Signatures of Large-scale Magnetic Field Disturbances and Switchbacks in Interplanetary Type III Radio Bursts by Daniel L. Clarkson et al.

Solar radio bursts are intrinsically linked to the motion of their emitting source through the coronal and heliospheric plasma. Electron transport is mostly confined to magnetic field lines. These electrons move at a substantial fraction of the speed of light and often generate radio emission via the plasma emission process. The resulting radio bursts, such as type III bursts from electrons streaming along open field lines, are an excellent diagnostic of the environment through which they propagate.

Tracing the spine of a radio burst by its peak intensity provides the frequency drift rate (e.g. Krupar et al. 2015; Azzollini et al. 2025). For an electron beam moving along a radial path, one would expect a drift rate that gradually decreases over time. Yet type III burst drift rates can vary on smaller frequency scales. For example, fine structures such as striae, which arise from density fluctuations along the beam path, can produce substantial variation in the drift rate over the burst lifetime. Moreover, for an emitter moving along a coronal loop, the drift rate can reduce to zero and then reverse (e.g. Reid et al. 2017; Zhang et al. 2024) and CESRA nuggets. This provides a clear example of how large-scale magnetic field structures affect burst morphology in dynamic spectra. Given the turbulent nature of the solar atmosphere, we test whether changes in type III burst drift rates can also be explained by magnetic field deviations such as switchbacks or large-scale deflections.

Figure 1. Simulations of propagating electron beams along perturbed field lines. (i) Perturbed (red) and unperturbed (white) magnetic field lines. (ii) Frequency over time experienced by propagating electron beams along the paths in panel (i), converted to distance in panel (iii). (iv) Deviation of perturbed path $r_perp$. (v) Change in the perpendicular field ratio $B_perp/B$. The open red circles show the frequencies probed by PSP/FIELDS.

To relate a change in drift rate to a magnetic field deflection, we connect fluctuations in the frequency drift to variations in distance, and map these to angular changes in the magnetic field via $B_perp/B = (dr_perp/dr) / sqrt{1 + (dr_perp/dr)^2}$, where $r_perp$ represents perpendicular deviations from a reference direction $r$. Figure 1 applies this procedure to a simulation of an electron beam propagating along a perturbed path (panel i), with the corresponding frequency–time profile shown in panel (ii). A clear reduction in drift rate is observed, which appears as a fluctuation in $B_perp/B$ in panel (v).

Figure 2. Numerical simulations of electron beam and Langmuir wave evolution along radial (a) and perturbed (b) fields. Each panel shows the generated type III burst. Panel (c) is reduced to the spectral resolution of PSP.

We also numerically simulate the evolution of an electron beam and the subsequent generation of Langmuir waves and type III radio emission (Kontar et al. 2001; Reid et al. 2021), both with and without field deviations. Figure 1a shows the smoothly decreasing drift rate for an electron beam propagating along a radial field. Panel (b) shows a type III burst produced along the perturbed path in Figure 1(i). Four observational signatures point to the presence of a field disturbance: a reduction in the frequency drift rate; a delay in the onset and decay of the burst; a break in the radio intensity; and an enhancement in intensity that appears as striae fine structure. This provides a new mechanism for the production of interplanetary striae, extending that proposed for coronal type IIIb bursts.

Following these findings, we analyse 24 interplanetary type III bursts observed by Parker Solar Probe (PSP) over one week. The peak frequencies are converted to distance and compared with a polynomial fit to determine $r_perp$. We estimate a noise level of 0.57 solar radii, so deviations above this threshold indicate real disturbances. Across the 24 events, 50% show deviations beyond this level, with an average displacement of 1.1 solar radii. These can be explained by density changes of 10%-30%, or magnetic field deviations of 23-88 degrees, over spatial scales of 1.8-6.4 solar radii. We further identify four type III bursts that exhibit some or all of the features seen in the simulations (e.g. Figure 3). The observed variations in these bursts are more plausibly explained by magnetic field deviations, such as switchbacks, than by unrealistically large density changes along the field.

Figure 3. Observational examples of a type III burst that exhibits the signatures of a magnetic field deviation such as a switchback.

These results show that variations in type III burst profiles can arise from both magnetic and density fluctuations, and highlight the value of type III bursts as remote probes of inner heliospheric structure at kilometre wavelengths.

Based on the recent paper by Daniel L. Clarkson and Eduard P. Kontar 2026, The Astrophysical Journal, 999, 134. [arXiv:2601.19687] DOI: https://doi.org/10.3847/1538-4357/ae3dae

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