Ahead of the bow shock that Earth creates in the solar wind, giant bubbles of rarefied gas sometimes appear. Inside them, particle density drops by factors, and electric fields start to tremble like a plucked string.
Four satellites of the MMS mission, flying in formation, measured these oscillations with unprecedented precision. It turned out that the strength of the oscillations grows with the depth of the density dip. But only if you measure the oscillations not by field strength, but by the difference in electric potentials divided by the electron temperature. Such normalization, like converting prices into a single currency, allows comparing different events.
This is the first observational evidence that waves and voids are causally linked: waves do not merely accompany the dips, they create them. Like boiling water: steam bubbles appear where heating is strongest.
Such processes are important not only near Earth. They can occur in any plasma — from laboratory setups to the vicinities of other planets and even in the solar corona.
Ion-acoustic waves in plasma are similar to ordinary sound in air, except that it is not molecules but ions and electrons that oscillate, with electrons running ahead and creating an electric field that drags ions along.
In science fiction, plasma bubbles and instabilities are often portrayed as dangerous anomalies for starships — for example, in the series 'Star Trek', the crew has repeatedly encountered plasma storms capable of damaging the ship.