Breakthrough Listen

Abstract:

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.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).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.Work on a measure of the strength of the RAFs is underway using the amplitude scintillation index S4. 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.ReferencesBoyde, 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.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.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.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.

Abstract:

The newest generation of radio telescopes is able to survey large areas with high sensitivity and cadence, producing data volumes that require new methods to better understand the transient sky. Here, we describe the results from the first citizen science project dedicated to commensal radio transients, using data from the MeerKAT telescope with weekly cadence. Bursts from Space: MeerKAT was launched late in 2021 and received ∼89 000 classifications from over 1000 volunteers in 3 months. Our volunteers discovered 142 new variable sources which, along with the known transients in our fields, allowed us to estimate that at least 2.1 per cent of radio sources are varying at 1.28 GHz at the sampled cadence and sensitivity, in line with previous work. We provide the full catalogue of these sources, the largest of candidate radio variables to date. Transient sources found with archival counterparts include a pulsar (B1845-01) and an OH maser star (OH 30.1–0.7), in addition to the recovery of known stellar flares and X-ray binary jets in our observations. Data from the MeerLICHT optical telescope, along with estimates of long time-scale variability induced by scintillation, imply that the majority of the new variables are active galactic nuclei. This tells us that citizen scientists can discover phenomena varying on time-scales from weeks to several years. The success both in terms of volunteer engagement and scientific merit warrants the continued development of the project, while we use the classifications from volunteers to develop machine learning techniques for finding transients.