Solar radio explosions are an indirect firm of accelerated electrons of the solar atmosphere. These rapid electrons generate Langmuir waves as they propagate through a decreasing plasma density and, ultimately, lead to bright broadband radio emissions with a quick frequency drift characteristic in dynamic spectra. Density turbulence in plasma can modulate this process, producing fine structures such as stretch marks and picos. These fine structures can also present a frequency drift that has been associated with the speed of the Langmuir wave group and coronal temperature (Reid et al., 2021See your equation 2).
As radio waves propagate, turbulence (which is anisotropic with respect to the environmental magnetic field) also leads to dispersion purposes, causing distortions in their position, size and time observed. The expansion of the time profile can also dilute the observed burst drift rate, which can be particularly significant for narrow bandwidth structures, which influence the driver’s interpretation.
Recent Lofar Observations of Clarksson et al. (2021, 2023) They found a non -radial source movement of fixed frequency of radio peaks with the time attributed to anisotropic dispersion in an environment with a non -radial magnetic field such as a coronal loop. In this work, we use an approximation of this magnetic structure (a dipole) with radio wave dispersion simulations (Kontart et al., 2019) To explain this movement. We also explore the consequences of the reduction induced by the dispersion of fine structure drift rates and how their dynamic spectra morphology can vary depending on the location of emission in non -radial magnetic fields.
Figure 1. The simulation results for a 35.2 MHz source injected into a 50 -degree heliocentric angle. (Upper row) Centroids of the plane of the sky, positions of Centroids X, y Y size and FWHM area, superimposed with the time profile. (Central row) Dispersion images (2D histograms) at different times corresponding to discontinuous lines in the upper panels. (Lower row) Images Convolted with a 2D Gaussian who mimics a Baja Lofar band antenna.
In a radially symmetrical magnetic field, anisotropic dispersion causes a radially far from the sun, projected in the plane of the sky. In a non -radial magnetic field, the trajectory of apparent origin (parallel to the local field) and the expansion of the axis (perpendicular to the local field) depends on the position of the emitter within the structure. Figure 1 shows a radio source under the apex of a coronal loop. Centroids of origin change vertically in the plane of the sky during the FWHM time, coinciding with the fixed frequency, the non -radial movement of a observed radio peak (Clarkson et al., 2021), both in position and in distance. For the sources located in the appex of the loop, the escape of the photons is produced throughout both directions of the Dipolo field, avoiding a clear central trajectory over time. The same mechanism leads to interesting results in certain conditions in which a single source of emission can produce two different components (Figure 2) in strong anisotropy regions due to high directivity along the guide field. As shown in the panels (E, F), said source fork may not be observed depending on the resolution of the instrument. The model also shows that the sources emitted along a field parallel to the plasma frequency surface remain in the strong dispersion region for a longer time, experiencing greater dispersion and absorption, which leads to longer durations and more sources and more sources weak than those of open field lines.
Figure 3 presents initial radio pulses convolutioned with a dispersed time profile for both instantaneous emission and for drifting. In both cases, the time profile in each frequency is extended and delayed. In the case of drift, the frequency derivation rate is diluted and given the dependence of the time profile in turbulent conditions and the location of the issuer in the field structure, the dilution level also depends on these conditions; That is, the weakest anisotropy and emission near the vertex of the loop produce a stronger drift rate reduction. The dilution also implies that the drift rate observed underestimated the speed of the LANGMUU wave group and the coronal temperature; For example, at 30 MHz, an observed drift rate of 10-20 kHz s $^{-1} $ implies $ t_e \ sim (0.3-0.6) $ mk instead of 1-1.5 mk once corrected by the dispersion .
Figure 3. (Left) Initial radio pulses and the convolution with a dispersed time profile. (Right) Initial radio pulses Convolted with dispersed time profiles of emission sources in different locations in a dipole. (Adapted from Clarksson et al. 2025).
Conclusion
The fine structures of the solar radio are presented a complex dynamic, however, its observed characteristics are further complicated by the propagation effects. The inclusion of a dipole magnetic field in radio wave dispersion simulations high At fixed frequencies, and that anisotropic dispersion can produce more than a single source. The drift rates observed are diluted depending on the dispersion contribution to the time profile. This may vary for sources in different locations in a given magnetic structure and should be taken into account if fine structure drift rates are used to infer characteristics of the environment and the emission process. The results show that both magnetic geometry and anisotropic dispersion play an important role in the way we interpret solar radio explosions.
Based on a recent article by Daniel L. Clarkson and Eduard P. Kontar Geometry of the magnetic field and anisotropic dispersion effects on Radio Solar Ráfaga observations (2025), APJ, 978, 73. doi: https://doi.org/10.3847/1538-4357/ad969c
References
Clarksson, DL, Kontar. EP, Gordovskyy, M. et al. 2021, APJL, 917, 2, L32
Clarksson, DL, Kontar. EP, Vilmer, N. et al. 2023, APJ, 946, 1, 33
Kontar, EP, Chen, X., Chrysaphi, N. et al. 2019, APJ, 884, 2, 122
Reid, Ha and Kontar, EP 2021, Nat. Astron., 5, 796-804
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