Forty-two years ago, NASA launched the twin Voyager spacecraft on a road that would make them enter the interstellar space.
Today, both Voyager 1 and 2 are somewhere near the outer edge of the solar system, transmitting valuable information about what their environment is like for scientists on earth.
In 2012, these researchers used data from Voyager 1's instrument to confirm that it had entered the interstellar space on August 25 of that year. Voyager 2 appeared to have achieved the same performance on November 5, 2018. This means that both Voyagers have passed the heliopause, the limit to which the sun's magnetic field extends. The helio break includes all the planets and part of the Kuiper Belt, where Pluto resides.
Now, after further analysis, scientists have made some exciting discoveries about the heliopause.
Sun's reach [1
Just as the Earth's magnetic field protects us from radiation from outer space, the sun's magnetic field protects us from cosmic radiation from interstellar space. Cosmic radiation particles are hundreds of times more energetic than the particles in the solar wind. The sun's magnetic field forms a protective bubble that blocks most of this cosmic radiation.
The outermost edge of this protective bubble is called heliopause, the boundary where the sun's magnetic field and the interstellar / galactic magnetic field meet. Thus, the heliopause also separates the hot solar plasma from the relatively cooler interstellar plasma.
As Voyager 1 and 2 flew through space, the data indicating the researchers transmitted the exact location of the helio pause. Voyager 1 found the heliopause at 122 AU and Voyager 2, at 119 AU. Thus, the heliopause is about three times the average Sun-Pluto distance.
However, the heliopause does not include the entire solar system. The average orbits for many dwarf planets and small bodies in the Kuiper Belt take them beyond the heliopause. Even after the Kuiper Belt lies the Oort cloud, which is where most of the comets that visit the sun come from.
Because Kuiper Belt is part of the solar system, Voyagers have not left the solar system yet. But they have certainly entered the interstellar space, where the particles in space have different energy, distribution, velocity, etc. relative to the heliopause.
The recently reported discoveries (published here and here) deal with this space region
Voyager 1 and 2 entered interstellar space in different orbits. Voyager 1 moves north on the planet where the planets orbit the sun, while Voyager 2 moves south on it. As such, the instruments on both spacecraft experienced different parts of the heliopause and interstellar space at different times.
A magnetic barrier
Researchers tracked how the magnetic field's strength in the probe environment changed before and after Voyagers passed the heliopause with a magnetometer on board. They found that the galactic magnetic field is much stronger than the solar field.
But the field strength did not suddenly increase. Shortly before turning off the helio break, Voyager 2 measured a 3x increase in the strength of the solar magnetic field. After crossing the heliopause, the local magnetic field was further enhanced as the galactic magnetic field revealed itself.
Researchers found that the magnetic transition region to the helio pause extended over 0.7 AU (Sun-Venus distance), confirming older predictions (this and this) that such a magnetic barrier exists. It is the result of interactions between solar and galactic magnetic fields at the helio pause.
But researchers found no such magnetic barrier in the Voyager 1 data, whose interstellar transition was smoother. But it experienced a broader heliopause than Voyager 2 and measured a weaker galactic magnetic field.
So scientists now have reason to believe that the heliopause is uniform.
Energetic charged particles
Voyagersen also recorded changes in the amount of charged particles as they approached the interstellar space. As both vessels crossed the heliopause, there was a sharp decline in the number of charged particles from the solar wind. At the same time, the amount of cosmic radiation increased sharply, by 20% and 30%, respectively, according to Voyager 1 and 2.
Again, the change was not sudden. Voyager 2 indicated that a significant portion of the solar wind particles "leaked" into the interstellar space. They were conducted along the magnetic field lines. After about half of the Sun-Earth distance, the amount of charged particles decreased and then stabilized. Researchers are now amazed why Voyager 1's instrument could not observe as a gradual outward decline, instead noting a steep drop.
More strikingly, before Voyager 1 crossed the heliopause, scientists found two "pocket regions" in the sun's bubble where the galactic magnetic field had moved in and brought with it energetic cosmic radiation. Voyager 2 saw nothing like that.
So while Voyager 2 saw a single but layered heliopause, Voyager 1 saw a different structure.
These measurements complement the magnetic field measurements nicely and reinforce the idea that the heliopause is constantly changing under the influence of complex interactions with the Milky Way magnetic field. This is no different from how the solar wind constantly shapes the Earth's magnetic field.
Researchers used the Plasma Wave Subsystem (PWS) onboard Voyagers to measure the density of electrons in plasma before and after the intersection of the heliopause. Both units thought that the electron density was low before the heliopause and 60x higher after.
Researchers had predicted this increase but were surprised that the transition to the higher density was not sharp. They found an area between the heliopause and the interstellar space that hosts intermediate electron densities. This transition region extended for a rather large 10 AU – Sun-Saturn distance.
In fact, researchers had seen evidence of just such a transition layer 25 years ago, when both Voyagers were studying the interstellar plasma beyond the heliopause from a distance. With direct measurements in hand, researchers are now confident that there is a large transitional region between the heliopause and the interstellar space.
Hot interstellar plasma
Unlike all other observations, only Voyager 2 made direct plasma measurements. The main Plasma instrument on Voyager 1 had failed in 1980, three years after its launch, forcing it to rely on indirect measurements from the PWS instrument.
Voyager 2 saw that at 1.5 AU (Sun-Mars distance) before heliopause, the plasma density doubled and the temperature increased. This greatly contrasts Voyager 1's indirect measurement, in which plasma density gradually decreased for about 6 AU until the helio pause came.
Researchers were also surprised that the interstellar space is much warmer than expected. Voyager 2 recorded temperatures between 30,000 ° C and 50,000 ° C. Scientific models had suggested a relatively cooler environment of 15,000-30,000 ° C.
Researchers believe that the interaction between the two magnetic fields – solar and galactic – compressed the surrounding plasma and warmed it. This may also explain the higher plasma temperature of Voyager 2 measured just before the heliopause.
Armed with Voyager data, the scientists now know that the heliopause changes in shape and size at different times and places, as the sun and galactic magnetic field dance around each other.
The Voyagers are expected to keep in contact with the earth until about 2025, after which the small nuclear generators on board will begin to decline. By now, researchers hope to accurately characterize the properties of interstellar space.
New Horizons, the first spacecraft to visit Pluto and a Kuiper Belt object (Ultima Thule), will cross the heliopause sometime after 2038. If its nuclear power source does not, then it cannot expire then it can help to improve what we know about the gate to the interstellar space. Space organizations did not plan for any other interstellar missions in the near future.
The Voyagers probes were actually part of a NASA experiment that continues to be productive four decades after its launch. The world has acquired a whole generation of researchers during this time.
Studying how solar and galactic magnetic fields interact is useful for understanding how stars affect their environment.
Voyager's observations have given us our first glimpse of what the outermost part of the protective bubble formed by our sun's magnetic field is like the interstellar space beyond. This is the beginning of the human project to map and characterize our largely uncharted interstellar neighborhood and lay the first cornerstone for future interstellar missions.
Jatan Mehta is a science writer and former science officer at TeamIndus Moon Mission. He has research experience in astrophysics and is passionate about space advocacy, science communication and open source. His portfolio is on jatan.space and he is on Twitter @ unsekerquark .