Power spectrum analysis of ionospheric fluctuations with the Murchison Widefield Array

Loi, Shyeh Tjing; Trott, Cathryn M.; Murphy, Tara; Cairns, Iver H.; Bell, M. E.; Hurley‐Walker, N.; Morgan, John S.; Lenc, E.; Offringa, A. R.; Lü, Feng; Hancock, P. J.; Kaplan, D. L.; Kudryavtseva, N. A.; Bernardi, G.; Bowman, Judd D.; Briggs, F.; Cappallo, R. J.; Corey, B. E.; Deshpande, A. A.; Emrich, D.; Gaensler, B. M.; Goeke, R.; Greenhill, L. J.; Hazelton, B. J.; Johnston‐Hollitt, M.; Kasper, J. C.; Kratzenberg, E.; Lonsdale, Colin J.; Lynch, M. J.; McWhirter, S. R.; Mitchell, D. A.; Morales, M. F.; Morgan, E.; Oberoi, D.; Ord, S. M.; Prabu, T.; Rogers, A. E. E.; Roshi, A.; Shankar, N. Udaya; Srivani, K. S.; Subrahmanyan, R.; Tingay, S. J.; Waterson, M.; Wayth, R. B.; Webster, R. L.; Whitney, A. R.; Williams, Anne; Williams, C. L.

doi:10.1002/2015rs005711

Cited by 38 publications

(60 citation statements)

References 88 publications

(138 reference statements)

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“…In each snapshot image, the ionosphere introduces a random displacement (typically 10-20 arcsec) in the source positions (Loi et al 2015). This smears out sources in the mosaicked image.…”

Section: Discussion and Future Workmentioning

confidence: 99%

The 154 MHz radio sky observed by the Murchison Widefield Array: noise, confusion, and first source count analyses

Franzen

Jackson

Offringa

et al. 2016

Mon. Not. R. Astron. Soc.

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We analyse a 154 MHz image made from a 12 h observation with the Murchison Widefield Array (MWA) to determine the noise contribution and behaviour of the source counts down to 30 mJy. The MWA image has a bandwidth of 30.72 MHz, a field-of-view within the half-power contour of the primary beam of 570 deg 2 , a resolution of 2.3 arcmin and contains 13,458 sources above 5σ. The rms noise in the centre of the image is 4 − 5 mJy/beam. The MWA counts are in excellent agreement with counts from other instruments and are the most precise ever derived in the flux density range 30-200 mJy due to the sky area covered. Using the deepest available source count data, we find that the MWA image is affected by sidelobe confusion noise at the ≈ 3.5 mJy/beam level, due to incompletely-peeled and out-of-image sources, and classical confusion becomes apparent at ≈ 1.7 mJy/beam. This work highlights that (i) further improvements in ionospheric calibration and deconvolution imaging techniques would be required to probe to the classical confusion limit and (ii) the shape of low-frequency source counts, including any flattening towards lower flux densities, must be determined from deeper ≈ 150 MHz surveys as it cannot be directly inferred from higher frequency data.

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“…In each snapshot image, the ionosphere introduces a random displacement (typically 10-20 arcsec) in the source positions (Loi et al 2015). This smears out sources in the mosaicked image.…”

Section: Discussion and Future Workmentioning

confidence: 99%

The 154 MHz radio sky observed by the Murchison Widefield Array: noise, confusion, and first source count analyses

Franzen

Jackson

Offringa

et al. 2016

Mon. Not. R. Astron. Soc.

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“…Most of our knowledge of the large‐scale ionosphere is derived from measurements from dedicated experiments with ionosondes and GPS satellites and receivers. Measurements of ionospheric structure have been done by radio telescopes before, for example, traveling ionospheric disturbances (TIDs) [ van Velthoven , ; Spoelstra , ; Helmboldt et al , ], turbulent like fluctuations [ Spoelstra , ; Cohen and Röttgering , ] and plasmaspheric irregularities [ Jacobson and Erickson , ; Helmboldt et al , ; Loi et al , ]. LOFAR can contribute to these measurements in a unique way.…”

Section: Introductionmentioning

confidence: 98%

Probing ionospheric structures using the LOFAR radio telescope

et al. 2016

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LOFAR is the LOw‐Frequency Radio interferometer ARray located at midlatitude (52°53′N). Here we present results on ionospheric structures derived from 29 LOFAR nighttime observations during the winters of 2012/2013 and 2013/2014. We show that LOFAR is able to determine differential ionospheric total electron content values with an accuracy better than 0.001 total electron content unit = 1016m−2 over distances ranging between 1 and 100 km. For all observations the power law behavior of the phase structure function is confirmed over a long range of baseline lengths, between 1 and 80 km, with a slope that is, in general, larger than the 5/3 expected for pure Kolmogorov turbulence. The measured average slope is 1.89 with a one standard deviation spread of 0.1. The diffractive scale, i.e., the length scale where the phase variance is 1rad2, is shown to be an easily obtained single number that represents the ionospheric quality of a radio interferometric observation. A small diffractive scale is equivalent to high phase variability over the field of view as well as a short time coherence of the signal, which limits calibration and imaging quality. For the studied observations the diffractive scales at 150 MHz vary between 3.5 and 30 km. A diffractive scale above 5 km, pertinent to about 90% of the observations, is considered sufficient for the high dynamic range imaging needed for the LOFAR epoch of reionization project. For most nights the ionospheric irregularities were anisotropic, with the structures being aligned with the Earth magnetic field in about 60% of the observations.

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“…Animations of the distortion vector field for each of the three nights are shown in Movies S1, S2, and S3 in the supporting information (see Loi et al [] for an explanation of the visualization scheme). In all plots, black solid lines are the geomagnetic field lines from L = 1.8, computed assuming a simple dipole model with the MWA at a geomagnetic latitude of 38.6°S and a magnetic declination of 0° (this is more accurately −0.153° [ Thébault et al , ], but we neglect this for simplicity of plotting).…”

Section: Resultsmentioning

confidence: 99%

“…These were obtained by taking the Fourier transform of the data cubes with all measurement values replaced by unity. Details of the method can be found in Loi et al [2015b]. The negative slope of the ringing features in the response functions arise from the drift of celestial sources as the Earth rotates, which produces diagonal tracks in the spatiotemporal sampling pattern.…”

Section: Appendix B: Power Spectrum Response Functionsmentioning

confidence: 99%

“…These include the Murchison Widefield Array (MWA) [ Lonsdale et al , ; Bowman et al , ; Tingay et al , ] and the Low Frequency Array [ van Haarlem et al , ]. Recent work using the MWA to map the regional TEC distribution [ Loi et al , , ] has demonstrated the success of the wide FOV, large N design of radio interferometer for surveying the near‐Earth plasma [ Coster et al , ].…”

Section: Introductionmentioning

confidence: 99%

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Density duct formation in the wake of a travelling ionospheric disturbance: Murchison Widefield Array observations

et al. 2016

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Geomagnetically aligned density structures with a range of sizes exist in the near‐Earth plasma environment, including 10–100 km wide VLF/HF wave‐ducting structures. Their small diameters and modest density enhancements make them difficult to observe, and there is limited evidence for any of the several formation mechanisms proposed to date. We present a case study of an event on 26 August 2014 where a travelling ionospheric disturbance (TID) shortly precedes the formation of a complex collection of field‐aligned ducts, using data obtained by the Murchison Widefield Array (MWA) radio telescope. Their spatiotemporal proximity leads us to suggest a causal interpretation. Geomagnetic conditions were quiet at the time, and no obvious triggers were noted. Growth of the structures proceeds rapidly, within 0.5 h of the passage of the TID, attaining their peak prominence 1–2 h later and persisting for several more hours until observations ended at local dawn. Analyses of the next 2 days show field‐aligned structures to be preferentially detectable under quiet rather than active geomagnetic conditions. We used a raster scanning strategy facilitated by the speed of electronic beamforming to expand the quasi‐instantaneous field of view of the MWA by a factor of 3. These observations represent the broadest angular coverage of the ionosphere by a radio telescope to date.

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Power spectrum analysis of ionospheric fluctuations with the Murchison Widefield Array

Cited by 38 publications

References 88 publications

The 154 MHz radio sky observed by the Murchison Widefield Array: noise, confusion, and first source count analyses

The 154 MHz radio sky observed by the Murchison Widefield Array: noise, confusion, and first source count analyses

Probing ionospheric structures using the LOFAR radio telescope

Density duct formation in the wake of a travelling ionospheric disturbance: Murchison Widefield Array observations

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