Research Article

A LOFAR observation of ionospheric scintillation from two simultaneous travelling ionospheric disturbances

¹ ASTRON – The Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
² Department of Electronic and Electrical Engineering, University of Bath, Claverton Down, BA2 7AY Bath, UK
³ School of Science and Technology, Nottingham Trent University, Clifton Lane, NG11 8NS Nottingham, UK
⁴ Space Environment and Radio Engineering, School of Engineering, The University of Birmingham, Edgbaston, B15 2TT Birmingham, UK
⁵ Space Research Centre, Polish Academy of Sciences, Bartycka 18A, 00-716 Warsaw, Poland
⁶ Space Radio-Diagnostics Research Centre, University of Warmia and Mazury, ul. Romana Prawocheskiego 9, 10-719 Olsztyn, Poland
⁷ Technische Universität Berlin, Institut für Geodäsie und Geoinformationstechnik, Fakultät VI, Sekr. H 12, Hauptgebäude Raum H 5121, Straße des 17. Juni 135, 10623 Berlin, Germany
⁸ GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
⁹ Shell Technology Center, 562149 Bangalore, India
¹⁰ Science and Technology B.V., 2616 LR Delft, The Netherlands
¹¹ RAL Space, UKRI STFC, Rutherford Appleton Laboratory, Harwell Campus, OX11 0QX Oxfordshire, UK
¹² Mt Stromlo Observatory, Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, ACT 2611 Weston Creek, Australia
¹³ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
¹⁴ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁵ Thüringer Landessternwarte, Sternwarte 4, 07778 Tautenburg, Germany
¹⁶ Jodrell Bank Centre for Astrophysics (JBCA), Department of Physics & Astronomy, University of Manchester, Alan Turing Building, Oxford Road, M139PL Manchester, UK
¹⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁸ LPC2E – Université d’Orléans/CNRS, 45071 Orléans Cedex 2, France
¹⁹ Station de Radioastronomie de Nançay, Observatoire de Paris, PSL Research University, CNRS, Univ. Orléans, OSUC, 18330 Nançay, France
²⁰ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²¹ Nikhef, Science Park 105, 1098 XG Amsterdam, The Netherlands
²² Vrije Universiteit Brussel, Astronomy and Astrophysics Research Group, Pleinlaan 2, 1050 Brussel, Belgium
²³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
²⁴ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
²⁵ ECAP, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erwin-Rommel-Str. 1, 91054 Erlangen, Germany
²⁶ DESY, Platanenallee 6, 15738 Zeuthen, Germany
²⁷ CIT, Rijksuniversiteit Groningen, Nettelbosje 1, 9747 AJ Groningen, The Netherlands
²⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²⁹ Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
³⁰ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
³¹ South African Radio Astronomy Observatory, 2 Fir Street, Black River Park, Observatory, 7925 Cape Town, South Africa
³² Department of Physics and Astronomy, University of the Western Cape, 7535 Cape Town, South Africa
³³ Department of Physics and Electronics, Rhodes University, PO Box 94, 6140 Makhanda, South Africa
³⁴ Jagiellonian University in Kraków, Astronomical Observatory, ul. Orla 171, 30-244 Kraków, Poland
³⁵ Department of Physics, Khalifa University, PO Box 127788, Abu Dhabi, United Arab Emirates
³⁶ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
³⁷ Joint Institute for VLBI-ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
³⁸ LESIA & USN, Observatoire de Paris, CNRS, PSL, SU/UP/UO, 92195 Meudon, France

^* Corresponding author: fallows@astron.nl

Received: 2 December 2019
Accepted: 13 February 2020

Abstract

This paper presents the results from one of the first observations of ionospheric scintillation taken using the Low-Frequency Array (LOFAR). The observation was of the strong natural radio source Cassiopeia A, taken overnight on 18–19 August 2013, and exhibited moderately strong scattering effects in dynamic spectra of intensity received across an observing bandwidth of 10–80 MHz. Delay-Doppler spectra (the 2-D FFT of the dynamic spectrum) from the first hour of observation showed two discrete parabolic arcs, one with a steep curvature and the other shallow, which can be used to provide estimates of the distance to, and velocity of, the scattering plasma. A cross-correlation analysis of data received by the dense array of stations in the LOFAR “core” reveals two different velocities in the scintillation pattern: a primary velocity of ~20–40 ms⁻¹ with a north-west to south-east direction, associated with the steep parabolic arc and a scattering altitude in the F-region or higher, and a secondary velocity of ~110 ms⁻¹ with a north-east to south-west direction, associated with the shallow arc and a scattering altitude in the D-region. Geomagnetic activity was low in the mid-latitudes at the time, but a weak sub-storm at high latitudes reached its peak at the start of the observation. An analysis of Global Navigation Satellite Systems (GNSS) and ionosonde data from the time reveals a larger-scale travelling ionospheric disturbance (TID), possibly the result of the high-latitude activity, travelling in the north-west to south-east direction, and, simultaneously, a smaller-scale TID travelling in a north-east to south-west direction, which could be associated with atmospheric gravity wave activity. The LOFAR observation shows scattering from both TIDs, at different altitudes and propagating in different directions. To the best of our knowledge this is the first time that such a phenomenon has been reported.