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Voyager: Pressure at the Edge of the System

One of these days we’ll have a spacecraft on a dedicated mission into the interstellar medium, carrying an instrument package explicitly designed to study what lies beyond the heliosphere. For now, of course, we rely on the Voyagers, both of which move through this realm, with Voyager 1 having exited the heliosphere in August of 2012 and Voyager 2, on a much different trajectory, making the crossing in late 2018. Data from both spacecraft are filling in our knowledge of the heliosheath, where the solar wind is roiled by the interstellar medium.

A new study of this transitional region has just appeared, led by Jamie Rankin (Princeton University), using comparative data from the time when Voyager 2 was still in the heliosheath and Voyager 1 had already moved into interstellar space. Leaving the heliosheath, the pressure of the Sun’s solar wind is affected by particles from other stars, and the magnetic influence of our star effectively ends. What the scientists found is that the combined pressure of plasma, magnetic fields, ions, electrons and cosmic rays is greater than expected at the boundary.

“In adding up the pieces known from previous studies, we found our new value is still larger than what’s been measured so far,” said Rankin. “It says that there are some other parts to the pressure that aren’t being considered right now that could contribute.”

Image: This is an illustration depicting the layers of the heliosphere. Credit: NASA/IBEX/Adler Planetarium.

Thus the Voyager data continue to be robust, giving us a look into a dynamic and turbulent region through which future missions will have to pass. The particular area that the study’s authors focused on is called a global merged interaction region, a wave of outrushing plasma produced by bursts of particles from the Sun in events like coronal mass ejections. Such an event is visible in Voyager 2 data from 2012, causing a decrease in the number of galactic cosmic rays, one that Voyager 1 would go on to detect four months later.

Traveling at nearly the speed of light, galactic cosmic rays are atomic nuclei from which all of the surrounding electrons have been stripped away. The difference between how this change in their numbers was detected by the two spacecraft is instructive. Still within the heliosheath at the time, Voyager 2 saw a decrease of galactic cosmic rays in all directions around the spacecraft, whereas at Voyager 1’s vantage beyond the heliosphere, only those galactic cosmic rays traveling perpendicular to the magnetic fields in the region decreased.

This intriguing asymmetry flags the crossing of the heliosheath, though the study’s authors are quick to point out that why this directional change in cosmic rays occurs remains unknown. They are able to calculate the larger than expected total pressure in the heliosheath, and discover that the speed of sound in the heliosheath is roughly 300 kilometers per second (remember that the speed of sound in any medium is simply the speed at which disturbances in pressure propagate, in this case the result of interactions in the solar wind).

Image: The Voyager spacecraft, one in the heliosheath and the other just beyond in interstellar space, took measurements as a solar event known as a global merged interaction region passed by each spacecraft four months apart. These measurements allowed scientists to calculate the total pressure in the heliosheath, as well as the speed of sound in the region. Credit: NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith.

“There was really unique timing for this event because we saw it right after Voyager 1 crossed into the local interstellar space,” Rankin said. “And while this is the first event that Voyager saw, there are more in the data that we can continue to look at to see how things in the heliosheath and interstellar space are changing over time.”

The paper is Rankin et al., “Heliosheath Properties Measured from a Voyager 2 to Voyager 1 Transient,” Astrophysical Journal Vol. 883, No. 1 (25 September 2019). Abstract.


Comments on this entry are closed.

  • Geoffrey Hillend October 11, 2019, 20:40

    If these cosmic rays entering the Heliosheath have all their electrons stripped, then they are completely ionized so that they cations or positively charged due to the loss of negatively charged electrons. Charged particles will be accelerated in a magnetic field, but only if their vectors are perpendicular to that field and only if the charged particle is moving. The cosmic rays are probably decelerated by their collision at a perpendicular angle to the magnetic fields in the Heliosheath. Maybe their direction change is the result of that.

    The speed of sound through a medium depends on it the medium’s density, and the gas is not very dense in the Heliosheath. We can’t hear that sound in space since the gas is too low density. The sound is very low frequency below the range of human hearing. For that reason, we can’t hear a high energy intense laser beam in space and we can’t see a visible light laser in space since the gas is not dense enough to be exited to the point of any scattering of light or Rayleigh scattering or air heated, thermal sounds. I still like Star Wars and Star Trek though.

  • Mike Serfas October 12, 2019, 9:06

    NASA has released lovely recordings of (largely electromagnetic) sounds in space ( https://soundcloud.com/nasa/enceladus-hiss-audio?in=nasa/sets/spookyspacesounds provides a compilation but very short and lacking in explanations). Is it feasible to launch a large number of tiny probes (whether Starshot or cubesats) capable of detecting such sounds in the weak interstellar medium, which would station themselves in the heliopause to take advantage of its relatively stationary medium of transmission? Could such probes be used to detect some sort of changes in these sounds that might be caused by incoming comets, asteroids, or perhaps starships penetrating at any point on the heliopause?

    • Alex Tolley October 12, 2019, 17:42

      The Cosmic Ray Subsystem on Voyager is of the order of 1 meter on a side, and is therefore too large or massive for a CubeSat. Which is not to say that smaller versions may now be possible. The bigger issue is how to cheaply launch these CubeSats and propel them fast enough so that they reach the edge of the solar system in a much shorter time than the Voyagers. Solar sails seem like the most likely way to do this, as they are lightweight, unlikely to fail, and relatively inexpensive. I would hope that as we gain experience, such sails will become readily available and low cost (preferably off-the-shelf) as propulsion systems for small spacecraft.

      • Mike Serfas October 13, 2019, 11:43

        I’m quite ignorant on these things, but my daydream was running along the line of a probe in a Juno-like series of orbits dropping many mothercubes that spit out baby probes to take advantage of the slingshot to get things started. But something more remarkable would be needed because they have to stop somehow – some sort of electric solar wind sail, perhaps? Maybe that would help get them there also. Once they arrived, the function of the electric sail might be reversed – measuring tiny currents to listen to the songs of the heliopause. Some of the same wires might also serve as an antenna to receive a small amount of power beamed from Earth periodically. Perhaps a wire might be kept charged and its deflection measured by watching the wiggling of a portion of the sun’s reflection (harvested from an axial position relative to the wire’s rotation) from a large distance away. The antenna system used for transmission might be simple, hopefully analog in real time, so that a very large radio telescope with a very sensitive receiver that has built up sophisticated simulations of the little probes’ individual wirings could steadily improve the quality of the ‘sound’ received from all the little probes it listens to. Ideally the devices would be able to avoid using lasers and digital processing altogether and be designed, once placed at the right position, velocity and rotation, to last for centuries.

  • Geoffrey Hillend October 12, 2019, 16:06

    Interesting but they are not actual sound waves but are translated into sound. I don’t know if a 300 kilometer an hour sound can be heard, but that seems fast enough to hear it. I imagine if it the sound from the Heliosheath are audible to the human ear, then it might be not that loud like what sounds on Mars might be through very thin air or slower and much quieter than on Earth.

    The comic rays hit the Heliosheath in the opposite direction as the solar wind which carries it’s own magnetic field, so a vector perpendicular to them might slow them down or decelerate them. The range of human hearing does include some space sounds, but a laser will still be silent and completely invisible in space do to the lack of atmosphere.

    • Robin Datta October 12, 2019, 17:38

      “300 kilometer an hour”: shouldn’t that be 300 kilometers per second?

  • wdk October 12, 2019, 20:10

    It is also stated above that the speed of sound through a medium depends on its density. Well, yes and no. If we compare wave transmission through solid materials such as within the Earth’s interior or a liquid such as water, I can see how that would be true -icomparison to a gas such as an atmosphere.

    But in the atmosphere, where the density varies with altitude, the temperature sonic speed has a relation to the square root of absolute temperature.

    My acquaintance with interstellar medium was only brief and far back, but what I recall is that the structures such as shock waves were modeled as very thick in terrestrial atmospheric terms (AUs) and quite tenuous. Mean free paths must have been large.

    But 300 km/sec would suggest a very high effective temperature. if it is related to hydrogen protons and 300 Kelvin with N2 is around ,333 km/sec, even with the mass ratio the temperature of the medium would be quite high. Thousands of kelvins?

  • Geoffrey Hillend October 13, 2019, 16:12

    Thankyou for catching that error Robin Datta. If the sound is three hundred kilometers per second, I don’t think we could hear that with the human ear. I read that solar wind sound waves have a very low frequency like many octaves below the lowest note on the piano so we can’t hear those sounds. I don’t remember where I read that. I think it was on Wikipedia. Paul Gilster wrote an interesting article on Centauri Dreams about the sounds in space I think was about the sounds of the solar wind, but I don’t remember the name of the article.

    The sound speed through our atmosphere IS DEPENDENT ON THE DENSITY OF THE AIR. Sound goes slower through thinner air. This is atmospheric science 101.

  • wdk October 15, 2019, 13:51

    Once again, I will grant that there are differences in sound propagation for different media and phases of matter, but your statement about sound speed through our atmosphere …

    If you look at an engineering description of a standard atmosphere or one compiled for meteorologists, there is direct correspondence between temperature and speed of sound. At about 12 kilometer altitude or 36,000 feet the temperature remains around 295 kelvin up to about 22 km. The density of the atmosphere reduces an order of magnitude over that interval.

    Aerospace engineering deals more directly with this phenomenon and the speeds of sound at altitude are listed as a result. A text such Houghton’s, the Physics of Atmospheres, does not bring this up. But typically there is discussion of the adiabatic lapse rate influenced by
    water vapor

    Speed of sound: a = sqrt ( gamma R/m T)
    gamma: specific heat ratio for the constituent gas or gases
    R the universal gas constant
    mass the mean molecular weight
    T for the absolute temperature.
    Specific heat ratio will change for monatomic or diatomic gases, etc.
    Mean molecular weight dependent on chemistry (N2, O2, etc.).
    There might be differences in physical modeling for the various shocks and transition layers where Voyager is in transit, but I based on my own experience with atmospheres and compressible fluid flow, consider it my contribution to attempting to characterize the phenomenon.

  • Geoffrey Hillend October 15, 2019, 15:50

    Wdk, let me apologize to you for not giving you enough information. One also has to know about sound as well as meteorology. When the air heats up, the air expands and the molecules become more widely spaced. The air becomes lighter and rises. This creates a low pressure zone. Sound travels faster through thick air because the molecules are closer together, than in thin air. The air molecules have to bump into each other due to the moving sound wave which vibrates. Consequently air will move faster through denser air than in thinner air. Sound always moves faster through a denser medium since the molecules are more closer together.

  • wdk October 15, 2019, 23:17

    It could be what is causing our ships to pass in the night is the acoustic expression of sound waves versus the phenomena that I have just described. The passage of a jet’s sonic boom will be transmitted across distances in accordance with what I described in terms of temperature and molecular properties of constituent gases. But at the same time, the wave properties are described in terms of partials with respect to pressure over the partials with respect to density. Which I have seen often enough but haven’t found as useful as the relation described above for engineering problems. I’ll keep this discussion in mind for reflection lest we get too far off topic. But I appreciate your elaboration on this. – Best regards, wdk.

  • Alex Tolley October 16, 2019, 12:44

    Re: Speed of sound.

    The original question was about the speed of sound in the heliosheath. I think the answer is that sound cannot be transmitted. IOW, “In space, no one can hear you scream”.

    Here is a quote from Wikipedia about the thermosphere of our atmosphere. (emphasis mine).

    The highly diluted gas in this layer can reach 2,500 °C (4,530 °F) during the day. Despite the high temperature, an observer or object will experience cold temperatures in the thermosphere, because the extremely low density of gas (practically a hard vacuum) is insufficient for the molecules to conduct heat. A normal thermometer will read significantly below 0 °C (32 °F), at least at night, because the energy lost by thermal radiation would exceed the energy acquired from the atmospheric gas by direct contact. In the anacoustic zone above 160 kilometres (99 mi), the density is so low that molecular interactions are too infrequent to permit the transmission of sound.

    Regarding the general idea of sound speeds and density. Sound does travel through different media at different speeds, with dense media having the fastest speeds. However, wdk is correct in saying that the speed of sound in air is temperature dependent, not density dependent. Sound actually travels faster in the upper atmosphere than in the stratosphere due to higher temperatures, even though the density is much lower. wdk – I think you have your stable temperature of teh atmosphere between 12 and 22 km of 295K wrong. By simple observation, the tops of snow-covered mountains must be, on average, at less than 273K. The data suggest that the approximately stable temperature is 216K.

    Atmospheric profile : temperature, density, pressure, and speed of sound

  • Geoffrey Hillend October 16, 2019, 17:03

    Alex Tolley. I don’t think we have an argument. Temperature and density are not mutually exclusive. Sound waves are the transmission of energy or the vibrations of the air molecules and that depends on the density in all cases. This is not my idea, but atmospheric physics 101 as I have said.
    Temperature can be involved also but it is the space between the air molecules or density that determine the speed of the sound. I never disagreed with temperature being involved. https://www.physicsclassroom.com/class/sound/u11l1c.cfm


    • Alex Tolley October 16, 2019, 21:22

      I cannot agree with your assertion. I posted the link to the graphic on our atmosphere for a reason. Between 10 and 20 km, the air temperature is constant, while air pressure and density decline as expected. By the speed of sound stays constant. This is as clear as it needs to be. The temperature variable is fixed, the density varies, but the speed of sound stays constant. Above 40 km or so, air density is now nearly constant or at least requiring a log scale to see the changes. But the temperature is changing and the speed of sound tracks that temperature.

      What about density? This table shows that the lighter the gas, the faster the speed of sound. Helium has a much higher speed of sound at 0C than air, or the even denser CO2. That is why one’s voice becomes high-pitched when the lungs (or just mouth and throat) are filled with helium.

      That liquids and solids have higher velocities for sound is also shown in the table, which is what you are referring to with sound and density as physics 101.

      Back to the heliosheath. As Robin corrected, the speed of sound is 300 kps. This is about 1000x that of air at 0C. Yet clearly the density is extremely low. So what accounts for the speed? The temperature is about 1 million C, which means the molecules are moving extremely fast.

  • Geoffrey Hillend October 16, 2019, 17:16

    The idea that the speed of sound changes when the density of air changes is not my idea but a principle. I got it from reading the internet. I think I first read it in a magazine about Mars which said that sound would go much slower on Mars due to the thin air. This is the correct link https://science.howstuffworks.com/sound-info2.htm

  • Ron S. October 16, 2019, 17:49

    Rather than worry so much about the speed of sound I think it worthwhile to consider the path loss (sound wave attenuation). In a sufficiently rarefied medium the path loss will be very high, so high that discussing sound’s speed may be superfluous.

    • Alex Tolley October 17, 2019, 10:08

      IOW, “In space, no one can hear you scream”. ;)

      The same applies to the high temperature of the thermosphere in the Earth’s atmosphere. The air is so rarified that it feels cold. This is similar to the lack of heat experienced when those “sparkler” fireworks touch your hand. Clarke once mentioned how an academic stated that rockets could never reach space because they would burn up in the thermosphere. Classic Clarke’s 1st Law.

      The same issue affects the Parker Solar probe that will reach the sun’s outer corona. The corona is perhaps 1 million degrees C, but its extremely low density means that the probe won’t burn up like a firefly entering a candle flame.

      • Ron S. October 17, 2019, 21:48

        Yes, this is the difference between temperature and heat. Many people incorrectly use the terms interchangeably.

        Again with regard to sound in a rarefied medium, if the mean free path is high enough you have a particle stream rather than a propagating pressure wave; that is, no sound. It’s important to understand the domains of bulk material physics and not use the formulas outside of those domains. Ohm’s law is similarly constrained, among other examples.

  • Geoffrey Hillend October 16, 2019, 18:25

    I forgot about the ideal gas law. It is the different chemical composition that makes sound go slower on Mars, but not the air density. Also pressure and density are “inversely related to the temperature and molecular weight” so temperature is most important as long as chemical composition and molecular weight don’t change and molecular weight.https://en.wikipedia.org/wiki/Speed_of_sound Consequently, the temperature is most important, but not the density which is canceled by molecular weight, so temperature is what matters. Wikipedia: The speed of sound. I stand corrected. The density of the medium does matter but only for different materials like gas, liquids and solids, but not air provided the air has same gases and molecular weight.


  • wdk October 18, 2019, 12:11

    GH, AT,
    Due to distractions beyond this site, I’ve been out of the fray for a few days, but it sounds like some progress was made on – what I think – is an interesting issue.

    Because, as GH points out, there is an element of change in density in sound waves – and yet in so many engineering applications, we treat the speed of sound as a temperature and averaged molecular weight dependency.

    Speaking of temperature, AT pointed out my error on temperature quotes. He is right. Speed of sound in meters per second was in a column adjoining the fractional values of T/Tref at the surface, and I confused the speed with the temperature. It should have been 216 degrees Kelvin. Numerically the English units are more distinguishable than the metric since Kelvin temperatures and sonic metric speeds in the atmosphere (1116 fps at the surface vs. ~ 518 Rankine). Usually I work with feet per second in the atmosphere and have had to load them into spreadsheets or data statements several times of late. But assuming many of our correspondents here prefer or are used metric – I picked up the nearest metric table at hand – with the resulting consequent error. Using metric does not necessarily make one more correct or less error prone.

    In the meantime, the cases of Mars and inhaling helium from a balloon came to mind as arguments, due to CO2 and helium media respectively.
    We would expect slow sonic speeds on Mars due to near double molecular weight, especially on a cold martian day or night. But then case of people talking like chipmunks at high pitch, as an argument seems to have a complication as well. Because, if I play a piano and hit a low key and then a high one, temperatures and air molecular weight have not changed, one could point that out, enlarging our debate about features of the interstellar transition.

    But there is in derivation the dP/d mu, P being pressure and mu being density. In deed a wave front hasa discontinuity in density and pressure; and certainly that is how we would characterize a shock. But temperatures fore and aft are also considerations ( ambient, stagnation, etc.).

    So I am glad that we explored this question. And I will attempt to examine it further on my own.


    • Alex Tolley October 18, 2019, 14:43

      Sounds of Mars

      This recording of wind on Mars supports your suggestion that the denser CO2 compared to air (and the cold temperature), reduces the speed of sound, thereby reducing the pitch.

  • ljk October 31, 2019, 10:59

    If nothing else, no one will ever accuse the makers of Voyager 1 and 2 with shoddy workmanship:


  • ljk October 31, 2019, 11:08

    NASA’s Voyager Missions Were Amazing. Now Scientists Want a True Interstellar Probe

    By Elizabeth Howell 2019-10-30 T 16:00:00

    Goodbye, heliosphere! Hello, interstellar space!

    WASHINGTON — Humanity should consider building an interstellar probe to see our neighborhood from an outside point of view, argued several scientists at a recent conference.

    NASA’s Voyager 1 and 2 spacecraft are the only machines that people have sent beyond our solar system. These 42-year-old spacecraft are still functioning well enough to send us information from interstellar space, and many of their insights have been surprising, according to Stamatios (Tom) Krimigis, the principal investigator of the low-energy charged particle experiment that is still working on both spacecraft.

    “The models have been wrong,” Krimigis told delegates on Oct. 25 at the International Astronautical Congress held here. One prominent example was the shape of the heliosphere, or the region of space in which the stream of charged particles emanates from our sun and wraps around the solar system. Until the 2010s, scientists thought it had a fan shape; the Voyagers, upon crossing the heliosphere in 2012 and 2018, revealed it is more like a bubble.

    Another surprise was finding out where cosmic rays (radiation from outside the solar system) are accelerated, he said. Before the Voyagers went into interstellar space, scientists thought these particles accelerate at the termination shock area, which is where the particles from the sun slow down to below the speed of sound. The Voyagers revealed the acceleration actually takes place in the heliosheath, the region of space just beyond the termination shock zone.

    Full article here:


    To quote:

    To keep mission managers happy, Alkalai suggested an interstellar mission should be built to do science before it reaches its destination. The probe’s ultimate goal would be to study interstellar space, but within the solar system, the spacecraft could, for example, image Kuiper Belt objects beyond Neptune or perform parallax measurements of small worlds to better calculate their distance.

    “There is plenty of science to do on the way on the interstellar medium,” Alkalai said.

    As the panel concluded, moderator Ralph McNutt, chief scientist of the space department at the Johns Hopkins University Applied Physics Laboratory, asked everyone born after 1990 to stand up. A third of the people in the room rose, and McNutt offered some career advice. He said that the standing-up group is the generation who would take over any interstellar missions launched by more experienced scientists today.

    “When you are in that position, don’t screw it up,” he joked.

  • Mike Serfas November 5, 2019, 15:09

    I was browsing a fluffy article today ( https://dailygalaxy.com/2019/11/the-two-voyagers-will-outlast-earth-now-in-the-realm-of-the-stars/ ) that says the two Voyagers will orbit the galaxy for billions of years. With current simulations putting odds at 12% that the Earth will be ejected entirely from a Milky Way-Andromeda merger galaxy in just a few billion years, plus presumably some chance that the collision remnant forms more than one galaxy, it seems quite possible that one of the Voyagers will eventually end up not being in the same galaxy as Earth … and therefore, may be considered to be humanity’s first intergalactic spacecraft.