Lessons about the ionosphere from the MWA Transients Survey
13 October 2015
Over the past two years, the Murchison Widefield Array (MWA) radio telescope has been used to conduct a general survey for transient and variable radio sources, the MWA Transients Survey (MWATS). Although the MWA’s large field of view alone gives it impressive survey speeds, capitalising on the fact that it can be repointed almost instantaneously in any direction lets us take this to a whole new level. In MWATS, the instrument cycles swiftly between three different pointings, gaining rapid coverage of the sky as the Earth rotates. Effectively, the telescope is used like a raster scanner, greatly expanding its angular coverage.
The low frequencies of operation mean that the MWA is highly sensitive to refractive distortions caused by the ionosphere. Combining all these properties makes the MWA useful for characterising ionospheric fluctuations over large expanses of sky (see our press release and video), a task important for understanding what the future Square Kilometre Array will experience. Gradients of electron density in the ionosphere produce displacements of point sources as a function of angular position and time, potentially affecting automated source extraction and association algorithms. Apparent brightnesses can also change, impacting how accurately radio light curves can be measured.
Former CAASTRO Honours student Cleo Loi (University of Sydney, now PhD student at the University of Cambridge, UK) and colleagues statistically analysed about 51 hours of MWATS data and revealed that at the MWATS observing frequency of 154 MHz, refractive displacements are typically 10-20 arcsec, consistent with the density gradients associated with atmospheric waves. This is several times smaller than the resolution of the telescope, and so is unlikely to affect spatial cross-matching. An upper bound on brightness fluctuations of ionospheric origin was placed at 1-3%, smaller than the contribution from other sources of noise under typical circumstances. The authors conclude that these results reassuringly suggest that the ionosphere is not a significant impediment to the goals of MWATS and other time-domain studies with the MWA at these frequencies.
In another publication by Cleo and her team, the same analyses used to extract ionospheric fluctuations uncovered an interesting event on the night of the 26th of August 2014. A large-amplitude travelling ionospheric disturbance, observed over a huge angular field of view, was seen to pass overhead, triggering the formation of a collection of density ducts in its wake. The event also evidences one possible mechanism for the formation of density ducts in the ionosphere: that they may be triggered by propagating disturbances. The ducts were aligned along the Earth’s magnetic field lines, a property expected for plasma density structures high in the atmosphere. Notably, geomagnetic conditions were very quiet at the time. This suggests that ionospheric activity does not necessarily correlate with global geomagnetic activity, making ionospheric forecasting potentially difficult.
Overall, what Cleo’s results have demonstrated is that studying ionospheric structure can be a fully commensal application of an instrument like the MWA alongside astronomical research. A general-purpose low-frequency survey like MWATS can reveal fascinating events happening high up in the Earth’s atmosphere. Ultimately, harnessing the MWA’s dual-purpose capabilities as an astronomical telescope and a geospace probe will allow it to realise its full scientific potential.