measured versus expected source positions of 1000 point sources in the field-of-view to estimate an ionospheric phase screen, and derive an activity metric that combines median source offset with source-offset anisotropy. Different thresholds are set for Phase I and Phase II datasets, where the reduced angular resolution of the latter array configuration yields a higher base activity level;…”

Data Analysis for Precision 21 cm Cosmology

“…This is typically viewed as problem for direction-dependent calibration. For example, one can use known positions of catalogued bright point sources to solve simultaneously solve for antenna gains and ionospheric phase shifts (Mitchell et al 2008;Jordan et al 2017;Gehlot et al 2018b). In some cases, Global Positioning Satellite measurements can be used to provide supplemental information to enhance solutions obtained under such a scheme (Arora et al 2015).…”

21 cm Observations: Calibration, Strategies, Observables

“…The wide field of view does not allow to isolate low foreground patches, but it allows to opt for drift scan observations or a mix of drift scan and pointed observations ( [83]), which have the advantage of more time stable primary beams. Wide field ionospheric effects are somewhat mitigated by the array compactness ( [38]). The MWA can therefore leverage on the strength of both redundant and traditional calibration and can adopt a mixture of foreground subtraction and avoidance strategies.…”