Hooked and U-Turned Radio Bursts Reveal Hidden Solar Magnetic Loops

The Sun’s corona is a seething maze of magnetic fields, and for decades scientists have relied on one particular kind of radio signal — the type III burst — to trace how electrons escape along open field lines into interplanetary space. Now a new study has turned the spotlight on the quieter, less glamorous cousins of those bursts: the J-shaped and U-shaped solar radio bursts that betray electrons racing along closed magnetic loops and turning back toward the Sun. By analyzing 38 such events recorded during the ascending phase of Solar Cycle 25, a team of researchers has shown that these curved spectral signatures can serve as powerful probes of coronal magnetic topology, reaching loop heights far beyond what standard magnetic field models can reconstruct.

The research, published in Astrophysics and Space Science, drew on high-resolution dynamic spectra from the e-CALLISTO network, a worldwide chain of inexpensive radio spectrometers that monitors the Sun continuously from multiple ground-based stations. Between June 2020 and December 2024, the team identified 18 J-type and 20 U-type bursts in the decametric frequency range of 10 to 200 megahertz, where these burst types are most prominent. Each event was screened through a multi-step process: candidate bursts were spotted visually in dynamic spectrograms, validated for consistent morphology and timing across independent stations, and rejected if their frequency drift did not follow the expected trajectory — a low-to-high frequency path for J-types, or a full low-to-high-to-low reversal for U-types.

The distinction between the two burst types lies in the geometry of the magnetic structures that guide the electron beams. A J-type burst appears in a dynamic spectrum as a hook-like feature: the emission frequency drifts downward as electrons climb through the corona, then terminates abruptly without reversing, suggesting that the beam dissipates or scatters somewhere along a loop anchored at a single footpoint. A U-type burst, by contrast, shows a symmetric frequency evolution — the drift reverses cleanly at a minimum frequency, marking the moment the electron beam passes over the apex of a closed loop and begins descending the opposite leg toward the solar surface. Because the emission frequency at any instant reflects the local plasma density through the plasma frequency relation, the shape of the burst on a spectrogram is effectively a map of the density — and therefore the height — along the electron’s path.

To convert those spectral curves into physical quantities, the researchers traced the backbone of each burst and applied the Newkirk density model, a classical empirical description of coronal electron density that falls off with heliocentric distance. From the frequency drift rate, they derived the radial velocity of the electron beams and the heights at which the emission originated. The results revealed a striking dichotomy. J-type bursts reached apex heights between 1.2 and 2.5 solar radii, with a mean near 1.6 solar radii, and displayed steep negative drift rates in their descending legs, typically ranging up to 14.6 megahertz per second. U-type bursts extended dramatically higher — up to 3.9 solar radii, with a mean around 2.5 solar radii — and showed smoother, more gradual drift patterns on their ascending branches, with drift rates spanning a broad range from 0.5 to 139.6 megahertz per second.

Those asymmetries carry real physical meaning. The differences between the ascending and descending branches of U-type bursts suggest that electron propagation is not symmetric around the loop apex, reflecting unequal density gradients, magnetic geometries, or beam conditions along the two legs of the closed structure. For J-type bursts, the steeper drifts point to rapid electron deceleration and confinement within short, low-lying loops, consistent with sharp density gradients and localized acceleration near compact reconnection sites. In effect, the spectral morphology of each burst encodes the geometry of the loop that produced it: J-types trace compact, often asymmetric structures, while U-types map long-lived, large-scale closed loops.

Perhaps the most surprising finding concerns the role of solar flares. Cross-referencing all 38 events with the GOES soft X-ray flare catalog, the team found that only 26 were associated with flares at all — and roughly 30 percent of the bursts occurred without any reported flare. Among the flare-associated events, C-class flares dominated, accounting for about 56 percent of J-type and 45 percent of U-type bursts, while a handful coincided with M-class flares and just one U-type event matched a B4.3 flare. No X-class flares appeared in the sample. This means the electron acceleration responsible for these bursts is not exclusively tied to impulsive flare energy release; smaller-scale reconnection, active region restructuring, or turbulence within coronal loops can also launch electron beams energetic enough to produce detectable radio emission.

To validate the loop heights inferred from the density model, the team turned to an entirely independent method: potential field source surface extrapolations built from photospheric magnetograms. This technique assumes the coronal field is current-free and divergence-free between the solar surface and a spherical source surface fixed at 2.5 solar radii, beyond which field lines are forced to be radial. Using GONG synoptic magnetograms matched to each event’s Carrington rotation, and the open-source pfsspy package wrapping a finite-difference solver, the researchers traced closed field lines through the three-dimensional solution and calculated the geometric height of each loop’s arc. Extreme ultraviolet images from SDO’s Atmospheric Imaging Assembly provided additional context, identifying the active regions and loop footpoints associated with the radio sources.

The comparison between the two methods was revealing but constrained. Of the 38 events, 15 had apex heights above the 2.5-solar-radius source surface boundary and lay outside the PFSS domain entirely, while four more originated near the solar limb where the magnetogram input is poorly constrained. That left 19 events with valid estimates from both approaches. For this subset, PFSS yielded loop heights from 0.36 to 2.5 solar radii with a mean near 1.6, while the Newkirk inversion ranged from 1.2 to 4.3 solar radii with a mean near 2.5 — a systematic offset of roughly 0.9 solar radii. Yet the event-to-event differences stayed within about half a solar radius, indicating reasonable agreement despite the models’ fundamentally different assumptions. The pattern was clear: PFSS reliably captured short, low-altitude loops typical of J-type bursts, while systematically underestimating the extended structures associated with U-type bursts, which the density-based method recovered up to 4.3 solar radii.

The study also extends the known scale of these phenomena. Earlier imaging spectroscopy with LOFAR had placed U- and J-type loop altitudes at roughly one solar radius; the new statistical analysis of 38 events pushes that ceiling considerably higher. Several U-type bursts even displayed double-lane or complex frequency structures, hinting at multiple electron injections or partial reflections within loops — features that make U-bursts uniquely valuable for investigating multi-episode acceleration and electron trapping, processes largely inaccessible through type III studies alone.

For space weather research, the implications are tangible. Closed-field electron acceleration governs how energetic particles are stored, scattered, or released from the corona, and knowing whether a burst originates from a compact low-lying loop or a sprawling structure extending several solar radii outward helps assess the connectivity between the Sun and interplanetary space. The authors argue that combining density-model inversions with magnetic extrapolations offers a robust dual diagnostic, and they point toward the next generation of instruments — LOFAR, the Murchison Widefield Array, the upcoming Square Kilometre Array, and in-situ measurements from Parker Solar Probe and Solar Orbiter — as the tools that will sharpen this picture further. As Solar Cycle 25 continues toward its maximum, every hooked and U-turned streak on a radio spectrogram becomes a free measurement of the corona’s hidden architecture, delivered by electrons that race along magnetic rails and broadcast their journey across the solar system.

Subject of Research: Closed-field electron acceleration and magnetic loop diagnostics using J- and U-type solar radio bursts in the solar corona

Article Title: Closed-field electron acceleration in the solar corona: insights from J- and U-type solar radio bursts

Article References: Soni, S. L., Luthra, V., Singh, J., Suklan, R., Bhatt, T. K. N., & Jain, R. (2026). Closed-field electron acceleration in the solar corona: insights from J- and U-type solar radio bursts. Astrophysics and Space Science, 371(9), Article 111. https://doi.org/10.1007/s10509-026-04639-4

Image Credits: AI Generated

DOI: 10.1007/s10509-026-04639-4

Keywords: solar radio bursts, J-type bursts, U-type bursts, solar corona, magnetic loops, electron acceleration, Solar Cycle 25, e-CALLISTO, PFSS extrapolation, Newkirk density model, space weather, solar flares