Broadband meter‐wavelength observations of ionospheric scintillation

Abstract

Intensity scintillations of cosmic radio sources are used to study astrophysical plasmas like the ionosphere, the solar wind, and the interstellar medium. Normally, these observations are relatively narrow band. With Low‐Frequency Array (LOFAR) technology at the Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) station in northern Finland we have observed scintillations over a three‐octave bandwidth. “Parabolic arcs,” which were discovered in interstellar scintillations of pulsars, can provide precise estimates of the distance and velocity of the scattering plasma. Here we report the first observations of such arcs in the ionosphere and the first broadband observations of arcs anywhere, raising hopes that study of the phenomenon may similarly improve the analysis of ionospheric scintillations. These observations were made of the strong natural radio source Cygnus‐A and covered the entire 30–250 MHz band of KAIRA. Well‐defined parabolic arcs were seen early in the observations, before transit, and disappeared after transit although scintillations continued to be obvious during the entire observation. We show that this can be attributed to the structure of Cygnus‐A. Initial results from modeling these scintillation arcs are consistent with simultaneous ionospheric soundings taken with other instruments and indicate that scattering is most likely to be associated more with the topside ionosphere than the F region peak altitude. Further modeling and possible extension to interferometric observations, using international LOFAR stations, are discussed.

Murchison Widefield Array Observations of Anomalous Variability: A Serendipitous Night-Time Detection of Interplanetary Scintillation

“…Ionospheric scintillation affects sources with angular sizes 10 ( Loi et al 2015b) and the latter usually not very significant (although see Fallows et al 2014). Therefore, ionospheric refraction should affect the thousands of unresolved sources in each image.…”

Separating Nightside Interplanetary and Ionospheric Scintillation With Lofar

“…The Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA; McKay-Bukowski et al (2014)), a station built using LOFAR hardware in arctic Finland, has been routinely monitoring the ionosphere, including ionospheric scintillation, since 2012 (e.g. Fallows et al 2014): The ionospheric scintillation conditions above KAIRA are naturally more severe than above LOFAR: At auroral latitudes, refractive index gradients due to fieldline elongated ionisation structures are stronger than in the case of middle latitudes structures. These observations can, therefore, be used to verify the effects of periods of strong ionospheric scintillation.…”

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

“…The first stage of analysis was the calculation of delay-Doppler spectra: these were created from the dynamic spectra using 5-min time slices, advancing every minute through the observation, following the methods described in Fallows et al (2014). To avoid regions more heavily contaminated by RFI, the frequency band used was restricted to 28.5-64.1 MHz.…”

“…The presence of two scintillation arcs likely indicates that scattering is dominated by two distinct layers in the ionosphere. A simple analysis, as described in Fallows et al (2014), can be used to estimate the altitude of the scattering region with a basic formula relating arc curvature C to velocity V and distance L along the line of sight to the scattering region (Cordes et al, 2006): The square term for the velocity illustrates the importance of gaining a good estimate of velocity to be able to accurately estimate the altitude of the scattering region via this method.…”

“…In observations of interstellar scintillation these spectra can exhibit discrete parabolic arcs which can be modelled to give information on the distance to the scattering "screen" giving rise to the scintillation and its velocity across the line of sight (Stinebring et al, 2001;Cordes et al, 2006). Broadband observations of ionospheric scintillation are not common, but such arcs have been observed using the Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA, McKay-Bukowski et al (2014) an independent station built using LOFAR hardware in arctic Finland) in a study by Fallows et al (2014). Model spectra produced by Knepp & Nickisch (2009) have also illustrated parabolic arc structures, particularly in the case of scattering from a thin scattering screen.…”