Solar type III radio burst time characteristics at LOFAR frequencies and the implications for electron beam transport

Abstract

Solar type III radio bursts contain a wealth of information about the dynamics of electron beams in the solar corona and the inner heliosphere; currently unobtainable through other means. However, the motion of different regions of an electron beam (front, middle and back) have never been systematically analysed before. We characterise the type III burst frequency-time evolution using the enhanced resolution of LOFAR in the frequency range 30 to 70 MHz and use this to probe electron beam dynamics. Methods. The rise, peak and decay times with a 0.2 MHz spectral resolution were defined for a collection of 31 type III bursts. The frequency evolution is used to ascertain the apparent velocities of the front, middle and back of the type III sources and the trends are interpreted using theoretical and numerical treatments. The type III time profile was better approximated by an asymmetric Gaussian profile, not an exponential as previously used. Rise and decay times increased with decreasing frequency and showed a strong correlation. Durations were smaller than previously observed. Drift rates from the rise times were faster than from the decay times, corresponding to inferred mean electron beam speeds for the front, middle and back of 0.2, 0.17, 0, 15 c, respectively. Faster beam speeds correlate with smaller type III durations. We also find type III frequency bandwidth decreases as frequency decreases. The different speeds naturally explain the elongation of an electron beam in space as it propagates through the heliosphere. The rate of expansion is proportional to the mean speed of the exciter; faster beams expand faster. Beam speeds are attributed to varying ensembles of electron energies at the front, middle and back of the beam.

15 pages, 14 figures, Accepted by Astronomy and Astrophysics

References in corpus (14)

• A Review of Solar Type III Radio Bursts
• Imaging Spectroscopy of Solar Radio Burst Fine Structures
• LOFAR tied-array imaging of Type III solar radio bursts
• Large scale simulations of solar type III radio bursts: flux density, drift rate, duration and bandwidth
• Dynamics of electron beams in the inhomogeneous solar corona plasma
• Onsets and spectra of impulsive solar energetic electron events observed near the Earth
• Imaging Spectroscopy of Type U and J Solar Radio Bursts with LOFAR
• Evolution of the Solar Flare Energetic Electrons in the Inhomogeneous Inner Heliosphere
• Statistical Survey of Type III Radio Bursts at Long Wavelengths Observed by the Solar TErrestrial RElations Observatory (STEREO)/Waves Instruments: Radio Flux Density Variations with Frequency
• The Low-High-Low Trend of Type III Radio Burst Starting Frequencies and Solar Flare Hard X-rays
• Langmuir Wave Electric Fields Induced by Electron Beams in the Heliosphere
• Radio Diagnostics of Electron Acceleration Sites During the Eruption of a Flux Rope in the Solar Corona
• Numerical consideration of quasilinear electron cloud dynamics in plasma
• Stopping Frequency of Type III Solar Radio Bursts in Expanding Magnetic Flux Tubes

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• Sub-second time evolution of Type III solar radio burst sources at fundamental and harmonic frequencies
• A Fokker-Planck Framework for Studying the Diffusion of Radio Burst Waves in the Solar Corona
• Imaging Spectroscopy of CME-Associated Solar Radio Bursts
• On Solar Recurrent Coronal Jets: Coronal Geysers as Sources of Electron Beams and Interplanetary Type-III Radio Bursts
• On the Source Position and Duration of a Solar Type III Radio Burst Observed by LOFAR
• First Results from the REAL-time Transient Acquisition backend (REALTA) at the Irish LOFAR station
• Solar Radio Spikes and Type IIIb Striae Manifestations of Sub-second Electron Acceleration Triggered by a Coronal Mass Ejection
• Deriving Large Coronal Magnetic Loop Parameters Using LOFAR J burst Observations
• Properties of Type III and Type IIIb Bursts in the Frequency Band of 8-80 MHz during PSP Perihelion at the Beginning of April 2019
• Forward Modeling of the Type III Radio Burst Exciter
• Revisiting the frequency drift rates of decameter type III solar bursts observed in July-August 2002
• Observation of solar radio burst events from Mars orbit with the Shallow Radar instrument
• Temporally resolved Type III solar radio bursts in the frequency range 3-13 MHz
• Interferometric imaging, and beam-formed study of a moving Type IV Radio burst with LOFAR
• Numerical simulations of a continuously injected relativistic electron beam relaxation into a plasma with large-scale density gradients
• Single-Mode Wave Decay