Search Results
Interferometric imaging of the type IIIb and U radio bursts observed with LOFAR on 22 August 2017
2023, Dabrowski, Bartosz, Mikuła, Katarzyna, Flisek, Paweł, Vocks, Christian, Zhang, PeiJin, Magdalenić, Jasmina, Warmuth, Alexander, Morosan, Diana E., Froń, Adam, Fallows, Richard A., Bisi, Mario M., Krankowski, Andrzej, Mann, Gottfried, Błaszkiewicz, Leszek, Carley, Eoin P., Gallagher, Peter T., Zucca, Pietro, Rudawy, Paweł, Hajduk, Marcin, Kotulak, Kacper, Sidorowicz, Tomasz
Context. The Sun is the source of different types of radio bursts that are associated with solar flares, for example. Among the most frequently observed phenomena are type III solar bursts. Their radio images at low frequencies (below 100 MHz) are relatively poorly studied due to the limitations of legacy radio telescopes. Aims. We study the general characteristics of types IIIb and U with stria structure solar radio bursts in the frequency range of 20-80 MHz, in particular the source size and evolution in different altitudes, as well as the velocity and energy of electron beams responsible for their generation. Methods. In this work types IIIb and U with stria structure radio bursts are analyzed using data from the LOFAR telescope including dynamic spectra and imaging observations, as well as data taken in the X-ray range (GOES and RHESSI satellites) and in the extreme ultraviolet (SDO satellite). Results. In this study we determined the source size limited by the actual shape of the contour at particular frequencies of type IIIb and U solar bursts in a relatively wide frequency band from 20 to 80 MHz. Two of the bursts seem to appear at roughly the same place in the studied active region and their source sizes are similar. It is different in the case of another burst, which seems to be related to another part of the magnetic field structure in this active region. The velocities of the electron beams responsible for the generation of the three bursts studied here were also found to be different.
A LOFAR observation of ionospheric scintillation from two simultaneous travelling ionospheric disturbances
2020, Fallows, Richard A., Forte, Biagio, Astin, Ivan, Allbrook, Tom, Arnold, Alex, Wood, Alan, Dorrian, Gareth, Mevius, Maaijke, Rothkaeh, Hanna, Matyjasiak, Barbara, Krankowski, Andrzej, Anderson, James M., Asgekar, Ashish, Avruch, I. Max, Bentum, Mark, Bisi, Mario M., Butcher, Harvey R, Ciardi, Benedetta, Dabrowski, Bartosz, Damstra, Sieds, de Gasperin, Francesco, Duscha, Sven, Eislöffel, Jochen, Franzen, Thomas M.O., Garrett, Michael A., Griessmeier, Jean-Matthias, Gunst, Andre W., Hoeft, Matthias, Horandel, Jorg R., Iacobelli, Marco, Intema, Huib T., Koopmans, Leon V.E., Maat, Peter, Mann, Gottfried, Nelles, Anna, Paas, Harm, Pandey, Vishambhar N., Reich, Wolfgang, Rowlinson, Antonia, Ruiter, Mark, Schwarz, Dominik J., Serylak, Maciej, Shulevski, Aleksander, Smirnov, Oleg M., Soida, Marian, Steinmetz, Matthias, Thoudam, Satyendra, Toribio, M. Carmen, van Ardenne, Arnold, van Bemmel, Ilse M., van der Wiel, Matthijs H.D., van Haarlem, Michiel P., Vermeulen, Rene C., Vocks, Christian, Wijers, Ralph A.M.J., Wucknitz, Olaf, Zarka, Philippe, Zucca, Pietro
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.
Interpretation of Radio Wave Scintillation Observed through LOFAR Radio Telescopes
2022, Forte, Biagio, Fallows, Richard A., Bisi, Mario M., Zhang, Jinge, Krankowski, Andrzej, Dabrowski, Bartosz, Rothkaehl, Hanna, Vocks, Christian
Radio waves propagating through a medium containing irregularities in the spatial distribution of the electron density develop fluctuations in their intensities and phases. In the case of radio waves emitted from astronomical objects, they propagate through electron density irregularities in the interstellar medium, the interplanetary medium, and Earth’s ionosphere. The LOFAR radio telescope, with stations across Europe, can measure intensity across the VHF radio band and thus intensity scintillation on the signals received from compact astronomical objects. Modeling intensity scintillation allows the estimate of various parameters of the propagation medium, for example, its drift velocity and its turbulent power spectrum. However, these estimates are based on the assumptions of ergodicity of the observed intensity fluctuations and, typically, of weak scattering. A case study of single-station LOFAR observations of the strong astronomical source Cassiopeia A in the VHF range is utilized to illustrate deviations from ergodicity, as well as the presence of both weak and strong scattering. Here it is demonstrated how these aspects can lead to misleading estimates of the propagation medium properties, for example, in the solar wind. This analysis provides a method to model errors in these estimates, which can be used in the characterization of both the interplanetary medium and Earth’s ionosphere. Although the discussion is limited to the case of the interplanetary medium and Earth’s ionosphere, its ideas are also applicable to the case of the interstellar medium.