Voyager Enters Final Frontier Of Solar System Options

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http://planetary.org/news/2005/voyager-upd...ation_0524.html

Voyager 1, the most distant human-made object in space, has crossed the termination shock, the last major threshold in the solar system, team members announced today at the annual American Geophysical Union (AGU) meeting in New Orleans, Louisiana.

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At the solar system's edge, more surprises from Voyager

Data from NASA's Voyager 1 spacecraft continues to provide new insight on the outskirts of our solar system, a frontier thought to be the last that Voyager will cross before becoming the first man-made object to reach interstellar space.

In papers published this week in the journal Science, scientists from the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and other Voyager partner institutions provide more clarity on the region they named the "magnetic highway" in December 2012. Cruising through what scientists describe as a curious, unexpected charged-particle environment, Voyager has detected, for the first time, low-energy galactic cosmic rays, now that particles of the same energy from inside the bubble around our Sun disappeared. As a result, Voyager now sees the highest level so far of particles from outside our solar bubble that originate from the death of other nearby stars.

"Voyager 1 may be months or years from leaving the solar system—we just don't know," says APL's Stamatios Krimigis, principal investigator for Voyager's Low-Energy Charged Particle (LECP) instrument. "But the wait itself is incredibly exciting, since Voyager continues to defy predictions and change the way we think about this mysterious and wonderful gateway region to the galaxy."

Voyager 1 and 2 were launched in 1977 and between them visited Jupiter, Saturn, Uranus and Neptune. Since 1990, the twin spacecraft have been on their Interstellar Mission, on track to leave the heliosphere, which is the bubble of magnetic field and charged particles the Sun blows around itself. On Aug. 25, 2012, when Voyager 1 was about 11 billion miles (18 billion kilometers) from the Sun, the spacecraft reached the so-called magnetic highway where charged particles from inside the heliosphere zoomed out along the magnetic field as cosmic rays from far outside zoomed in. The lack of a detectable change in the direction of that magnetic field, however, convinced scientists that Voyager remained within the Sun's influence.

The new Science papers focus on observations from the summer and fall of 2012 by LECP as well as Voyager 1's Cosmic Ray and Magnetometer instruments, with additional LECP data through April 2013.

"The most dramatic part was how quickly the solar-originating particles disappeared; they decreased in intensity by more than 1,000 times, as if there was a huge vacuum pump at the entrance ramp onto the magnetic highway," says Krimigis. "We have never witnessed such a decrease before, except when Voyager 1 exited the giant magnetosphere of Jupiter, some 34 years ago."

"Surprisingly, the traveling direction of the 'inside' charged particles in this region made a difference, with those moving straightest along the magnetic field lines decreasing most quickly. Those that moved perpendicular to the magnetic field did not change as quickly," adds LECP Co-investigator Robert Decker, also of APL. The cosmic rays from outside, moving along the field lines, were somewhat more intense than those moving perpendicular to the field, and this imbalance varied significantly with time during the eight months since "It is this time-varying behavior of the cosmic rays that tells us that we're still in a region controlled by our Sun," says APL's Edmond Roelof, also an LECP co-investigator.

The multidimensional measurements speak to the unique abilities of the LECP detector, designed at APL in the 1970s. It includes a stepper motor that rotates the instrument through 45-degree steps every 192 seconds, allowing it to gather data in all directions and pick up something as dynamic as the solar wind and galactic particles. The device, designed and tested to work for 500,000 steps and last four years, has been working for nearly 36 years and well past 6 million steps.

Magnetic Field Observations as Voyager 1 Entered the Heliosheath Depletion Region

Magnetic fields measured by Voyager 1 (V1) show that the spacecraft crossed the boundary of an unexpected region five times between days 210 and ~238 in 2012. The magnetic field strength B increased across this boundary from ≈0.2 nT to ≈0.4 nT, and B remained near 0.4 nT until at least day 270, 2012. The strong magnetic fields were associated with unusually low counting rates of >0.5 MeV/nuc particles. The direction of B did not change significantly across any of the 5 boundary crossings; it was very uniform and very close to the spiral magnetic field direction, which was observed throughout the heliosheath. The observations indicate that V1 entered a region of the heliosheath (“the heliosheath depletion region”), rather than the interstellar medium.

Search for the Exit: Voyager 1 at Heliosphere’s Border with the Galaxy

We report measurements of energetic (>40 keV) charged particles on Voyager 1 (V1) from the interface region between the heliosheath (HS), dominated by heated solar plasma, and the local interstellar medium (LISM) expected to contain cold nonsolar plasma and the galactic magnetic field. Particles of solar origin at V1, located at 18.5 billion km (123 AU) from the Sun, decreased by a factor >103 on 25 August 2012, while those of galactic origin (cosmic rays) increased by 9.3% at the same time. Intensity changes appeared first for particles moving in the azimuthal direction and were followed by those moving in the radial and antiradial directions with respect to the solar radius vector. This unexpected heliospheric "depletion region" may form part of the interface between solar plasma and the galaxy.

Voyager 1 Observes Low-Energy Galactic Cosmic Rays in a Region Depleted of Heliospheric Ions

On 25 August 2012, Voyager 1 was at 122 astronomical units when the steady intensity of low-energy ions it had observed for the last 6 years suddenly dropped for a third time and soon completely disappeared as the ions streamed away into interstellar space. Although the magnetic field observations indicate that Voyager 1 remained inside the heliosphere, the intensity of cosmic ray nuclei from outside the heliosphere abruptly increased. We report the spectra of galactic cosmic rays down to ~3 × 106 electron volts per nucleon, revealing H and He energy spectra with broad peaks from 10 × 106 to 40 × 106 electron volts per nucleon and an increasing galactic cosmic ray electron intensity down to ~10 × 106 electron volts.