Speaker
Description
For the long-wave radio spectrum in which LOFAR operates, the ionosphere is of key importance in the context of radiation transfer and is a source of distortion in the recorded signal. Therefore, good knowledge and predictability related to the shape and dynamics of the ionosphere is extremely important for the LOFAR system [1].
However, for astrophysical research it is a disadvantage, it can provide an additional and independent source of information from standard techniques (like GNSS) in the processes of studying and modelling the ionosphere. Furthermore, the remarkable sensitivity of the LOFAR system makes it an extremely sensitive tool for observing flux changes in scintillation processes in the ionosphere. This is exploited by observing bright sources such as CasA or CygA. See for example [2]. At the same time, LOFAR is a powerful instrument for radio diagnostics of ionospheric plasma.
Observations of pulsars carried out since the very beginning of the LOFAR Radio Telescope prove that it is one of the best instruments for this type of research (see, for example, [3]). However, most of the analysis is based on data obtained at significant integration times. In a previous paper [4], we showed the qualitative influence of the ionosphere on changes in the relative flux of pulsar profiles. The present work, based on the dynamical spectra of bright pulsars, highlights in many cases an unusual variability that may be – as we assume – caused by the ionosphere. Moreover, this variability is well explained by models of fine-scale variations in the ionosphere.
In our studies, dynamic spectra are constructed based on temporal S/N (signal-to-noise) values. The analysis of variability based on wavelet analysis, the study of the second-order spectrum and the comparative analysis of such a signal with the results of measurements based on GNSS data allow us to determine the dynamics of changes in the direction of the observed source. Additional aspect is the determination of the PSI - Pulsar Scintillation Index, based on the S4 index described in [5,6].
During the presentation, we will show the observed phenomena and the presence of quasi-periodic variations visible in the dynamical spectra. We will discuss the possible correspondence between the characteristics of observed spectra revealed from LOFAR data and theoretical characteristics of wave excitations in the atmosphere-ionosphere. We will also present the results of correlations between the dynamical spectra of selected pulsars observed simultaneously by the Polish LOFAR stations PL612 and PL611.
References:
1.M. P. van Haarlem et al., “LOFAR: The LOw-Frequency ARray,” A&A, 556 ,2013, A2, DOI: https://doi.org/10.1051/0004-6361/201220873
2. R. A. Fallows et al., “Broadband meter-wavelength observations of ionospheric scintillation”, J. Geophys. Res. SpacePhysics,119, 10,544–10,560, 2014 doi:10.1002/2014JA020406.
3. A. Noutsos et al., 2015 Pulsar polarisation below 200 MHz: Average profiles and propagation effects, A&A, 576, A62, 2015 DOI: 10.1051/0004-6361/201425186.
4. L.P. Błaszkiewicz et al., “Finding the Ionospheric Fluctuations Reflection in the Pulsar Signals’ Characteristics Observed with LOFAR”. Sensors.; 21(1):51, 2021, DOI: https://doi.org/10.3390/s21010051.
5. B. Forte et al., Interpretation of Radio Wave Scintillation Observed through LOFAR Radio Telescopes, Astroph. Journal, Supplement Series, Vol. 263, Issue 21, 2022, DOI: 10.3847/1538-4365/ac6deb
6. P. Flisek et al., "Towards the possibility to combine LOFAR and GNSS measurements to sense ionospheric irregularities", J. Space Weather Space Clim. Vol. 13, 2023, DOI: https://doi.org/10.1051/swsc/2023021
