Signatures of Large-Scale Magnetic Field Disturbances and Switchbacks in Interplanetary Type III Radio Bursts

Abstract

Type III solar radio bursts are driven by non-thermal electron beams travelling along heliospheric magnetic fields, with the radio emission frequency drift-rate determined by the beam speed and the plasma density profile. Analysing beam kinematics inferred from the drift-rate reveals behaviour inconsistent with the emitter moving radially through smooth, monotonically decreasing density. We examine whether these features are driven by disturbances in the guiding magnetic field direction, such as switchbacks, rather than plasma inhomogeneities along the beam path. Using simulations and remote observations of 24 interplanetary type III bursts observed by Parker Solar Probe, we relate measured drift-rate variations to local field deflections. In 50% of events, we identify disturbances above a $2σ$ noise level that can be attributed to perpendicular deflections of the field between (0.7-1.7) R$_\odot$, over scales (1.8-6.4) R$_\odot$ at heliocentric distances (9-30) R$_\odot$. The features correspond to either density changes of (10-30)%, or deflections of the field direction by (23-88)$^\circ$. Further, beam transport simulations show field direction perturbations produce additional observational signatures in type III bursts: delayed emission, intensity breaks, and enhanced emission resembling stria fine structures. In addition, we identified four bursts where the observed variations are more plausibly explained by field deflections, possibly in the form of magnetic switchbacks, than by unrealistically large density changes along the field line. The results show that variations in type III burst profiles can arise from magnetic as well as density fluctuations, and demonstrate the value of type III bursts as remote probes of inner-heliospheric structure at kilometric wavelengths.