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

TL;DR

This work addresses how interplanetary Type III radio burst drift-rates respond to upstream magnetic-field disturbances versus plasma-density gradients. The authors combine Parker Solar Probe observations of 24 bursts with 1D kinetic simulations of beam-Langmuir-wave dynamics and targeted field-line perturbation models to link drift-rate variations to magnetic deflections. They find that half the events show drift-rate variations not easily explained by radial density changes alone, with disturbances characterized by scales of $1.8-6.4\,R_igodot$ and field deflections of $|\theta|\approx 23^{\circ}-88^{\circ}$ (mean ~ $47^{\circ}$); in several cases, magnetic switchback-like disturbances provide a more plausible explanation than large along-field density fluctuations. The simulations predict additional observable signatures—delayed emission, intensity breaks, and stria-like enhancements—that align with some PSP bursts, supporting the view that type III bursts can diagnose inner-heliospheric magnetic structure at kilometric wavelengths and complement in-situ measurements.

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.

Figures (14)

• Figure 1: Cartoon showing the influence of a magnetic field disturbances on a type III radio burst profiles. In each column, a radio emitting source propagating along the magnetic structure would generate the radio burst profile displayed in the lower panels. (a) Large-scale magnetic loop. (b) A 90$\arcdeg$ deflection. In this case, a $\theta=\pm90\arcdeg$ deflection of the field in either direction would produce the same reduction in frequency drift-rate on the burst profile in dynamic spectra, shown in the lower panel. The red arrows show the radial density gradient with lighter color representing lower density.
• Figure 2: A $90\arcdeg$ field perturbation (a) and a large-scale magnetic loop (b). The electrons propagate along the radial and perturbed fields with an initial speed of $v_0=0.1c$ and decelerate as $v(r)\propto r^{-0.3}$. (i) Paths of each magnetic field overlaid onto a 2D, radially symmetric heliospheric density map constructed using equation \ref{['eq:Nparker']}. The plasma density is shown by the colour gradient. The segments of each path analysed are shown as solid lines. The radial grid lines are separated by $5\arcdeg$. (ii) The emission frequency experienced along each path. (iii) The distance corresponding to each frequency from panels (ii). (iv) Perpendicular deviation $r_\perp$ between the radial and curved paths. The dotted line shows the $2\sigma$ noise level determined in Appendix \ref{['appendix:noise']}. (v)$B_\perp/B$ along the curved path.
• Figure 3: As in figure \ref{['fig:kink_Jburst_sim']} but for two field perturbations determined by (a) an arctan function and (b) a double Gaussian function. The upper panels show the field lines for a Parker spiral (white dashed lines) and the perturbed field line (red). The segment of the nominal Parker spiral used to compare to the perturbed field line are shown as solid white lines. The background gradient represents a 2D plasma density field. The lower panels show $f(t)$, $r(t)$, $r_\perp$ and $B_\perp/B$ over time. The dashed black lines represent the $2\sigma$ noise level from Appendix \ref{['appendix:noise']}. The open red circles in all panels represents the distances probed by PSP due to the spectral resolution of the FIELDS instrument.
• Figure 4: Spacecraft orbits during 14-21 January 2024 (thick black lines) in HEE coordinates. The green circle denotes Earth. Additional spacecraft orbits (Solar Orbiter and STEREO-A) are also shown. The arrows represent the approximate source longitude for 15 of the type III bursts from multi-spacecraft intensity fitting where the colour represents dates increasing from dark to light red.
• Figure 5: An interplanetary type III burst observed by PSP on 2024 January 20 near 02:29 UT. (a) Dynamic spectrum. The white circles mark the time of each intensity peak. (b)$r(t)$ profile (white circles). The black line shows the polynomial best fit, and the red lines mark the length of $r_\perp$. The grey region shows the fit corridor from 1000 realisations generated by Monte Carlo sampling of the measurement uncertainties. The black line in panel (a) shows the polynomial best fit converted to frequency space. (c)$r_\perp(t)$. The dotted lines represent the $2\sigma$ noise level (Appendix \ref{['appendix:noise']}). Open circles show values of $r_\perp$ larger than the noise, and closed grey points denote values indistinguishable from noise. Blue circles and connected blue line show a perturbation where a scale can be defined. (d)$|\Delta{n}/n|$ over heliocentric distance. (e)$|B_\perp/B|$ over heliocentric distance. Since this quantity is determined by the differences in $r_\perp$, the values are plotted against against the midpoint of the two times used for each difference.