A query by Ian Tresman, transferred from the "Deep Impact Caused a Great Gush of Water Vapour" thread. I put it here lest I violate the rules of the other part of the forum, by continuing an ATM discussion there.
The "it" in Ian's query is my reference as follows:
Originally Posted by iantresman
X-Ray and extreme ultraviolet emissions from comets
Krasnopolsky, Greenwood & Stancil
Space Science Reviews 113(3): 271-374, August 2004
There is significant progress in the observations, theory, and understanding of the x-ray and EUV emissions from comets since their discovery in 1996. That discovery was so puzzling because comets appear to be more efficient emitters of x-rays than the Moon by a factor of 80 000. The detected emissions are general properties of comets and have been currently detected and analyzed in thirteen comets from five orbiting observatories. The observational studies before 2000 were based on x-ray cameras and low resolution (E/deltaE ≈ 1.5-3) instruments and focused on the morphology of xrays, their correlations with gas and dust productions in comets and with the solar x-rays and the solar wind. Even those observations made it possible to choose uniquely charge exchange between the solar wind heavy ions and cometary neutrals as the main excitation process. The recently published spectra are of much better quality and result in the identification of the emissions of the multiply charged ions of O, C, Ne, Mg, and Si which are brought to comets by the solar wind. The observed spectra have been used to study the solar wind composition and its variations. Theoretical analyses of x-ray and EUV photon excitation in comets by charge exchange, scattering of the solar photons by attogram dust particles, energetic electron impact and bremsstrahlung, collisions between cometary and interplanetary dust, and solar x-ray scattering and fluorescence in comets have been made. These analyses confirm charge exchange as the main excitation mechanism, which is responsible for more than 90% of the observed emission, while each of the other processes is limited to a few percent or less. The theory of charge exchange and different methods of calculation for charge exchange are considered. Laboratory studies of charge exchange relevant to the conditions in comets are reviewed. Total and state-selective cross sections of charge exchange measured in the laboratory are tabulated. Simulations of synthetic spectra of charge exchange in comets are discussed. X-ray and EUV emissions from comets are related to different disciplines and fields such as cometary physics, fundamental physics, x-rays spectroscopy, and space physics.
The constant discussion of double layers is one of the things which make the EU model look so bad. That's because the only thing that the EU proponents actually know about double layers is that they happen, but nothing more. So, they blithely assume that double layers must be everywhere. What they really need to do is show some good physical reason why there should be double layers anywhere near a comet.
Here is a reference that does the study that the EU "scientists" have never done:
Dust-acoustic double layers - Ion inertial effects
Richard Mace & Manfred Hellberg
Planetary and Space Science 41(3): 235-244, March 1993
Space and astrophysical plasmas often comprise a number of massive ion components in addition to a tenuous, negatively charged dust component and an electron component. Stationary electrostatic double layers in a dusty plasma are investigated in a model treating the ion components as Boltzmann-distributed (inertialess) fluids. On comparison with the inertialess theory, one finds considerably reduced double layer existence parameter regimes. Significantly, highly nonlinear double layers are ruled out when ion inertia is incorporated. However, in the restricted parameter regimes in which the inertial theory predicts double layers for small ion/dust mass ratios (about 10-15
) there is good qualitative agreement with inertialess theory. The reasons for these, and other discrepancies and similarities, are discussed.
In this paper the authors use more complete physics in analyzing the existence of electrostatic double layers in dusty plasmas, as applied to comets. They find that the previous, simpler models, overestimate the probability that a double layer can form. It turns out that the double layers that can form are also limited to relatively weak potential differences. So it does not look like one should expect double layers to be significant in a cometary environment.
The paper by Krasnopolsky, Greenwood & Stancil does not mention double layers, but the reason is that they are outside the scope of the paper, which is a study of charge exchange reactions. Double layers were assumed to exist, and accelerate electrons to charge the dust in Hill & Mendis, 1980. But the study of dusty plasmas by Mendis & Rosenberg, 1994, does not mention double layers at all (and they continue to use inertialess models in studying wave instabilities).
I don't see any papers which challenge the conclusion of Mace & Hellberg. Double layers in dusty, cometary plasmas are inhibited by the inertia of the heavy ions in the solar wind. Those that do form are relatively weak, and unlikely to accelerate electrons to produce X-ray emission.
But it also needs to be pointed out that the X-ray emission we see is readily explained by physics that does not include double layers. We already know about charge exchange from the the cited study by Krasnopolsky, Greenwood & Stancil. We know that X-rays are generated by a cometary bow shock (Wegmann & Dennerl, 2005). And we know that X-rays can be generated by charging the surface of the comet (Djuric, et al., 2005). So why do we need double layers to explain anything? How do you overcome the limitations imposed by Mace & Hellberg? And, finally, keep the point in mind that double layers could never explain line emission, only bremsstrahlung. Double layers at comets appear to be a losing proposition for the EU.