Vasili Kharchenko

CHANDRA OBSERVATIONS OF THE ''DARK'' MOON AND GEOCORONAL SOLAR WIND CHARGE TRANSFER

We have analyzed data from two sets of calibration observations of the Moon made by the Chandra X-Ray Observatory. In addition to obtaining a spectrum of the bright side that shows several distinct fluorescence lines, we also clearly detect time-variable soft X-ray emission, primarily O vii Kand O viii Ly� , when viewing the optically dark side. The apparent dark-side brightness varied in time by at least an order of magnitude, up to � 2 � 10 � 6 photons s � 1 arcmin � 2 cm � 2 between 500 and 900 eV, which is comparable to the typical 3 keV-band background emission measured in the ROSAT All-Sky Survey. The spectrum is also very similar to background spectra recorded by Chandra in low- or moderate-brightness regions of the sky. Over a decade ago, ROSAT also detected soft X-rays from the dark side of the Moon, which were tentatively ascribed to continuum emission from energetic solar wind electrons impacting the lunar surface. The Chandra observations, however, with their better spectral resolution, combined with contemporaneous measurements of solar wind parameters, strongly favor charge transfer between highly charged solar wind ions and neutral hydrogen in the Earths geocorona as the mechanism for this emission. We present a theoretical model of geocoronal emission and show that predicted spectra and intensities match the Chandra observations very well. We also model the closely related process of heliospheric charge transfer and estimate that the total charge transfer flux observed from Earth amounts to a significant fraction of the soft X-ray background, particularly in the ROSAT 3 keV band.

Energy transfer in collisions of oxygen atoms in the terrestrial atmosphere

The total and differential cross sections of elastic O(3P) + O(3P) collisions have been obtained from quantal calculations, and the rate of energy relaxation of fast oxygen atoms in atmospheric oxygen gas has been evaluated using realistic differential cross sections of oxygen-oxygen collisions. All the electronic energy curves of O2 separating to the atomic ground states have been incorporated in calculations of the scattering phase shifts. The total and differential cross sections averaged over a statistical population of O(3P) fine structure levels have been used for calculations of the self-diffusion coefficient, the viscosity and thermal conductivity of oxygen gas. The kernel of the Boltzmann equation for the energy relaxation process has been determined. The rate of energy losses of energetic oxygen atoms in the upper terrestrial atmosphere are calculated for typical thermospheric conditions. The energy distribution function of the recoil oxygen atoms is constructed using the computed energy relaxation kernel.

Complex scattering lengths in multi-channel atom–molecule collisions

Abstract It is shown that in the presence of inelastic scattering, zero energy elastic and inelastic scattering can be characterized by a complex scattering length, the imaginary part of which is related directly to the total inelastic scattering cross section. Collisions between H atoms and vibrationally excited H 2 molecules are investigated. Zero energy cross sections for all vibrational states of H 2 and the corresponding complex scattering lengths are reported using accurate quantum mechanical calculations. We obtain large values of the elastic cross sections which we attribute to s -wave bound and quasi-bound states of H⋯H 2 ( v ). The energies and lifetimes of the quasi-bound states are extracted from the complex scattering lengths.

Mesoscopic molecular ions in Bose-Einstein condensates.

We study the possible formation of large (mesoscopic) molecular ions in an ultracold degenerate bosonic gas doped with charged particles (ions). We show that the polarization potentials produced by the ionic impurities are capable of capturing hundreds of atoms into loosely bound states. We describe the spontaneous formation of these hollow molecular ions via phonon emission and suggest an optical technique for coherent stimulated transitions of condensate atoms into a specific bound state. These results open up new possibilities for manipulating tightly confined ensembles.

THE SOLAR WIND CHARGE-TRANSFER X-RAY EMISSION IN THE 1/4 keV ENERGY RANGE: INFERENCES ON LOCAL BUBBLE HOT GAS AT LOW Z

We present calculations of the heliospheric solar wind charge-exchange (SWCX) emission spectra and the resulting contributions of this diffuse background in the ROSAT 1/4 keV bands. We compare our results with the soft X-ray background (SXRB) emission detected in front of 378 identified shadowing regions during the ROSAT All-Sky Survey. This foreground component is principally attributed to the hot gas of the so-called Local Bubble (LB), an irregularly shaped cavity of ~50-150 pc around the Sun, which is supposed to contain ~106 K plasma. Our results suggest that the SWCX emission from the heliosphere is bright enough to account for most of the foreground emission toward the majority of low galactic latitude directions, where the LB is the least extended. On the other hand, in a large part of directions with galactic latitude above 30°, the heliospheric SWCX intensity is significantly smaller than the measured one. However, the SWCX R2/R1 band ratio differs slightly from the data in the galactic center direction, and more significantly in the galactic anticentre direction where the observed ratio is the smallest. Assuming that both SWCX and hot gas emission are present and their relative contributions vary with direction, we tested a series of thermal plasma spectra for temperatures ranging from 10 5 to 10 6.5 K and searched for a combination of SWCX spectra and thermal emission matching the observed intensities and band ratios, while simultaneously being compatible with O VI emission measurements. In the frame of collisional equilibrium models and for solar abundances, the range we derive for hot gas temperature and emission measure cannot reproduce the Wisconsin C/B band ratio. This implies that accounting for SWCX contamination does not remove these known disagreements between data and classical hot gas models. We emphasize the need for additional atomic data, describing consistently EUV and X-ray photon spectra of the charge-exchange emission of heavier solar wind ions.

Charge Abundances of the Solar Wind Ions Inferred from Cometary X-Ray Spectra

An emission spectrum of X-rays from comet McNaught-Hartley (C/1999 T1) detected with the Chandra X-ray telescope is analyzed on the assumption that it results from electron captures by heavy ions of the solar wind colliding with the atoms and molecules of the cometary atmosphere. An accurate value of the relative abundance of O+8 and O+7 ions is obtained by comparing the spectrum derived from the measurement data with the predicted spectrum. A close match is found for an abundance ratio of O+8 to O+7 of 0.34 ± 0.07, the uncertainty reflecting the measurement statistics. A similar analysis of the contribution from captures by Ne+9 ions leads to an abundance ratio of Ne+9 to O+7 of 0.02 with an uncertainty of 40%. The spectrum consists of many unresolved lines. The brightest feature is composed mostly of the 2 3S1 → 1 1S0 spin-forbidden line of O+6 at 561 eV with 74% of the intensity and the 2 3P1 → 1 1S0 intercombination line at 568 eV with 12%. The strongest of the O+7 lines is the Lyα transition at 654 eV with an intensity of 29%, relative to the O+6 lines. The sensitivity of cometary X-ray spectra to variations in the abundances of highly charged oxygen ions in the solar wind is investigated.

Variability of Cometary X-Ray Emission Induced by Solar Wind Ions

X-ray emission spectra induced by the interaction of the slow and fast solar wind components with cometary gas are calculated for typical solar wind compositions. The emission spectra arising from the charge transfer mechanism are shown to be in good agreement with observational data. The identities and intensities of the brightest spectral lines that include forbidden transitions are obtained for the slow and fast solar winds. Differences in emission spectra of individual comets occur because of variations in the solar wind composition. Comparisons with observational data for comets Levy and Hale-Bopp indicate that they were subjected to the slow solar wind. The spectra at photon energies above 500 eV fluctuate with the solar wind composition because of the varying presence of fully stripped oxygen ions. Mechanisms of X-ray photon emission at energies above 0.9 keV are discussed.

Slowing of energetic O(3 P) atoms in collisions with N2

Energetic atoms influence the composition, the thermal structure and the evolution of the upper atmosphere. We investigate the slowing of energetic O(3P) atoms by elastic and inelastic collisions with N2. Results of quantum mechanical and semiclassical calculations of energy and angle-resolved cross sections are presented for elastic and inelastic collision of O(3P) with N2. A general analytical expression is developed for the kernel of the Boltzmann equation for the energy distribution function which is valid for elastic and inelastic collisions and incorporates both the angle and the energy dependence of the cross sections. The Boltzmann kernel for the energy relaxation of fast O(3P) atoms is evaluated using the computed cross sections. We report values of the collision frequency, the average energy loss in elastic and inelastic collisions with N2, and the mean time and the number of collisions required to thermalize initially energetic O(3P) atoms. We compare the efficiencies of elastic and inelastic collisions in slowing down fast O(3P) atoms. These data are basic to the development of a reliable model of the atmospheric effects of the hot oxygen atoms produced by dissociative recombination, collisional quenching, photodissociation and photoelectron impact dissociation.

Kinetics of thermalization of fast nitrogen atoms beyond the hard sphere approximation

Abstract The thermalization of fast nitrogen atoms through elastic collisions with thermal oxygen atoms is studied by obtaining the energy distribution function of fast nitrogen atoms from a direct numerical solution of the Boltzmann kinetic equation. The kernel of the Boltzmann equation is constructed using quantum mechanically evaluated differential scattering cross sections. The contributions to the cross sections that arise from the six lowest NO molecular potential curves exhibit a marked dependence on both the energy and the scattering angle. It is shown that the classical scattering cross sections are an excellent approximation to the smooth curve obtained by averaging the quantum mechanical cross sections. The differences between the results of calculations of the collision kernel for the hard sphere cross sections and the actual cross sections are discussed and the steady state distribution function of the fast N atoms is compared with the results of kinetic calculations with the hard sphere approximation.

Spectra of the X-ray emission induced in the interaction between the solar wind and the heliospheric gas

Spectra of the heliospheric EUV and X-ray emission induced in the charge transfer collisions of the highly charged solar wind ions with the interstellar gas have been calculated. Cascading photon spectra of individual Oq+, Cq+, Nq+, and Neq+ ions have been constructed using recent data on ion radiative transition probabilities and the state-selective population cross sections for charge transfer collisions of the most abundant heavy solar wind ions with H and He atoms. Emission spectra have been calculated for slow and fast solar winds interacting with the heliospheric H and He gas. Relative intensities of the brightest lines have been predicted. The volume power distribution of the charge transfer EUV and X-ray emission has been computed for simplified models of the solar winds and the interstellar gas. X-ray images of the heliosphere have been composed for the region inside 10 AU from the Sun.