Observations made using the LOw Frequency ARray (LOFAR) between 10:15 and 11:48 UT on the 15th of September 2018 over a bandwidth of approximately 25-65 MHz contain discrete pseudo-periodic features of ionospheric origin. These features occur with a period of approximately ten minutes and collectively last roughly an hour. They are strongly frequency dependent, broadening significantly in time towards the lower frequencies, and show an overlaid pattern of diffraction fringes. By modelling the ionosphere as a thin phase screen containing a wave-like disturbance, we are able to replicate the observations, suggesting that they are associated with small-scale travelling ionospheric disturbances (TIDs). This modelling indicates that the features observed here require a compact radio source at a low elevation, and that the TID or TIDs in question have a wavelength ~30 km. Several features suggest the presence of deviations from an idealised sinusoidal wave form. These results demonstrate LOFAR's capability to identify and characterise small-scale ionospheric structures.

The signal received at a ground-based radio antenna from a distant natural radio source is a convolution of the source and a time-and frequency-dependent variability in amplitude and phase induced by scattering from plasma irregularities in the ionosphere. These effects can be broadly divided into diffractive scintillation (Carrano et al., 2011;Singleton, 1970) and refractive scattering (e.g., Forte, 2008), and chiefly depend on the scale sizes of the plasma irregularities and the wavelength of the radio waves received (Booker, 1981;Booker & Majidiahi, 1981). Observing the various manifestations of these signal variations therefore permits remote characterization of ionospheric behavior. When examining such variations in the high frequency (HF) to very high frequency (VHF) band, with corresponding radio wavelengths ranging between ∼100 and 1 m, one can observe a wide variety of ionospheric scattering characteristics that span the refractive and diffractive domains (e.g., Fallows et al., 2014Fallows et al., , 2020.Ionospheric scintillation is the rapid variation (∼1-s) in received amplitude and phase of radio waves transiting the ionosphere, due to diffractive scattering of the radio waves by small-scale plasma density features in the raypath (e.g.,

<p>The Low Frequency Array (LOFAR) is designed to observe the early universe at radio wavelengths. When radio waves from a distant astronomical source traverse the ionosphere, structures in the plasma affect the signal. The high temporal resolution available (~10 ms), the large range of frequencies observed (10-80 MHz & 120-240 MHz) and the large number of receiving stations (currently 52 across Europe) mean that LOFAR can observe the effects of the midlatitude ionosphere in an unprecedented level of detail.</p><p>The observational programme LT16_002 began in September 2021 and observations from the first 15 months of this programme are used to investigate ionospheric structures. A variety of patterns in the received signal intensity have been observed. Some of these appear to be similar to features reported previously, such as Spectral Caustics seen in solar observations (Koval et al., 2017) using the Nançay Decametric Array, as well as observations inferred from LOFAR of Travelling Ionospheric Disturbances (TIDs) at large- and medium-scales (Fallows et al., 2020), small scale TIDs (Boyde et al., 2022) and sporadic E (Wood et al., 2022). Other structures appear to be previously unreported. Collectively, we refer to these structures as Radio Alteration Features (RAFs).</p><p>In order to investigate the occurrence and origin of RAFs, 1092 hours of observations from LT16_002 were analysed. If the intensity of the received signal rose to 20% above the median value for the observation in a given hour then, within this study, this hour was classified as containing a RAF. RAFs were observed in 382 hours of observations. RAFs are primarily a night-time phenomenon and are more common in summer. They do not appear to have a statistically-significant relationship to geomagnetic activity as measured by a variety of geomagnetic indices, but there is some evidence that they are more common during times of enhanced solar activity or when a CME encounters the Earth.</p><p>Work on a measure of the strength of the RAFs is underway using the amplitude scintillation index S<sub>4</sub>. New observations from LT16_002 mean that the database is continually expanding. Comparisons of the climatology of RAFs to the climatology of other features, such as TIDs, is planned to give an insight into the driving processes. The latest developments in this work will be reported.</p><p><strong>References</strong></p><p>Boyde, B., Wood, A. G., Dorrian, G. D., Fallows, R. A., Themens, D. R., et al. (2022). Lensing from small-scale travelling ionospheric disturbances observed using LOFAR. J. Space Weather Space Clim. 12, 34. https://doi.org/10.1051/swsc/2022030.</p><p>Fallows, R. A., et al. (2020), A LOFAR Observation of Ionospheric Scintillation from Simultaneous Medium- and Large-scale Travelling Ionospheric Disturbances, J. Space Weather Space Clim. doi.org/10.1051/swsc/2020010.</p><p>Koval, A., et al. (2017), Traveling ionospheric disturbances as huge natural lenses: Solar radio emission focusing effect, J. Geophys. Res. Space Physics, 122, 9092–9101, doi:10.1002/2017JA024080.</p><p>Wood, A. G., Dorrian, G. D., Boyde, B. and Fallows, R. A. (2022), Terrestrial drivers of rapidly changing plasma structures observed with the International LOFAR Telescope, 3rd URSI AT-AP-RASC.</p>

No abstract