Some 12 billion miles from Earth, a frontier marks the edge of the Sun’s realm and the start of interstellar space. When NASA’s Voyager 2 space probe crossed this boundary more than 40 years after its launch it beamed-back a weak signal from deep space scientists have now decoded.
University of Iowa researchers confirmed Voyager 2’s passage into the Interstellar Medium (ISM) by noting a peculiar spike in plasma density detected by a plasma wave instrument.
This increase in plasma density is evidence of Voyager 2 journeying from the hot, lower-density plasma characteristic of the solar wind to the cool, higher-density plasma of interstellar space.
The results echo the plasma density jump experienced by Voyager 1 when it too entered interstellar space.
The University of Iowa’s Don Gurnett, corresponding author on the study, said: “In a historical sense, the old idea that the solar wind will just be gradually whittled away as you go further into interstellar space is simply not true.
However, they crossed into the ISM at basically the similar distances from the Sun.
That provides valuable information about the structure of the heliosphere, the bubble created by the sun’s wind as it extends to the boundary of the solar system.
Dr Bill Kurth, University of Iowa research scientist and a co-author on the study, said: “It implies that the heliosphere is symmetric, at least at the two points where the Voyager spacecraft crossed.
“That says that these two points on the surface are almost at the same distance.”
Data from the Iowa instrument on Voyager 2 also gives additional clues to the thickness of the heliosheath, the outer region of the heliosphere and the point where the solar wind piles up against the approaching wind in interstellar space, likened to the effect of a snowplow on a city street by Professor Gurnett.
The Iowa scientists say the heliosheath has varied thickness, based on data showing Voyager 1 sailed 10 AU farther than its twin to reach the heliopause.
Some researchers predicted Voyager 2 would make that crossing first, based on models of the heliosphere.
Dr Kurth added: “It’s kind of like looking at an elephant with a microscope.
“Two people go up to an elephant with a microscope, and they come up with two different measurements.
“You have no idea what’s going on in between. What the models do is try to take information that we have from those two points and what we’ve learned through the flight and put together a global model of the heliosphere that matches those observations.”