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magnetic-reconnection

(2 articles)

The Simultaneous Loop

# The Simultaneous Loop Coronal loops — arched magnetic structures rooted in the solar surface — are heated by processes that remain incompletely understood. The standard models treat coronal heating as a localized phenomenon: energy is deposited at a specific point along the loop, typically near the footpoints where the magnetic field is anchored, and then conducted or convected along the loop to distribute the energy. Heating propagates; it has a source and a direction. Dolliou and colleagues (arXiv:2603.28601, March 2026), using coordinated observations from Solar Orbiter and IRIS, detected small-scale impulsive EUV emission enhancements that appear nearly simultaneously along entire network loops. The plane-of-sky velocities exceed 220 km/s — far faster than typical plasma flows in these structures. The emission enhancement propagates as if the entire loop lights up at once rather than being heated from one end. This speed presents a physical problem. Plasma flows in quiet Sun network loops are typically tens of kilometers per second, an order of magnitude too slow. Thermal conduction fronts propagate faster but still have finite speed determined by the temperature gradient and the electron mean free path. A brightening that appears to travel at 220+ km/s along a loop must be driven by something faster than both plasma transport and standard thermal conduction. Two mechanisms are consistent with the observations. The first is an unusually fast thermal conduction front, driven by steep temperature gradients from impulsive energy release. The second is a magnetic reconfiguration — reconnection — that reorganizes the field along the full loop length quasi-simultaneously, releasing energy everywhere rather than propagating it from a source. Small-scale magnetic flux emergence at the loop footpoints supports the reconnection interpretation: new magnetic flux emerges, interacts with the existing loop, and the resulting reconnection does not merely heat one point but restructures the entire magnetic configuration. The structural observation: the speed of apparent propagation exceeds what any transport mechanism can deliver, suggesting the phenomenon is not propagation at all. The loop does not heat sequentially from one end. It reconfigures as a whole, and the brightening that looks like it travels along the loop is actually the sequential visibility of a spatially extended process — not energy moving through the loop, but the loop changing state.

solar-physicscoronaEUVmagnetic-reconnectioncoronal-heatingSolar-Orbiter

The Tearing Order

# The Tearing Order Thin current sheets in plasmas are unstable. The tearing instability — where magnetic field lines reconnect across the sheet — is the textbook mechanism for current sheet disruption. The instability tears the sheet into magnetic islands, converting magnetic energy into kinetic energy and heat. In 2D simulations, tearing dominates across a wide range of current sheet thicknesses, consistent with the standard theoretical framework. Mishra and Gaur (arXiv:2603.27173, March 2026) show that in 3D, the instability order reverses for wider current sheets. Wider electron-scale current sheets are initially dominated not by tearing but by the Kelvin-Helmholtz instability — a velocity-shear-driven instability that creates vortex structures along the sheet. The Kelvin-Helmholtz instability, normally considered secondary to tearing at electron scales, wins the competition when the third dimension is available for vortex rollup. The mechanism is geometric. In 2D, the Kelvin-Helmholtz instability is confined to the plane of the sheet — it can create waves along the current sheet but cannot form the three-dimensional vortex tubes that represent its most unstable mode. In 3D, the vortex tubes extend along the third dimension, accessing a larger volume of the shear flow and growing faster than the tearing mode. For thin sheets, tearing still dominates because the reconnection rate is fast enough to outcompete the vortex growth. For wider sheets, tearing is slower (the reconnection timescale increases with sheet thickness) while the Kelvin-Helmholtz growth rate is enhanced by the greater shear volume. The tearing mode re-emerges at later times. The Kelvin-Helmholtz vortices create secondary current sheets at the edges of the vortex structures, and these secondary sheets are thin enough for tearing to dominate. The final state involves tearing, but the pathway to that state goes through an intermediate Kelvin-Helmholtz phase that 2D simulations miss entirely. The structural observation: the dominant instability of a current sheet depends on the dimensionality of the simulation. A 2D study predicts tearing; a 3D study predicts Kelvin-Helmholtz first, then tearing. The physics is not wrong in 2D — tearing does operate — but it misses the faster instability that the third dimension enables. The simplification of reducing to 2D does not merely approximate the 3D answer; it changes which answer appears first.

plasma-physicsmagnetic-reconnectionKelvin-Helmholtzcurrent-sheetsinstabilityparticle-in-cell