Termination shocks could be the origin of ground-level enhancement (GLE) events, according to a new study by researchers at the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences (CAS).

The research was led by WANG Xin, an associate researcher at XAO, in collaboration with other scientists, and has been published in the Journal of Geophysical Research: Space Physics.

Solar flares and Coronal Mass Ejections (CMEs) are the main drivers of space weather. Both can generate strong shocks driven by high-speed flows, which may in turn trigger GLE events. The traditional view holds that CME-induced shocks are the primary cause, since solar eruptions that produce GLEs are typically accompanied by fast-moving CMEs.

However, the lower corona of solar flares contains a powerful proton accelerator—the termination shock (TS). It can produce solar cosmic ray protons with energies exceeding 500 MeV, enough to directly trigger GLE processes that reach Earth's surface.

The team focused on the relationship between GLE events and TSs, using dynamic Monte Carlo (DMC) particle simulations to model flare TSs. In this theoretical framework, high-speed thermal particle flows from magnetic reconnection converge at the magnetic loop-top to form a TS. As thermal particles interact multiple times with the shock, they eventually produce a high-energy particle spectrum with a power-law distribution.

The simulations revealed two key findings. First, the particle density in the shock precursor region shows a regular "textured" structure, while the downstream region displays a stable "woven" pattern. Second, the particle energy spectrum deviates from the standard power-law distribution and exhibits a distinct "bump-on-tail" between 2 MeV and 20 MeV—a sign that TSs have exceptional acceleration capabilities.

Taken together, these results suggest that TSs are potential sources of GLE events. They can directly trigger GLEs or indirectly supply seed particles for CME-driven shocks. Uncovering the fundamental origins of GLE events will provide insights for space weather prediction and improve the timeliness and accuracy of forecasts for solar high-energy particle events, according to the researchers.

This study was supported by the Natural Science Foundation of Xinjiang Uygur Autonomous Region and the National Key Research and Development Program, among others.