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

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.

Apparent radio transients mapping the near-Earth plasma environment

“…To analyse this constraint in more detail, we must first note that the scintillation zones are tens of degrees across in the sky, but as we will show below, the individual lensing plasma bubbles are much smaller. However, scintillation studies done with LOFAR by Fallows et al (2020) indicate that a fair approximation is that a scattering screen with turbulent cells moves past our line of sight rather faster than that it changes in its own rest frame, as in the case of interstellar scintillation. Therefore we will assume that the angular velocity we measure for the overall region also applies to the individual turbulent cells.…”

“…In a recently published study of a strong scintillation episode in Cas A with LOFAR, Fallows et al (2020) perform a detailed analysis of dynamic spectra and of the time sequence in which the scintillation hits different LOFAR stations. They find that this episode is caused primarily by two layers of the ionosphere, the D region at around 80 km, where the plasma speed was about 110 m s −1 and the F region at 300-400 km altitude, where that speed was 20-40 m s −1 at the time.…”

Climatology Characteristics of Ionospheric Irregularities Described with GNSS ROTI

“…Rapid changes in ionospheric plasma density can significantly affect the performances of such instruments as the Low Frequency Array (LOFAR)-a European radio interferometer operating on a frequency range of 20-240 MHz [15]. Such sensitive instruments as LOFAR provide, also, some possibilities of ionospheric irregularity observations [16].…”

Study of the equatorial ionosphere using the Giant Metrewave Radio Telescope (GMRT) at sub-GHz frequencies

“…Observations are therefore recommended during day-time hours, especially for the LOFAR-EoR project. Recently, Fallows et al (2020) with a combined analysis of Global Navigation Satellite System (GNSS), ionosondes and LOFAR reveals large-scale travelling ionospheric disturbances (TIDs) and small-scale TIDs travelling perpendicular to each other. These cause instabilities which breakdown large-scale structures into smaller scales during a weak sub-storm at high latitudes.…”