Solar Radio Echoes Reveal High-Altitude Particle Acceleration
Scientists have identified 613 repeating radio burst pairs in the solar corona, providing new insights into magnetic reconnection and particle acceleration at altitudes far higher than conventional models predict.
Key Findings
- Primary bursts are brief and intense, with a median duration of 0.31 seconds, while delayed components are longer and more diffuse, lasting a median of 1 second.
- The median delay between primary and delayed bursts is 4.3 seconds.
- Both components share the same central frequency and similar bandwidth.
- Primary bursts show rapid negative frequency drifts; delayed bursts have reduced drift rates.
- Spatial mapping reveals primary bursts are concentrated above the negative-polarity region of the active source, while delayed emissions are displaced outward into the upper corona.
- Observations suggest that magnetic reconnection and electron acceleration occur at altitudes near one solar radius, higher than conventional models predict.
Methodology
Researchers used the Low Frequency Array (LOFAR) in its low-band outer configuration (30–80 MHz), employing tied-array beamforming with over 200 synthesized beams. A semi-automated screening process identified isolated burst pairs.
Spatial origins were determined via 2D Gaussian fitting on calibrated maps, with corrections for ionospheric distortions. EUV data from SDO's AIA and magnetic field measurements from HMI were combined with PFSS models to reconstruct 3D magnetic fields.
Implications
The study, published in Nature Communications, found that secondary bursts are echo-like emissions from reflections off dense plasma structures, consistent with magnetic reconnection and particle acceleration at high altitudes.
The findings provide a tool for probing coronal density gradients and scale heights, enhancing understanding of plasma conditions affecting energetic particle propagation. The research also addresses why radar echoes from the Sun are weaker than expected—attributed to anisotropic scattering redirecting radio waves away from the viewing geometry.