A Multispacecraft Analysis and Modeling of Type III Radio Burst Exciter Deceleration in Inhomogeneous Heliospheric Plasma

TL;DR

This work exploits simultaneous four-spacecraft observations (PSP, STA, SolO, Wind) to localize Type III burst sources and accurately derive exciter velocities and accelerations, correcting for source–spacecraft geometry and light-travel time. The analysis shows exciter speeds scaling roughly as $u \propto f^{0.32 \pm 0.12}$ and accelerations as $a \propto r^{-1.71 \pm 0.20}$, consistent with a deceleration driven by a decreasing-density heliospheric plasma. A simple gas-dynamic model predicts $u(x)/u(x_0) = (\omega_{pe}(x)/\omega_{pe}(x_0))^{1/4}$, leading to $u(r) \propto r^{-0.29}$ and $a(r) \propto r^{-1.58}$ for $n(r) \propto r^{-2.3}$, in good agreement with observations. The study also finds inter-spacecraft differences in measured drift rates likely due to radio-wave scattering, underscoring the importance of multi-distance measurements for Type III analyses. These results reinforce beam–plasma interactions in inhomogeneous plasmas as the main deceleration mechanism and provide a framework for future statistical investigations.

Abstract

Electron beams accelerated in solar flares and escaping from the Sun along open magnetic field lines can trigger intense radio emissions known as type III solar radio bursts. Utilizing observations by Parker Solar Probe (PSP), STEREO-A (STA), Solar Orbiter (SolO), and Wind spacecrafts, the speeds and accelerations of type III exciters are derived for simple and isolated type III solar bursts. For the first time, simultaneous four spacecraft observations allow to determine positions, and correct the resulting velocities and accelerations for the location between the spacecraft and the apparent source. We observe velocities and acceleration to change as $u(r) \propto r^{-0.37 \pm 0.14}$ and $a(r) \propto r^{-1.71 \pm 0.20}$ with radial distance from the Sun $r$. To explain the electron beam deceleration, we develop a simple gas-dynamic description of the electron beam moving through plasma with monotonically decreasing density. The model predicts that the beam velocity decreases as $u(f)\propto f^{1/4}(r)$, so the acceleration changes $\propto r^{-1.58}$ (and speed as $\propto r^{-0.29}$) for the plasma density profile $n(r)\propto r^{-2.3}$. The deceleration is consistent with the average observation values corrected for the type III source locations. Intriguingly, the observations also show differences in velocity and acceleration of the same type III observed by different spacecrafts. We suggest the difference could be related to the additional time delay caused by radio-wave scattering between the spacecraft and the source.

Figures (7)

• Figure 1: Dynamic spectra (left) and frequency-time (right) on the 11 July 2020 by the PSP, STEREO-A and SolO spacecraft (from top to bottom). For each spacecraft, the peak-flux frequencies (and the fit) are plotted on the right for the times-frequencies selected by the green dashed box, containing peak flux points (green 'X' symbols), along with their fitted curve (green dashed line), while the fitted positions of the emitter as a function of time and the normalized residuals from the fit are shown on the right. Blue and red lines correspond, respectively, to the fundamental and harmonic components.
• Figure 2: Type III burst peak fluxes measured by four different spacecrafts (left) and spacecraft positions (right) in HEE coordinates during the 11 July 2020 (2:30 UT) event. The direction of maximum directivity is found by fitting Equation \ref{['eq:musset']} for the peak fluxes from STEREO-A, PSP, Wind and SolO at 979 kHz. This frequency was selected on the assumption that for $\,\hbox{$\sim$}\hbox{$>$}, 1$ MHz, the Sun's magnetic field is approximately radial, meaning that the observed radio sources will mainly have been shifted radially due to scattering. On the left peak fluxes are plotted as a function of HEE Longitude. The red dashed line shows the position of the radio source as revealed by the directivity fit. On the right are the position of Solar Orbiter, Parker Solar Probe, STEREO-A and Wind projected in the plane of the HEE coordinate system.
• Figure 3: Type III exciter propagating from position ${r_1}$ to ${r_2}$ with constant velocity $v_s$ at an angle $\phi$ to the line of sight. This simple representation allows us to correct for the source-to-spacecraft light travel time (Equation \ref{['eq:delta_t']}).
• Figure 4: Exciter velocities and accelerations from the 11 July 2020 type III burst for the PSP, STEREO-A and SolO spacecraft (from top to bottom). Blue and red lines correspond, respectively, to the fundamental and harmonic components. Velocity as a function of frequency is shown on the left, where the shaded areas show velocity deduced from 2015AA...580A.137K by STEREO-A and STEREO-B data. Median values are shown with transparent solid (STEREO-A) and dashed (STEREO-B) lines. On the right, velocity and acceleration of the exciter are plotted as a function of distance. Black dashed lines correspond to the result from Equation \ref{['eq:u_x']}, where $x_0$ corresponds to the location where the highest analysed frequency is emitted.
• Figure 5: Velocities deduced by PSP, STEREO-A and SolO data are represented by lines different color shades, with darker to lighter shades being associated to PSP, STEREO-A and SolO, respectively. Blue and red shades are associated to fundamental and harmonic emission, respectively. The top row displays fundamental emission, while the bottom row represents the harmonic component. The events of July 11, 2020, and July 21, 2020, are distinguished using dots and crosses as markers, respectively. Shaded regions represent the results from the 2015AA...580A.137K analysis, with median values showcased by the transparent solid (STEREO-A) and dashed (STEREO-B) lines.