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A Galactic glitch

Filed under: — rasmus @ 10 March 2008

Knud Jahnke and Rasmus Benestad

After having watched a new documentary called the ‘Cloud Mystery’ – and especially the bit about the galaxy (approximately 2 – 4 minutes into the linked video clip) – we realised that a very interesting point has been missed in earlier discussions about ‘climate, galactic cosmic rays and the evolution of the Milky Way galaxy.

It is claimed in ‘The Cloud Mystery’, the book ‘The Chilling Stars’, and related articles that our solar system takes about 250 million years to circle the Milky Way galaxy and that our solar system crosses one of the spiral arms about every ~150 million years (Shaviv 2003).

But is this true? Most likely not. As we will discuss below, this claim is seriously at odds with astrophysical data.

Here is a little background on the Milky Way: The arms of spiral galaxies are not constant entities in time. They are results of gravitational instabilities in the disk or are induced by external companions. These instabilities are moving mass ‘overdensities’ containing old stars and gas, but also newly formed stars recently created from local collapse of the overdense gas.

Arms move around a spiral galaxy with a pattern speed that is defined by the mass distribution. This pattern speed differs from the motion of individual stars, just like the speed of an ocean wave differs from the movement of water particles. Estimating the pattern speed is difficult, as it is not coupled to the motion of individual stars but can only be inferred indirectly. For this reason it has not yet been reliably measured for our Milky Way – unlike for some other spiral galaxies, for which our clear and unobstructed view from the outside allows an estimate.

So how did Shaviv come up with this number?

Measuring the rotational velocity of stars in the Milky Way disk or other spiral galaxies is straightforward. The rotation is not rigid, but depends on the encircled mass inside the orbit of a star, including the Dark Matter, a yet unknown but solidly established source of gravitational attraction. It is easy and a standard technique to measure rotation curves of galaxies as a function of radius, and this is also possible for the Milky Way.

The two different rotating velocities of arms and stars have a different radial dependence – to first order the arms get preserved as entities while the stars further out have much smaller angular velocities than stars further inside – so the relative velocity of a star with respect to the nearest spiral arm will depend on its distance from the centre of the galaxy. At a certain radius, the radius of co-rotation, the two velocities are identical and a star at this radius has zero relative velocity with respect to the spiral arm pattern. It stays “forever” in the same spiral arm – or outside of it.

What are the best estimates for the relative velocity of the Sun with respect to the spiral arm pattern of the Milky Way? As mentioned, the pattern speed of the spiral arm in the Milky Way has not been firmly established.

When investigating other spiral galaxies, however, it was found that almost independently of the wide range of possible assumptions on which the pattern speed estimate was based, the radius of co-rotation follows a simple law: rcorot=r0 * (3.0 +/- 0.5), where r0 is the scale length of the exponential disk of the galaxy (the surface brightness of spiral galaxies drops very close to exponentially from the center to the outside, setting a characteristic size scale). This was measured by Kranz et al. 2003.

Since the Milky Way is a completely normal spiral galaxy, we can apply this result to it. The scale length of the Milky Way disk has recent estimates ranging from 2.6 kilo-parsec (kpc, 1pc=3.3 light years) from the SDSS survey (Juric et al. 2008), through 2.8 kpc (Ohja 2001) to 3.5 kpc (Larsen & Humphreys 2003).

We also know the Sun’s distance to the galactic center well, 7.9 +/- 0.4 kpc (Eisenhauer et al. 2003), which means that the range of values for rcorot=9.1 +/- 1.9kpc. In other words, from this calculation the co-rotation radius of the Milky Way is between 7 and 11 kpc, and at 8 kpc our Sun is close to or at the radius of co-rotation. It almost certainly is not 6 kpc further inside, as Shaviv (2003) claims.

Shaviv (2003) lists in his Table 3 a number of values for the pattern speed of the spiral arms, taking from publications ranging from 1969 to 2001, two years before his article. In these papers the derived relative motion of the Sun relative to the arms ranges from Omegarel=+13.5 km/s/kpc to -4km/s/kpc, and includes estimates that are close to zero (-4km/s/kpc < than Omegarel < +7), i.e. a location near the radius of co-rotation in the majority of the publications, and most of the more recent ones. However, he selectively disregards most of these results.

If we add the above evidence that the radius of co-rotation lies at 9kpc distance and not further out, and convert this to relative velocities, e.g. by using the Milky Way rotation curve by Merrifield 1992, we obtain Omegarel =+3.2 km/s/kpc with an error range from -2.5 to +7.1km/s/kpc, and including zero. Shaviv’s derived “period for spiral arm crossing” of p=134 +/- 25Myr for four spiral arms is well outside the range derived from these values.

So it seems that Shaviv’s “periodicity” estimate for crossing of spiral arms by the sun does not hold up under scrutiny when using current astronomical results as the work by Kranz et al. This comes in addition to the previously shown fact that the correlation of cosmic ray flux with paleoclimatic data proposed by Shaviv and Veizer (2003) only arises “by making several arbitrary adjustments to the cosmic ray data” (Rahmstorf et al. 2004).

Note also that the question of current climate change is quite another matter from that over time scales of many millions of years – despite Shaviv’s remarkable press-release claims that “The operative significance of our research is that a significant reduction of the release of greenhouse gases will not significantly lower the global temperature”. As we repeatedly pointed out over the years: that global warming over the past decades is not linked to cosmic rays is clear from the fact that the cosmic ray measurements over the past 50 years do not show any trend (Schiermeier 2007).

Remarkably, the poor scientific basis of the galactic cosmic ray hypothesis seems to be inversely related to the amount of media backing it is getting. At least 3 documentaries (‘The Climate Conflict’, the ‘Global Warming Swindle’, and now ‘The Cloud Mystery‘) have been shown on television – all with a strong thrust of wanting to cast doubt on the human causes of global warming.

References:

Eisenhauer et al. 2003, ApJ, 597, 121; http://adsabs.harvard.edu/abs/2003ApJ…597L.121E

Kranz et al. 2003, ApJ, 586, 143; http://www.journals.uchicago.edu/doi/abs/10.1086/367551

Juric et al. 2008, ApJ, 673, 864; http://adsabs.harvard.edu/abs/2008ApJ…673..864J

Larsen & Humphreys 2003, AJ, 125,1958; http://adsabs.harvard.edu/abs/2003AJ….125.1958L

Merrifield 1992, AJ, 103, 1552; http://adsabs.harvard.edu/abs/1992AJ….103.1552M

Ohja 2001, MNRAS, 322, 426; http://adsabs.harvard.edu/abs/2001MNRAS.322..426O

Rahmstorf, S., et al., 2004: Cosmic rays, carbon dioxide and climate. Eos, 85(4), 38, 41.

Schiermeier, Q., No solar hiding place for greenhouse skeptics. Nature, 2007. 448: p. 8-9.

Shaviv, N., 2003, NewA, 8, 39; http://adsabs.harvard.edu/abs/2003NewA….8…39S

Shaviv, N. and J. Veizer, Celestial driver of Phanerozoic climate? GSA Today, 2003. 13(7): p. 4-10.

378 Responses to “A Galactic glitch”

  1. 351
    Hank Roberts says:

    Mark, we both studied this in school, sometime after Ernst Mach wrote the equation.

    I tried a well footnoted example and cannot get it past the spam filter here. This is cut way down, last try:

    Simple answer — no sound if you can’t hear anything:
    http://www.qrg.northwestern.edu/projects/vss/docs/space-environment/1-is-there-sound-in-space.html
    __________________________

    College physics answer (from the TOC):

    Astrophysical Hydrodynamics (Second Edition)
    Published Online: 25 Feb 2008
    Author(s): Prof. Steven N. Shore
    Print ISBN: 9783527406692 Online ISBN: 9783527619054

    … * Supersonic Jets
    … * Bending of Jets by a Supersonic Cross-Flow

  2. 352
    Mark says:

    Yeah, Hank. And Ernst Mach wasn’t considering the interstellar medium.

    Also, although I did do some astrophysics in O- and A-level Physics, there wasn’t any discussion about sound in the interstellar medium.

    I had to to a degree in astrophysics.

    =====

    “Supersonic jets”

    As I said. How can sound go faster than sound?

    Supersonic waves are waves that travel through a medium that is at that differential incompressible.

    In the case of jets, it is a jet that is ejecting material faster than the speed of sound in that ejecta, not in the interstellar medium.

    Here’s some figures for density/mach speeds:

    (see http://en.wikipedia.org/wiki/Speed_of_sound)

    Material Density mach

    Aluminum 2.7 5000
    Water 1.0 1402
    Methyl 0.85 1130
    Air .001292 331.3

    Now what do you think the speed of sound will be in a medium at the density of 10^-27?

    (PS Ray, #350, the sound pressure level and the velocity all go up as density increases. When you have a sound wave propogate, the charged particle will push the other particles *aside* as much as *onward*. And sideways propogation of sound waves is dispersive).

  3. 353
    Mark says:

    PPS Ray, 350. What does the energy of cosmic rays have to do with the interstellar medium and its ability to propogate longitudinal pressure waves (i.e. sound) within it?

    The force between two protons .5 cm apart is ~10^-23 N. To move them .5 cm would require an energy of about 10^-23 J.

    That’s the energy pertinent to sound in the interstellar medium.

    MeV’s it isn’t. More like 10^-7 eV.

  4. 354
    Hank Roberts says:

    http://books.google.com/books?id=qrWQiBTepsUC&pg=PA136&lpg=PA136&dq=Bending+of+Jets+by+a+Supersonic+Cross-Flow&source=web&ots=dNsfSXkMVz&sig=ZVF2BJAu0jStj9B7Eh3oOuicMC8&hl=en&sa=X&oi=book_result&resnum=2&ct=result#PPA137,M1

    Mark, you’re telling me you have a degree in this subject. Are you saying the published work I find is simply incorrect? I can’t figure out what you’re arguing _with_ let alone _about_. What’s your point?
    And why are you arguing about it in this thread? Just saying the word is used wrong by other scientists isn’t helping me as an ordinary reader figure out what you’re trying to say here.

  5. 355
    Hank Roberts says:

    > what do you think …?

    I tried pasting your question into Google Scholar; this among others turned up:

    http://www.sciencedaily.com/releases/2008/07/080703113646.htm
    _________________
    ReCaptcha: TOURS ex-

  6. 356
    Mark says:

    No, Hnak you’re misreading the published work.

    Rather like when denialists point out “you scientists were saying that there’d be an ice age!” or “there’s 30% more ice in the north pole!!”.

    The supersonic jets are supersonic within the ejecta. Not in the interstellar medium.

    If you take the interstellar medium as an ideal gas (and I’ll show you why this is wrong soon), the speed of sound is about 40m/s. Just about ANYTHING is moving faster than that. So saying “supersonic” means naff all when you’re in a medium with a “sonic” limit of 80 miles an hour.

    Now, as to why it’s wrong to deal with it as an ideal gas: basically there’s not enough density to count it as a gas.

    When you put sound in a medium, the maximum sound pressure you can get is equal to the pressure in the medium. At 1 Hydrogen atom per cc, that’s about 10^-18 pa.

    So the maximum pressure is double that and the minimum 0.

    If you pushed the medium harder than that, there’s no way the gas can come back into equilibrium for the next pressure increase.

    The maxmimum frequency depends on the pressure too (since the particles in the gas must have time at the nominal pressure to return back to the sound source), but I can’t find this one too easily. However look at the pressure. Not fast.

    Now, take just counting statistics: what is the maximum number of atoms per cc if there’s 1 per cc in a 1m^3 volume? 1,000,000 per m^3 means 1,000. Pressure depends on N, so the pressure is 1,000 times bigger in that one exceptional cubic cm. Not based on any sound passing through but just on the fact that these atoms are randomly traveling within the mass as per ideal gas law.

    Remember that we had +/- 1 pressure.

    Randomness wipes out any information in pressure in a medium this thin.No sound. NOTE: the reason why counting statistics works here is that the numbers are so small that even a million in a cubic cm doesn’t cause significant dispersal forces to even out the density within even seconds.

    So given that even the star itself is traveling “supersonically” if you take the interstellar medium, why is calling an ejecta jet “supersonic” done? I’ll answer that one for you: it’s not supersonic wrt the medium. That would be redundant.

    If you want, go to a university, pick a professor and ask if there’s sound in the interstellar medium. That WAS my original point in #342. Whether there’s a paper on “supersonic jets” doesn’t tell you that there’s sound in space.

    Until you’ve read up and come up with something, leave it alone.

  7. 357
    Martin Vermeer says:

    Mark #352:

    The answers are here:

    http://en.wikipedia.org/wiki/Speed_of_sound

    Density has nothing to do with speed of sound — temperature (mean molecular velocity) has. For solids it’s more complicated. For plasmas, look at the electrons.

    Whatever physics you studied must have been a long, long time ago :-)

    …and I wonder why we discuss speed of sound anyway in the context of spiral arms. The mechanism there is quite different: gravitational instability combined with “detonation” of the galactic medium in OB star formation, quickly leading to supernovas. It’s an active mechanism. The dark gas and dust are at the leading edge of the arm, the OB associations, HII clouds and supernova remnants on the trailing edge.

  8. 358
    Pat Cassen says:

    Not sure where this discussion of waves in the interstellar medium started or where it’s going, but here’s more:

    The galactic medium (neutral gas, ionized gas and stars) supports a variety of physically relevant waves: MHD, “rotational”, self-gravitational, etc. Each obeys a dispersion relation which may reduce to the one for acoustic (e.g., sound) waves in some limit, usually when thermal energy dominates other (magnetic, gravitational, etc.) factors. But the full dispersion relations are complex and interesting, usually supporting “slow”, “fast” and other (e.g., Alfven) modes. Self gravity is usually important for spiral density waves in rotating media (as in the galaxy or even the particles of saturn’s rings); these waves can “shock” in a manner analogous to acoustic waves.

    The solar wind, near its termination shock, is very “super-fast mode” and very “super-alfvenic”. It obeys equations very similar to those of a gas flow expanding to supersonic speed through a nozzle, complete with an “exit” shock as the flow adjusts to the downstream pressure boundary condition.

    Martin, yes, the observed spiral structure is a dynamic affair, as you say. But gravitational density waves may initiate the whole business.

    Can you “hear” these waves? Sure, you just have to have the right kind of ears!

  9. 359
    Hank Roberts says:

    Mark, I’m aware of this level of answer, which is what you’re fixed on:

    Is there sound in space?
    In deep space, the large empty areas between stars and planets, there are no molecules to vibrate. There is no sound there. …
    http://www.qrg.northwestern.edu/projects/vss/docs/space-environment/1-is-there-sound-in-space.html

    That answer is based on the size of human ears. Seriously.

  10. 360
    Mark says:

    Martin, you’re still off track. If I’m allowed, I’ll go into what those papers are talking about but get back to the point. Hank is getting back to the point but he’s still working like a climate denialist.

    Hank, I can find you the link to the absolutely 100% correct treatise on the Middle Ages Warm Period.

    Middle ages: we didn’t have cars.
    Warm: well if it was remembered this long, it MUST be warmer than now.

    You KNOW that’s wrong. Yet you’re doing the same thing here.

    I’ll start with the handwaving and if this doesn’t convince you, do the maths. I’ll tell you how later.

    In a sound wave, what is happening? Pressure waves. Rarefaction and compression. The medium transporting the sound is traveling back and forth. Now what makes the sound travel back? The pressure differential between peak and trough. Which is VERY small. But what’s the pressure difference between two areas in the medium? I’ve already shown you that. Much bigger. Still tiny, but much bigger. So the signal is absolutely 100% impossible to take from the noise.

    Think of it this way: if something pushes the atom in one direction, why would it go back to where it was? Gravity in denser clouds, but that’s nothing here. So what brings it (or a compatriot, but at some point, you’re going to run out of them unless there’s something bringing them) back here so that the cycle can begin again?

    Now if that’s not good enough, do the maths.

    We’ve actually got a great test of modelling here. Go *right* back to basics. We have computers that are fast, we have very simple maths to work with and only a small number of particles to consider.

    Take a piston box. Walls are proof. The empty space is 1m cubed and one side has a piston which can compress and expand the volume. It is filled with 1 million hydrogen atoms. The atoms start off with random energies and velocity vectors (selected so that they obey the relevant gas laws) and are at 3K. The piston moves 10cm (limit the compression so we don’t have to deal with carnot cycles and adiabatic losses) at whatever frequency you like.

    This is sound. This does not even depend on the presence of human ears.

    What happens to the time-dependent velocity variations of the atoms in that box.

    How long would it take you to discern the frequency of the piston from the velocity variations random noise gives? At a rough order of magnitude, I’d say the age of the universe. That does assume that the speed of the piston is not orders of magnitude more than the velocity of the particles and no relativistic effects take place.

    Do the modelling.

    Oh, and while we’re at this, please explain how the energy of a cosmic ray changes how ionising it is to an atom. It doesn’t. Unless you get a bullseye and collide. So the only thing that is important is how close the cosmic ray has to get to an atom in the interstellar medium to rip the electron out. Note: this is why OIII is much more prevalent: you don’t need so much pull to take the electron off.

    Now what’s the cross-section of an Hydrogen atom? Multiply that by 10,000 and then by 5 (the number of cosmic rays per cc) and that is how likely a hydrogen atom is to be ionised. Therefore, that the cosmic rays outnumber the hydrogen atoms doesn’t make all the hydrogen ionised (cf #348).

    Take the cosmic flux and that tells you how long a hydrogen atom will have between hits to re-acquire an electron.

    If this were prevalent, we’d be seeing a LOT MORE H-I lines in the sky than we do, and astronomy would be a LOT more difficult.

    But that was kind of a side rant.

  11. 361
    Mark says:

    Pat, MHD waves, self-gravitation waves, etc. all require a certain level of density.

    That density is not 1H/cc.

    These do exist and I know about them: Hell, all you GET from astronomy is light from the available spectrum, from which you use astrophysical application of known physics to explain how they happened. Solar physics relies EXTENSIVELY on Alfven waves to explain things that go on.

    But what density level is it when these effects become useful?

    A lot higher than 1H/cc.

    Now “the interstellar medium” is a good word. However, you’re jumping to conclusions. The interstellar medium include damn near vacuum (heck, on the cm scale, most of the gaps between stars is a vacuum devoid of any material particle not travelling at near-light), it includes dense (cold) nebulae. It includes dense hot emmision regions. It includes gas clouds that are the birthplace of stars.

    And because “the interstellar medium” contains dense nebulae, they also contain regions that can support sound waves. That doesn’t mean ALL of the interstellar region can. E.g. black holes. Wandering planets. Comets. Failed stars. Vacuum.

    In fact, most of the gaps between stars can’t support sound waves.

    Oh, and maybe an analogy will help here.

    During a hurricane, drop a stone in the sea. Look for ripples. Hey, it works in a calm sea!

  12. 362
    Pat Cassen says:

    Mark – Alfven waves propagate very nicely in a magnetized medium of only a few particles/cc and less. Self-gravitating density waves propagate in Saturn’s rings (pretty good vacuum between solid particles), as well as in a medium made only of stars and NO gas! And magneto-acoustic waves propagate in the solar wind where the density is only a few protons/cc. Pure sound waves do need collisions, but the relevant measure is mean-free-path/wavelength of the disturbance, no?

    But I think we basically agree.

  13. 363
    Hank Roberts says:

    Ok, and all this relates to the topic, how?

  14. 364
    Pat Cassen says:

    “…relates to the topic, how?” — Beats me.

  15. 365
    Ray Ladbury says:

    Mark, while seriously off topic, the point was:
    1)The interstellar medium can support waves, including magnetoacoustic waves, alfven waves
    2)You are the one who brought up the speed of GCR (I was just pointing out that their flux is not negligible). I pointed out that the mean energy is less than you might think.

    If you look at the interstellar medium, its density is nonzero. Its particles interact–the average proton scatters 3 times from the Sun to Earth–so there IS a pressure. I see no limitation in the acoustic equation on density or pressure. Do you? Yes space is an excellent vacuum. No, it is not empty.

  16. 366
    Hank Roberts says:

    Anyhow, it started with
    Brian H Says: 25 July 2008 at 1:25 PM
    which Mark immediately contradicted.

    Here’s Scholar using the terms.
    http://scholar.google.com/scholar?hl=en&lr=&safe=off&q=+%2B%22speed+of+sound%22+%22density+wave%22+GALACTIC+ARM%22&btnG=Search

    I still don’t see how it matters for the topic. Enough!

  17. 367
    Mark says:

    Hank, #363. b) It’s a little late to be asking NOW isn’t it?

    b) It goes back to 344. Which, I think, is yours.

    Pat, #362. Right, but the magnetic fields are string enough to make the body bind. In air and most “thick” gases where an ideal gas is appropriate, mfp is the big factor: the molecules keep smacking into each other, meaning they mix and transfer energy readily. And you can treat the medium not as a bunch of random particles but approximate it as a continuous fluid.

    Saturns rings are also well constrained: the gravity of Saturn and the energy of their orbital does that.

    And magnetic fields require quite strong magnetic fields and ionised particles to be moving along them. Constrained again.

    But a strong orbital is not a feature of the basically cold and inert gases that occupy the majority of the interstellar distances.

    Likewise, ionised hydrogen is not a big feature over most of the volume of space and neither are sufficiently strong magnetic waves (though I’m somewhat reaching at the end there). Most of this rarified medium is diffuse, so the “law of large numbers” doesn’t work. E.g at 1cm, the electrostatic energy of two protons is less than 1/10th (something like 1/50th) of the *average* kinetic energy of the medium even at 3K. It isn’t well constrained by any forces and the energies involved mean that random issues are more important and any attempt to get a wave started in there much more likely to disperse the cloud as opposed to set up a wave of sound.

  18. 368
    Hank Roberts says:

    > 344
    Nope, 339, Brian. Brian, you still here? What is the relevance of that to the topic?
    ——-
    Recaptcha: Strugglers of ….

  19. 369
    Mark says:

    Hank, #368

    You were the one started on about how Brian on 339 was right, contrary to my response. I was just correcting his misapprehension. You started the argument.

    You started it by using the name of a paper that names “supersonic jets” and used that as proof of there being sound in the interstellar medium.

    Brian was just incorrect. You were stating a stupidity and I was trying to educate you. As much as what you were doing to astrophysics is the same as really rabid denialists use “gut feeling” to change the names of things that are real into something they can use to believe they are right.

    Something I pointed out several times yet you remained silent on them.

    Another denialist tactic.

    Hence my continuing attempts to correct you.

  20. 370
    Mark says:

    Dang. Ray, #365.

    you did bring up the energy of the cosmic rays. I didn’t. Or did you think that the interstellar medium is of an average energy of 10MeV?

  21. 371
    Ray Ladbury says:

    Mark, From your #349: “Ray, the cosmic rays are going near lightspeed.” A 300 MeV gcr proton has a velocity of only about 0.6 C–not all that relativistic and roughly half are less energetic.

    As to relevance, it’s better than the diversion with Gusbob. ;-)

  22. 372
    Hank Roberts says:

    OK, we can track the term back to much earlier in the thread:
    Robert A. Rohde Says:
    10 March 2008 at 7:37 PM

    Re #2: The spiral pattern is created by a “supersonic” “shock wave” in the gas of the galaxy. The shock front moves at its own speed seperate from the velocities of stars, much the same way that ripples on a pond move faster than the water itself.

    I’m still curious why it matters whether or not the word “supersonic” is used — is there any difference in the physics? Are cosmic rays associated with “supersonic” “shock waves” more energetic, so passing through the spiral would show a variation in the energy of cosmic rays? (Is there another kind of shock wave?)

    Is this correlated with the the signal these guys are claiming to find that distinguishes passage through the spiral arm?

    Aside — Mark, I appreciate your persistence but could you focus on the physics _behind_ the terminology? Clearly people use various terms and you don’t like the one others introduced here. But regardless of what you call them, there they ARE — do the spiral arms make a distinct cosmic ray signal as claimed in the paper being discussed? what’s the difference in the imact on Earth?

  23. 373
    Pat Cassen says:

    Hank, here’s my take on it.

    Martin Vermeer’s #357 is relevant. The arms of spiral galaxies (like ours) are made visible by the bright O and B stars formed in them. These stars have lifetimes on the order of 10^6 years, much shorter than the roughly 10^8 year interval between passages of the longer-lived stars (like the Sun) through a spiral arm. So, from the perspective of the Sun, passage through a spiral arm is like going through a wave of O-B stars turning on and quickly (10^6 years) dying in supernova explosions.

    Now it is thought that cosmic rays originate in energetic environments like supernovae explosions and the shock waves caused by the powerful outflows from O and B stars (among other sources). If this is true, it is plausible that the Sun would experience an enhanced cosmic ray flux for the million years or so that it would take to go through the spiral arm, due simply to proximity to cosmic ray sources. Of couse this conclusion is depends on how fast the cosmic rays diffuse throughout the galaxy (pretty fast, I imagine), inhomogeneities in the arm itself, etc.

    Without getting into the semantics, the term “supersonic shock wave”, in this context, is not helpful.

    I should say that it seems to me unlikely that any of this is relevant to climate. The correlation with meteorite data that Shaviv claimed has been shown to be false (I can find the reference if you want), and that data has long been explained satisfactorily by meteoriticists as associated with exposure ages since asteroidal break-up. I don’t know anything about relevant paleo-climate data.

  24. 374
    Hank Roberts says:

    Gotcha, thanks Pat.
    > find the reference

    Gack. I tried to. There’s a lot out there, e.g. searching very recent papers in Scholar (I don’t have the papers, very curious to know if Gavin’s point about timing at top of this thread informs any of them)

    http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=1851772

    The galactic cycle of extinction
    M Gillman, H Erenler – International Journal of Astrobiology, 2008 – Cambridge Univ Press

    Scholar’s bit of excerpt:
    … Although this agrees with the periodicity of Fe–Ni meteorite exposure ages (143¡10, Shaviv (2003)) and the 140 Myr signal from the fossil record (Rohde & …

    and

    A long-term association between global temperature and biodiversity, origination and extinction in …
    PJ Mayhew, GB Jenkins, TG Benton – Proceedings of the Royal Society B: Biological Sciences, 2008 – The Royal Society
    … However, the age of meteorites is likely to be linked causally to cosmic ray flux 4 2 0 … B (2008) Page 5. (Rohde & Muller 2005), and the latter (Shaviv & Veizer …

    and

    Cyclicity in the fossil record mirrors rock outcrop area – all 2 versions »
    AB Smith, AJ McGowan – Biology Letters, 2005 …
    In a recent article, Rohde & Muller (Rohde & Muller 2005 Nature 434, 208–210 …

    A Gauss-Vaníek Spectral Analysis of the Sepkoski Compendium: No New Life Cycles…
    M Omerbashich – COMPUTING IN SCIENCE & ENGINEERING, 2006 – doi.ieeecomputersociety.org
    … (Figure reprinted from RA Rohde and RA Muller, “Cycles in Fossil Diversity,” Nature, vol. 434, 10 Mar. 2005, pp. 208 210.) Rohde and Muller based their …

  25. 375
    Hank Roberts says:

    Er, Knud and Rasmus’s, not Gavin’s, point at top of thread. Seems this is a lively area. I’ll sit back and read and, yes, try to learn something.

  26. 376
    Mark says:

    Pat, not what I read as precisely, but doable. A bit like “mean free path”. It’s a good handle on it, but the precisionist (or pedant, depending on taste) is itching to teach you a better way. :-)

    Hank, there’s a reason why I didn’t make a complaint about inaccuracy on #2: I didn’t feel it inaccurate.

    I did comment (like you did with the “carbon doesn’t grow on trees”) about the speed of sound of the interstellar medium. And that was because it was wrong. Very wrong.

    Then in #344 YOU came out and said that there was too sound in space. Now if you wanted to keep this on track for climate work, you shouldn’t have made that call in #344. If you wanted it now, you shouldn’t have made the call in #372. Why? Because that didn’t get us back on track and added yet another message to the tangential discourse: yours.

    If I didn’t think this place was somewhere where incorrect statements and lack of knowledge in a particular area was appropriate (or if I thought you unable to learn), I would have just said “you’re wrong”.

  27. 377
    Pat Cassen says:

    Hank, this is the reference I was thinking of (you might recognize some names):

    Cosmic Rays, Carbon Dioxide, and Climate
    EOS, TRANSACTIONS AMERICAN GEOPHYSICAL UNION, VOL. 85, NO. 4, doi:10.1029/2004EO040002, 2004
    by Rahmstorf, Archer, Ebel, Eugster, Jouzel, Maraun, Neu, Schmidt, Severinghaus, Weaver and Zachos

    Unfortunately the abstract is not informative (it’s just the first paragraph of the paper), and, sadly, you need a subscription to EOS to see the rest. Send me an email at pcassen@mail.arc.nasa.gov if you want a pdf of it.

    [Response: It’s available here. – gavin]

  28. 378
    Hank Roberts says:

    So is it density, or temperature? Sources disagree.
    http://www.google.com/search?q=%22speed+of+sound+of+the+interstellar+medium%22
    Clearly somebody is wrong on the Internet.

    It appears not to matter at all for this topic.
    If it does I’m sure someone will correct me.