Radio waves from a distant pulsar travel through the interstellar medium, and along their path they encounter invisible magnetic structures the size of the distance from the Sun to Earth. One such structure is a current sheet, where the magnetic field abruptly changes direction, like a compass needle flipping from north to south. Previously, seeing such a flip directly was impossible: standard methods average the signal over the entire path of hundreds of light-years, and the fine detail is lost.
But pulsars have a special trait: their radio signal twinkles, like the light of a distant star in a turbulent atmosphere. Astronomers have learned to reconstruct the waveform from this twinkling with phase accuracy — the position of the oscillation within its cycle. By comparing the phases of two polarizations (left and right circular), one can extract the rotation measure difference — a quantity that indicates how much the magnetic field has rotated the plane of polarization. This technique was applied to archival observations of pulsar B0834+06 made in 2005.
In the data, a “1-millisecond lens” emerged — a plasma structure that delays the signal by a thousandth of a second. It split into two branches, and for six low-frequency sub-bands, the rotation measure difference between the branches was measured. The result was a signal at the 2.9σ level — not yet a discovery, but a serious claim. Converting the measured difference into a model, the authors obtained a magnetic field jump of about 4.4 microgauss. This value agrees with estimates for the warm ionized medium, where thermal and magnetic energies are roughly equal. This suggests the structure is not a random fluctuation but a long-lived element of interstellar turbulence.
Previously, it was unknown whether current sheets with field reversals exist on such scales. Now it has been shown that the pulsar scintillation method with phase recovery can detect them. If the method is scaled to other pulsars, it could map magnetic fields on small scales and clarify how cosmic rays wander through the galaxy. This, in turn, could also help in the search for the gravitational-wave background using pulsar timing arrays.
The size of the current sheet that was 'seen' is about 0.6 astronomical units — smaller than the distance from the Sun to Earth. Yet the pulsar itself is hundreds of parsecs away, meaning millions of times farther.
The idea that invisible magnetic structures in the interstellar medium can influence particle trajectories resembles the concept of 'gravitational lenses' from science fiction, but here the lensing is done by plasma and magnetic fields, not gravity.