LOFAR reveals a new class of burst pairs linked to high-altitude magnetic reconnection

The sun does not transmit radio signals in silence. Every time a population of non-thermal electrons is accelerated through a magnetized, turbulent plasma — which happens constantly in the solar corona — the electromagnetic consequences can be detected from Earth by sensitive low-frequency arrays. What those arrays have been hearing for decades are bursts: abrupt, narrowband flashes of radio energy that drift in frequency as the electrons that generate them travel outward through a corona of declining plasma density.

But a new paper published in Nature Communications on June 9, 2026, shows that some of those bursts are doing something that had never been documented before. They are echoing. Not metaphorically — physically, measurably, with characteristic timing, a consistent spatial displacement, and a mechanism that can now be explained from first principles.

The study, led by Suli Ma (University of Chinese Academy of Sciences and University of Glasgow) together with Eduard P. Kontar, Daniel L. Clarkson, Huadong Chen, Yihua Yan, and collaborators, used the LOw Frequency ARray (LOFAR) to discover and characterize over a thousand instances of a previously unknown fine structure in solar radio emission: the spike-like repeating burst pair.

The solar corona as a radio laboratory

The solar corona — the outermost layer of the solar atmosphere, extending millions of kilometers into space — is a plasma: a gas of ionized particles embedded in a magnetic field, constantly in motion, intermittently explosive. It is in this environment that some of the most energetically powerful processes in the solar system occur: solar flares, coronal mass ejections, magnetic reconnection events. Each of these releases energy in ways that leave signatures across the electromagnetic spectrum.

At low radio frequencies — tens to hundreds of megahertz — the corona is a remarkably structured emitter. Electrons accelerated through reconnection events generate Langmuir waves in the surrounding plasma, and those waves convert to electromagnetic radiation at the local plasma frequency and its harmonics. Because plasma frequency decreases with the electron density of the ambient medium, and density decreases with altitude, radio emission at successively lower frequencies is produced progressively higher in the corona. This is why solar radio bursts drift from high to low frequencies over time — they trace the outward trajectory of the electron beam that generates them.

LOFAR observes precisely this frequency range, between roughly 10 and 90 MHz, with temporal and spectral resolution sufficient to resolve fine structures lasting fractions of a second.

A new type of burst: the repeating pair

On July 9, 2017, LOFAR observed an active region on the sun — designated AR12665 — during a two-hour interval. The observation captured numerous well-known burst types: broadband type III bursts produced by fast electron beams, and their fine-structure variants known as type IIIb bursts, with individual sub-components called striae. But amid this expected activity, something else appeared repeatedly in the dynamic spectra: pairs of short, narrowband radio spikes occurring at the same frequency and separated by an interval of approximately four seconds.

The team identified and statistically characterized 613 of these burst pairs from the observation window. Each pair consists of two components. The first — designated the «earlier» (E) component — is a brief, bright, narrowband spike lasting typically around 0.3 seconds with a spectral width of about 70 kilohertz, occurring at frequencies between 30 and 50 MHz. Approximately four seconds later, at exactly the same frequency, a second spike appears — the «delayed» (D) component. It is dimmer (roughly half the peak flux of the E burst), longer in duration (around 1 second), and exhibits a markedly reduced frequency drift rate compared to its counterpart.

The consistency of the four-second delay across more than a thousand independently occurring pairs is itself a remarkable result. This is not statistical scatter. The timing is constrained by physics — specifically by the geometry of the corona through which the radio emission propagates.

The echo mechanism: anisotropic scattering at work

The central interpretive breakthrough of the study is the identification of the delayed burst not as an independent emission event, but as a turbulent echo of the earlier burst.

The mechanism works as follows. When the earlier spike is generated — at the harmonic frequency of local plasma emission, twice the local plasma frequency — its radiation propagates outward in two directions simultaneously. Some of it escapes directly upward through the corona toward the observer. But some is directed initially downward, toward a layer of denser plasma deeper in the corona where the plasma frequency equals the emission frequency. At this layer, the downward-traveling radiation cannot propagate further and is effectively reflected, sent back outward through the corona. This reflected radiation, scattered and broadened by the turbulent plasma it traverses on the return journey, arrives at the observer approximately four seconds later.

The four-second delay is not arbitrary. It corresponds to the time required for light to travel down to the reflecting layer and back up through a density scale height of roughly one-quarter to one-third of a solar radius. The delay encodes a measurement of coronal density structure.

What makes this scattering and reflection possible is the anisotropy of the coronal plasma turbulence. Density fluctuations in the corona are not isotropic — they are elongated preferentially along magnetic field lines. This field-aligned structure means that radio waves scatter differently depending on their direction of travel relative to the field. With an anisotropy parameter between approximately 0.1 and 0.2 (quantified as the ratio of parallel to perpendicular wavenumbers of the density fluctuations), the simulations developed by the team reproduce the observed four-second delay, the reduction in drift rate of the delayed burst, and the spatial displacement between the two components.

The simulations also demonstrate something diagnostically important: the observed timing and spatial offsets are consistent only with harmonic emission, not fundamental emission. If the bursts were generated at the fundamental plasma frequency rather than its second harmonic, the expected time delay would be only about one second and the D component would not be spatially displaced from the E component. The data unambiguously select the harmonic interpretation.

What the imaging revealed: spatial echoes in the corona

One of the most powerful features of LOFAR is its ability to perform imaging spectroscopy — constructing radio images of the sun at specific frequencies and times, rather than simply recording integrated fluxes. This capability proved decisive in understanding the repeating burst pairs.

Imaging analysis of 309 burst pairs reveals that the E and D components are not co-spatial. The earlier burst originate from a region concentrated above and near the active region’s negative magnetic polarity — a spatially compact, well-defined source. The delayed component, by contrast, appears displaced by up to 400 arcseconds in the sky plane, spread over a larger area, and distributed farther from the active region core.

This pattern is exactly what the anisotropic scattering model predicts. The reflected, scattered harmonic emission is redirected along magnetic field lines, emerging from an apparent location offset from the true source by the full path of the downward and upward propagation. The directionality imposed by anisotropic turbulence concentrates the scattered echo along the field geometry, producing a systematic rather than random displacement.

The images also reveal that the burst sources reside within a cone-shaped magnetic structure arching over the active region — a geometry consistent with a funnel of closed field lines on one side and open field lines on the other, providing both a confinement structure for the initial emission and a pathway for the reflected echo to escape.

Reconnection at altitude — what the burst location tells us

The frequencies at which the repeating burst pairs are observed — 30 to 50 MHz — correspond, under standard coronal density models for active regions, to altitudes of approximately one solar radius above the photosphere. This is significantly higher than the low-lying regions typically associated with solar flares.

The implication is direct: the electron acceleration and magnetic reconnection driving these bursts are happening high in the corona, in a domain that standard flare models have historically not emphasized. The active region AR12665 exhibited continuous mini-jets and eruptions during the observation interval, visible in extreme ultraviolet imaging from NASA’s Solar Dynamics Observatory. These small-scale reconnection events, launching outflows and jets from the upper corona, are the probable physical trigger for the electron beams that produce the repeating pairs.

This elevation of the reconnection site has consequences for how we model the transport of energetic particles through the heliosphere. Electrons accelerated at high coronal altitudes encounter a different plasma environment on their journey into the solar wind than do electrons accelerated near flare ribbons at low altitudes. The burst pairs provide a new observational handle on this high-altitude acceleration regime.

Why radar echoes from the sun are so weak

The study raises an unexpected connection to a decades-old problem in radio science: the extreme weakness of radar echoes from the sun.

Since the 1960s, astronomers have attempted to detect the reflection of Earth-based radar signals from the solar corona. These experiments involve transmitting a powerful radio signal toward the sun and searching for the echo returning to Earth. At 50 MHz, radar echoes from the sun should be detectable in principle — but they have proven to be far weaker than models of simple reflection would predict.

The repeating burst pairs offer a natural explanation. The same anisotropic scattering that produces the four-second echo delay in the natural solar radio bursts is also responsible for redirecting radar reflections away from the Earth-Sun axis. The anisotropic turbulence channels reflected radiation preferentially along magnetic field directions — which are generally not aligned with the observer on Earth. What should be a specular-like echo is instead scattered into a broad cone aligned with the field, most of which misses Earth entirely.

The observation of a naturally occurring echo from the corona — the delayed burst component — provides the first direct empirical demonstration of this suppression mechanism, and opens a path toward using the statistical properties of repeating pairs to quantify the anisotropy parameters that govern radar reflection losses.

A new diagnostic for the corona

The discovery of spike-like repeating burst pairs by LOFAR opens a new class of observational diagnostic for the solar corona. Each pair carries embedded information about the local plasma density (through the emission frequency), the density scale height (through the four-second delay), the anisotropy of coronal turbulence (through the spatial and temporal properties of the echo), and the altitude of magnetic reconnection (through the comparison of emission frequencies with density models).

Future solar radio observatories — including extensions to the LOFAR network, the Square Kilometre Array (SKA), and next-generation solar-dedicated instruments — will be able to monitor these structures with even greater temporal and spatial resolution, tracking how the anisotropy and density structure of the corona evolve during active periods, flares, and solar cycle phases.

The solar corona has been speaking in radio for billions of years. LOFAR has now shown that some of what it is saying is, in a very precise physical sense, its own reply.

This article was produced by Homer Dávila Gutiérrez, FRAS, director of SKYCR.ORG, based on a study published in Nature Communications (2026). Original study: Suli Ma et al., «Imaging spectroscopy reveals spike-like repeating radio burst pairs in the solar corona,» Nature Communications, vol. 17, article 5131 (2026). DOI: 10.1038/s41467-026-74137-2.

© 2026 SKYCR.ORG | Homer Dávila Gutiérrez, FRAS. All rights reserved. Reproduction in whole or in part without express written permission is prohibited. More information: Nature DOI: 10.1038/s41467-026-74137-2