Posted on 07/03/2008 6:20:41 AM PDT by Abathar
Hurtling through space 31 years after its launch, the Voyager 2 spacecraft has sent back the most detailed view yet of the shock wave that marks the thinning of the solar wind, the charged particles streaming from the sun.
Researchers say the crossing confirms that the heliosphere—the region swept out by the solar wind—is actually lopsided, perhaps due to a tilted magnetic field in local interstellar space.
The shock wave, or heliospheric termination shock, occurs when the supersonic wind thins to the point that it can no longer rebuff the denser haze of charged particles flowing through interstellar space. Instead, the solar wind suddenly collapses in on itself.
Researchers say the phenomenon is sort of like the edge of a stream of tap water after it hits the sink [see image]. Solar wind is swept along by the sun's magnetic field, which means it cascades like a fluid instead of crashing like billiard balls.
Data from Voyager 2, described in a series of papers today in Nature, show that the craft entered the termination shock on August 31, 2007, at a distance from the sun of about eight billion miles (13 billion kilometers) and crossed it the next day.
That's 10 percent closer to the sun than when the craft's sister ship, Voyager 1, passed through the same shock wave in late 2004 heading outbound from the solar system in a different direction.
That far from the sun, the density of solar wind is, at most, a couple of protons and electrons per gallon, astrophysicist J.R. Jokipii of the University of Arizona Lunar and Planetary Laboratory in Tucson says. "It's almost impossible to measure. You have to give it to these experimenters," he says.
Voyager 2 will now follow its twin into the heliosheath, the region of slower-moving wind beyond the termination shock.
Besides confirming earlier research that hinted at the lopsided heliosphere, the crossing provides new details, including the energy and speed of the solar wind, that Voyager 1 could not pick up because its plasma detection instrument had stopped functioning.
According to the new data, the wind downstream of the shock was cooler and faster moving than researchers had anticipated. The interpretation, says Jokippi, who wrote an editorial accompanying the Nature reports, is that the solar wind is imparting energy to neutral atoms from the interstellar gas and causing them to ionize.
These "pickup" ions are then accelerated to speeds of hundreds of miles (kilometers) per second, exerting a strong effect on the structure of the shock, he says.
The twin Voyager craft set out for deep space in 1977 to study Jupiter and Saturn, but after their primary mission was completed, they kept on going. In 10 to 20 years after reaching the termination shock, NASA expects the craft to cross the heliopause, the outer edge of the heliosheath.
That would mean they have exited the solar system and entered the interstellar medium. NASA engineers estimate that both probes' plutonium power packs have the potential to keep them broadcasting data until 2025.
If we're lucky, Jokippi says, they'll let us know what they find.
No.
There is no wind in a vacuum.
There is no sound in a vacuum.
Therefore the term “supersonic” is dead wrong.
The solar wind isn't a vacuum. The article is about the behavior of a stream of particles; the presence of those particles themselves makes it, by definition, not a vacuum.
The terms known as "vacuum" and "zero-gravity" are good enough for the vast majority of uses, but "near-vacuum" and "microgravity" are more strictly correct.
The behavior described is the same as that of air particles in the Earth's atmosphere. The particles at that speed can't get out of each other's way, forming the shock wave that's known as a sonic boom. Can the resulting vibrations be called "sound"? That's a semantic question. The solar wind is like a tree falling in the woods with no one to hear it.
Insofar as there is really no perfect vacuum in nature (even in deep space hydrogen particles exist even if far apart) I still say that the use of the term supersonic is ridiculous.
The solar wind is nothing like atmospheric wind. There is nothing even remotely approaching the density, and even the term supersonic would vary with density as the speed of sound does.
The speed in units is irrelevant; what's at issue is where the shockwave phenomenon occurs. The equivalence between "supersonic" and 340.3m/sec. is only valid at room temperature and sea level, but it's close enough that airplane and spacecraft speeds are expressed in Mach units.
I agree that more precise terms exist, but "supersonic" is not flat-out wrong -- just vague to a degree that I would not consider acceptable in a scientific journal, but which is good enough for an article directed at a general audience.
You’re splitting hairs, needlessly. You know they’re using “the speed of sound” to represent the accepted constant of 1088 feet per second used in mach units.
check out
spacesounds.com
1) I am not splitting hairs needlessly. It is an asinine term to use and does nothing but confuse people, especially any kids who might be reading it. Hollywood already does a wonderful job of ignoring the vacuum of space.
2) I let it go almost a full day ago.
Michelson-Morley, anyone?
Funny thing is that I read they actually did find a difference, but it was deemed by the “establishment” at that time to not be enough to be statistically significant.
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