The frequency of the noise is associated with a specific density of plasma. The higher the frequency, the denser the plasma.



The only trouble is that something has to excite the plasma for it to "ring," something like a large solar flare. Waiting for a solar flare can take years during a solar minimum (a period of low solar activity).



Also, when a flare does occur, it can take as long as a year for the shock wave to reach Voyager 11.6 billion miles away.



Fortunately for Voyager scientists, the antennas picked up two long-lasting oscillations. The first was in October and November of 2012 and the second was in April and May of 2013. In both cases, the frequency suggested that the plasma was cold and dense. Voyager was in interstellar space.



"It was key evidence," Stone said. "We really needed to measure plasma to know if we were inside or outside the heliosphere. Everything else is more of a proxy."



Gurnett and his colleagues arrived at the crossing date of Aug. 25 by extrapolation.



Plasma density was increasing in a linear fashion as Voyager moved further from the heliosphere and into the interstellar medium. The frequency measured in the fall of 2012 was 2.2 kilohertz, and by the spring of 2013 it had risen to 2.6 kilohertz. Previous research told Gurnett that the frequency of the radio signal at the crossing point should be 2 kilohertz, and so by plotting each point on a line, he was able to arrive at a date.