Voyager 1, the unmanned probe launched toward the outer planets in 1977 by the National Aeronautics and Space Administration, has offered added proof it's the first spacecraft from Earth to reach interstellar space.

The robotic observer, operated from the space agency's Jet Propulsion Laboratory in Pasadena, California, reported a "ringing" phenomenon as it passed through a dense wave of plasma ejected about a year ago by the sun.

"Normally, interstellar space is like a quiet lake," Ed Stone, the mission's project scientist since 1972, said in a news release. "But when our sun has a burst, it sends a shock wave outward that reaches Voyager about a year later. The wave causes the plasma surrounding the spacecraft to sing."

In other words, as the probe flies through such solar "tsunamis," as Stone and other mission specialists call them, ionized matter surrounding Voyager 1 vibrates at frequencies expected in interstellar space.

Occurring last March, the latest plasma "tsunami" was the third such encounter for the probe; a similar ringing heard in the fall of 2013 announced to NASA scientists Voyager 1 had, in fact, reached the vast region between the sun's direct influence and the closest star.

Research from NASA say the Voyager 1 mission has not left the solar system yet, as it still has to reach a final halo of comets surrounding the sun, but it did cross the solar wind-blown bubble of far-flung particles known as the heliosphere.

"All is not quiet around Voyager," said Don Gurnett of the University of Iowa, the principal investigator of the plasma wave instrument on Voyager 1. "We're excited to analyze these new data."

Our sun, the NASA Website explains, experiences periods of increased activity, where it explosively ejects material from its surface. Those events -- coronal mass ejections -- generate pressure, or shock waves.

The shock waves in turn push against cosmic rays, the energetic charged particles that emanate from nearby stars in the Milky Way galaxy, pushing them around like buoys in an enormous ocean tidal wave.

"The tsunami wave rings the plasma like a bell," said Stone. "While the plasma wave instrument lets us measure the frequency of this ringing, the cosmic ray instrument reveals what struck the bell -- the shock wave from the sun."

Voyager 1 and its twin, Voyager 2, were launched 16 days apart in 1977 and both probes flew by Jupiter, Saturn and their respective moons.

Voyager 2, the first of the duo sent into space and now the longest continuously operated spacecraft in history, is expected to as well reach interstellar space in a few years.