A Weak Shock Wave on the Sun Accelerates Electrons to 122 keV express
A shock wave in the Sun's corona is like the sonic boom from an airplane, except instead of air there is scorching plasma, and instead of sound there is a jump in density and magnetic field. On March 10, 2024, such a wave swept across the western part of the solar disk. It was so weak that its speed exceeded the local speed of magnetic waves by only half a percent (Mach number about 1.005). You'd think such a 'quiet' wave couldn't accelerate anything.
But when the wave reached a region where the magnetic field lines did not loop back to the Sun but extended into interplanetary space, everything changed. There, the wave struck the open field lines almost perpendicularly. At that moment, radio telescopes caught bursts that looked like frequent 'fir trees' on the dynamic spectrum: that's what beams of electrons ejected along magnetic field lines look like. From the tilt of these 'fir trees,' the electron energy was estimated: from 75 to 122 kiloelectronvolts. For comparison, the energy of particles in a candle flame is fractions of an electronvolt.
This case is important: previously, it was thought that strong shock waves from powerful flares were needed to accelerate particles. Here, the wave was barely noticeable, but the field geometry, open lines perpendicular to the front, did the trick. So even weak disturbances in the corona can contribute to the flux of energetic particles that later reach Earth and affect space weather.
So a quiet wave, almost indistinguishable from the background, reminded us: in solar physics, it's not just the strength of the impact that matters, but also where it's directed. An open door sometimes matters more than a fist.
Original abstract
as written by the authors arXiv:2603.23335
Extreme ultraviolet (EUV) waves are global waves in the solar corona which can accelerate particles. The efficiency of the acceleration depends on local plasma characteristics e.g. Alfv\'en speed and the geometry of the magnetic field. This shock-driven particle acceleration can produce radio signatures such as Type II radio bursts and herringbone emission. Here we investigate signatures of particle acceleration by a weak coronal shock on 10 March 2024. In particular, we combine EUV images with radio imaging and spectral observations to determine how and where this weak shock could accelerate energetic particles. A potential field source surface extrapolation was used to examine the pre-eruption ambient magnetic field while the evolution of the global wave was probed using running difference and base difference EUV images. The EUV images enabled the speed and Alfv\'en Mach number of the EUV wave to be characterised. The combination of radio images and dynamic spectra provide evidence of beams of shock-accelerated electrons localised to a dimming region at the time the EUV wave passes through it. The speeds and energies of these electrons were estimated from the drift rates of their herringbones. The EUV wave initially propagated West, channelled by loop systems, before changing direction northward. From the EUV intensity jump at the wavefront, the Alfv\'en Mach number was estimated to be approximately 1.005 at the time that the herringbones were produced. The herringbone drift rates revealed accelerated electron energies of 75-122 keV, using Newkirk density models with scaling factors of 1.3-2.6. These observations suggest that the weak lateral shock impacted quasi-perpendicular open field in a dimming region, enabling localised particle acceleration. This indicates that the geometry of the ambient magnetic field relative to the shock strongly governs where particles can be accelerated.
An energy of 122 keV means the electron is traveling at about 60% of the speed of light. If such an electron could run along a wire, it would circle the Earth in a second.
In Arthur C. Clarke's novel 'Solar Storm,' a stream of accelerated particles from the Sun knocks out technology on Earth, just the scenario studied through such observations.