By Emil Lenc

In early April 2025, I investigated a sustained degradation in the quality of ASKAP images collected over several consecutive days to determine whether the issue arose from the telescope or the data processing pipeline. After close inspection, it turned out neither was at fault. Instead, the effect was traced to the aftermath of a major solar storm, during which powerful solar flares interacted with Earth’s upper atmosphere and triggered a strong geomagnetic response. Such events are known to disrupt radio communications and GPS systems and to enhance auroral activity. The interaction produces a complex, multi layered plasma environment within the magnetosphere, including the ionosphere and plasmasphere. The plasmasphere contains cylindrical plasma density structures aligned with Earth’s magnetic field, which can induce small positional shifts in astronomical radio sources. These shifts are typically subtle and difficult to detect with narrow field or higher frequency instruments. Similar effects were previously observed with the Murchison Widefield Array during the 2013 solar cycle. In the ASKAP data, the same phenomenon is evident. The animation above shows the shifts of background astronomical objects seen in the ASKAP data. Red and blue show shifts in opposite directions, the length of the arrow is the magnitude of the shift, and the grey lines are the direction of the Earth’s magnetic field lines (these slowly rotate over the course of the observation as the telescope tracks the observed field). Over the 20-minute interval shown, disturbances aligned with the magnetic field propagate through the field of view, producing coherent, wave-like motion – it’s as if the astronomical objects are performing a “Mexican Wave”. It is believed that this is the first time such an effect has been noted at Gigahertz frequencies.