Photo voltaic radio bursts are an oblique signature of accelerated electron beams from the photo voltaic ambiance. These quick electrons generate Langmuir waves as they propagate by way of a reducing plasma density and finally result in the intense broadband radio emissions with a attribute quick frequency drift in dynamic spectra. Density turbulence within the plasma can modulate this course of, producing superb buildings equivalent to sub-second, narrowband striae and spikes. These superb buildings might also current a frequency drift that has been related to the Langmuir wave group velocity and coronal temperature (Reid et al., 2021, see their equation 2).
Because the radio-waves propagate, the turbulence (which is anisotropic with respect to the ambient magnetic area) additionally results in scattering results, inflicting distortions of their noticed place, measurement, and timing. The broadening of the time profile might also dilute the noticed burst drift-rate, which might be notably important for slim bandwidth buildings, influencing the interpretation of the driving force.
Latest LOFAR observations by Clarkson et al., (2021, 2023) discovered non-radial, fixed-frequency supply movement of radio spikes over time that was attributed to anisotropic scattering in an setting with a non-radial magnetic area equivalent to a coronal loop. On this work, we use an approximation of such a magnetic construction (a dipole) with radio-wave scattering simulations (Kontar et al., 2019) to clarify this movement. We additional discover the implications of the scatter-induced discount of superb construction drift-rates and the way their dynamic spectra morphology can differ relying on the emission location in non-radial magnetic fields.
Determine 1. Simulation outcomes for a 35.2 MHz supply injected at a heliocentric angle of fifty levels. (High Row) Sky-plane centroid movement, X, Y centroid positions, and FWHM measurement and space, overlaid with the time profile. (Center row) Scatter photos (2D histograms) at totally different instances akin to the dashed traces within the higher panels. (Backside row) Pictures convolved with a 2D Gaussian mimicking a LOFAR Low Band antenna.
In a radially symmetric magnetic area, anisotropic scattering causes a supply shift radially away from the Solar, projected into the sky-plane. In a non-radial magnetic area, the obvious supply trajectory (parallel to the native area) and broadening axis (perpendicular to the native area) depends upon the emitter’s place inside the construction. Determine 1 reveals a radio supply under the apex of a coronal loop. The supply centroids shift vertically within the sky-plane over the FWHM time, matching the fastened frequency, non-radial movement of a LOFAR noticed radio spike (Clarkson et al., 2021), each in place and distance. For sources situated on the loop apex, photon escape happens alongside each instructions of the dipole area, stopping a transparent centroid trajectory over time. The identical mechanism results in attention-grabbing ends in sure situations the place a single emission supply can produce two distinct elements (Determine 2) in areas of robust anisotropy owing to excessive directivity alongside the guiding area. As proven in panels (e,f), such supply bifurcation might not be noticed relying on the instrument decision. The mannequin additional reveals that sources emitted alongside a area parallel to the plasma-frequency floor stay within the robust scattering area for longer, experiencing elevated scattering and absorption, which results in longer durations and fainter sources than these alongside open area traces.
Determine 3 presents preliminary radio pulses convolved with a scattered time profile for each instantaneous emission, and drifting emission. In each circumstances, the time profile at every frequency is broadened and delayed. For the drifting case, the frequency drift-rate is diluted, and given the dependence of the time profile on the turbulent situations and the emitter location within the area construction, the dilution stage can also be depending on these situations; that’s, weaker anisotropy and emission close to the loop apex produces a stronger drift-rate discount. The dilution additionally implies that the noticed superb construction drift-rate underestimates the Langmuir wave group velocity and coronal temperature; for instance, at 30 MHz, an noticed drift-rate of 10-20 kHz s$^{-1}$ implies $T_esim(0.3-0.6)$ MK quite than 1-1.5 MK as soon as corrected for scattering.
Determine 3. (Left) Preliminary radio pulses and the convolution with a scattered time profile. (Proper) Preliminary radio pulses convolved with scattered time profiles from emission sources at totally different places in a dipole. (Tailored from Clarkson et al. 2025).
Conclusion
Photo voltaic radio burst superb buildings current advanced dynamics, but their noticed traits are additional sophisticated by propagation results. The inclusion of a dipolar magnetic area in radio-wave scattering simulations highlights how the obvious sources transfer alongside the path of the magnetic area traces, permitting for puzzling non-radial supply movement of radio bursts at fastened frequencies to be reproduced, and that anisotropic scattering can produce greater than a single supply. The noticed superb construction drift-rates are diluted relying on the scatter contribution to the time profile. This will differ for sources in numerous places in a given magnetic construction and must be accounted for if superb construction drift-rates are used to deduce traits of the setting and emission course of. The outcomes present that each magnetic geometry and anisotropic scattering each play an necessary position in how we interpret photo voltaic radio bursts.
Based mostly on a current paper by Daniel L. Clarkson and Eduard P. Kontar Magnetic Subject Geometry and Anisotropic Scattering Results on Photo voltaic Radio Burst Observations (2025), ApJ, 978, 73. DOI: https://doi.org/10.3847/1538-4357/ad969c
References
Clarkson, D. L., Kontar. E. P., Gordovskyy, M. et al. 2021, ApJL, 917, 2, L32
Clarkson, D. L., Kontar. E. P., Vilmer, N. et al. 2023, ApJ, 946, 1, 33
Kontar, E. P., Chen, X., Chrysaphi, N. et al. 2019, ApJ, 884, 2, 122
Reid, H. A. S. and Kontar, E. P. 2021, Nat. Astron., 5, 796-804
