On December 31, 2023, the Sun unleashed a flare of the highest class — one that releases the energy of billions of atomic bombs in just a few minutes. Along with it, a huge prominence, a cloud of superheated gas, tore away from the surface and shot outward at nearly 3000 kilometers per second. Like a speedboat at full throttle, this ejection plowed a shock wave through the solar corona.
At that moment, radio telescopes on Earth were catching signals from the Sun. And in those signals, a Christmas tree pattern emerged: short radio bursts that, on a frequency-versus-time plot, branch upward and downward like limbs. Such patterns are called 'fishbone' structures. They occur when a shock wave sweeps up electrons and accelerates them to tremendous speeds — up to a quarter of the speed of light. These accelerated electrons emit radio waves, and their frequency reveals where the acceleration happened.
The scientists noticed that the 'branches' of the Christmas tree appeared not uniformly but with a periodicity of about 20 seconds. This means the shock wave accelerated electrons not continuously but in bursts, as if someone were rhythmically pressing the gas pedal. This intermittency helps us understand how the flare's energy is transferred to particles.
This event is especially intriguing because the 'Christmas tree' coincided with the moment the shock wave was just forming. The scientists were able to measure the height at which this occurred — about three-quarters of the Sun's radius above the surface. And this height exactly matched the position of the ejection's front. So radio signals act as a kind of 'echo sounder,' showing where the shock wave is at any given moment and how it accelerates particles.
Such observations help us better understand how solar flares affect space weather. After all, accelerated electrons and protons can reach Earth and damage satellites or even disrupt radio communications. And if we learn to read the 'Christmas trees' on the Sun, we can issue early warnings for such events.
The plasma ejection in this event traveled at about 2850 km/s — nearly 1% of the speed of light. In one second, the cloud covers the distance from Moscow to London.
In Arthur C. Clarke's story 'The Solar Wind,' a giant ejection from the Sun carries charged particles toward Earth, and the heroes must reckon with their invisible but destructive force.