Forty-two years ago, NASA launched the twin spaceship Voyager on a route into interstellar space.
Today, both Voyager 1 and Voyager 2 are near the outer edge of the solar system, sending valuable information about how their environment looks to scientists on Earth.
In 2012, these scientists used data from Voyager 1 instruments to confirm that they entered interstellar space on August 25 of that year. Voyager 2 seemed to have done the same feat on November 5, 2018. This means that both Voyagers have crossed the heliopause, the limit to which the sun's magnetic field extends. The heliopause covers all planets and part of the Kuiper belt on which Pluto is located.
After further analysis, scientists have now made some fascinating discoveries about the heliopause.
The Range of the Sun [1
Just as Earth's magnetic field shields us from space radiation, the Sun's magnetic field protects us from cosmic rays 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 extreme edge of this protective bubble is called Heliopause, the boundary between the sun's magnetic field and the interstellar / galactic magnetic field. The heliopause also separates the hot solar plasma from the relatively cooler interstellar plasma.
As Voyager 1 and 2 flew through space, they transmitted data that showed the scientists the exact location of the heliopause. Voyager 1 found Heliopause at 122 AU and Voyager 2 at 119 AU. The heliopause is thus about three times the average Sun-Pluto distance.
The heliopause, however, does not cover the entire solar system. The average orbit of many dwarf planets and small bodies in the Kuiper belt leads beyond the heliopause. Even after the Kuiper Belt lies the Oort Cloud, from which most of the comets that visit the Sun come.
Since the Kuiper belt is part of the solar system, the Voyager have not yet left the solar system. However, they have certainly entered the interstellar space in which the particles in space have a different energy, distribution, velocity, etc. than in the heliopause.
The newly reported discoveries (published here and here) deal with this region of space.
Voyager 1 and 2 entered interstellar space on different trajectories. Voyager 1 moves north of the plane where the planets orbit the sun while Voyager 2 moves south of it. As such, the instruments of both spacecraft at different times experienced different parts of the heliopause and the interstellar space.
A Magnetic Barrier
Scientists tracked how much the magnetic field around the probes was changed before and after passing the heliopause through the Voyager with a magnetometer on board. They found that the galactic magnetic field is much stronger than the solar one.
The field strength, however, did not increase abruptly. Shortly before the heliopause approached, Voyager 2 measured a 3-fold increase in the strength of the solar magnetic field. After crossing the heliopause, the local magnetic field further amplified as the galactic magnetic field showed.
Scientists found that the heliopause magnetic transition region spanned 0.7 AU (Sun-Venus distance), confirming earlier predictions (this and that) that such a magnetic barrier exists , This is the result of interactions between the solar and galactic magnetic fields in the heliopause.
However, scientists did not find such a magnetic barrier in the Voyager 1 data whose interstellar transition was gentler. However, a larger heliopause occurred than Voyager 2 and a weaker galactic magnetic field was measured.
Scientists now have reason to believe that heliopause is uneven.
Energetically charged particles
The Voyagers also registered changes in the amount of charged particles as they approached interstellar space. As both vehicles crossed the heliopause, there was a sharp decrease in the number of charged particles by the solar wind. At the same time, according to Voyager 1 and 2, the amount of cosmic radiation increased dramatically by 20% and 30%, respectively.
Again, the change was not abrupt. Voyager 2 pointed out that a significant portion of the solar wind particles "leaked" into the interstellar space. They were performed along the magnetic field lines. After about half the distance from sun to earth, the amount of charged particles decreased and then stabilized. The scientists are now confused as to why Voyager 1 instruments could not observe a gradual drop to the outside rather than a steep drop.
Even more striking: Before Voyager 1 crossed the Heliopause, the scientists found two "pocket" regions in the solar bubble where the galactic magnetic field had entered and brought with it energetic cosmic radiation. Voyager 2 did not see such a thing.
While Voyager 2 saw a single but stratified heliopause, Voyager 1 saw an uneven structure.
These measurements complement the magnetic field measurements very well and underpin the idea that it is a heliopause constantly changing under the influence of complex interactions with the magnetic field of the Milky Way. This does not differ from the way the solar wind constantly shapes the earth's magnetic field.
Scientists used the Plasma Wave Subsystem (PWS) aboard the Voyagers to measure the electron density in the plasma before and after crossing the heliopause. Both units found that the electron density was low before heliopause and then 60x higher.
Scientists had expected this increase, but were surprised that the transition to higher density was not sharp. They found a region between the heliopause and the interstellar space, in which middle electron densities are located. This transitional region extended over a fairly large distance of 10 AU – Sun-Saturn.
In fact, scientists had seen signs of such a transitional layer 25 years ago when both Voyager remotely examined the interstellar plasma outside the heliopause. With the direct measurements available, scientists are now certain that there is a large transition region between heliopause and interstellar space. Hot Interstellar Plasma
Unlike all other observations, only Voyager 2 has performed direct plasma measurements. The main plasma instrument on Voyager 1 had failed in 1980, three years after launch, and was forced to rely on indirect measurements of the PWS instrument.
Voyager 2 saw that at 1.5 AU (Sun-Mars distance) before the heliopause, the plasma density doubled and the temperature rose. This is in sharp contrast to the indirect measurement of Voyager 1, where the plasma density gradually decreased for about 6 AU until the heliopause arrived.
The scientists were also surprised that the interstellar space is much hotter than expected. Voyager 2 registered temperatures between 30,000 ° C and 50,000 ° C. Scientific models had suggested a relatively cooler environment of 15,000 to 30,000 ° C.
Scientists believe that the interaction between the two magnetic fields – solar and galactic – compressed and heated the surrounding plasma. This could also explain the higher plasma temperature that Voyager 2 measured shortly before heliopause.
Equipped with Voyager data, scientists now know that heliopause varies in shape and size at different times and locations, both in the Sun and in the galaxy. Magnetic fields dance around each other.
Voyagers are expected to remain in contact with Earth until about 2025, after which the small nuclear generators on board will decrease. Until then, scientists hope to accurately characterize the properties of interstellar space.
New Horizons, the first spaceship to visit Pluto and an object of the Kuiper Belt (Ultima Thule), will cross the Heliopause after 2038. It could help improve knowledge of the gateway to interstellar space. Space agencies are not planning any more interstellar missions in the near future.
In fact, the Voyagers probes were part of a NASA experiment that is still productive four decades after launch. The world has won a generation of scientists at this time.
Studying the interaction of solar and galactic magnetic fields is useful for understanding how stars affect their environment.
The observations of Voyager have given us a first glimpse into what the outermost part of the protective bubble formed by the magnetic field of our Sun is like the interstellar space that lies behind it. This is the beginning of the human project to map and characterize our largely unknown interstellar neighborhood and lay the foundation for future interstellar missions.