X-ray and extreme ultraviolet emissions from comets

Abstract

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/δE ≈ 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.

Line ratios for soft-x-ray emission following charge exchange betweenO8+and Kr

“…On the other hand, multi-electron processes are possible, such as double capture, transfer ionization, triple capture, etc. Since transfer ionization cross sections fall off exponentially with decreasing collision energies, the X-ray spectra contribution from transfer ionization can be neglected here [22]. We note that, with aid of the Molecular Classical over-Barrier Model (MCBM) [23], the estimated cross sections for double capture even triple capture are roughly comparable to that of single capture for the present system.…”

“…and interplanetary neutrals (H, He, etc.) [4,5]. Cravens [6] developed an analytic model that predicted 25% -50% of the soft X-ray background could come from SWCX.…”

Cometary emissions induced by scattering and fluorescence of solar X-rays

“…Nano-sized dust and ice particle densities and composition ratios are not as well-established because of a lack of precise observational data for such small masses (Utterback & Kissel 1990). Nanoparticles also efficiently scatter X-rays because their grain geometric sizes are on the same scale as the photon wavelengths (Krasnopolsky et al 2004;Draine 2003) and are therefore the focus of our scattering calculations. For our model, we infer that dust particles would maintain a similar ratio to those observed for atoms, with 85% H 2 O, 10% C, and 5% heavier elements like N and Si.…”

“…Instead, the empirically established proportionality for mass loss rates q dust 1.5q gas is used to estimate the total number of nanoparticles present within the atmosphere (McDonnell et al 1987;Krasnopolsky 1997;Krasnopolsky et al 2004). This relationship has previously been shown to be valid for micron-sized dust particles down to 0.1 μm (Fink & Rubin 2012) and is extrapolated down to nanoparticles for our analysis.…”

“…Collisions between highly charged, heavy SW ions, such as C 6+ , N 6+ , O 7+ , O 8+ , Mg 9+ , and Ne 9+ , and neutral cometary particles have been identified as the major source of cometary X-ray photons with energies below 1 keV, as defined by the electron energy levels of such ions (Kharchenko et al 2003). Scattering and fluorescence (SF) of solar X-rays also contribute to the observed X-ray intensities, but their contributions at photon energies below 1 keV have been estimated to be a minor fraction of the total flux of cometary X-rays (Krasnopolsky 1997;Krasnopolsky et al 2004).…”