X Rays From the Solar System and Beyond II
Presiding: T E Cravens, University of Kansas; R Hoekstra, University of Groningen
P42A-01 INVITED 10:30h
Cometary, Martian and Venusian X--rays
In 1996 an unexpected discovery was made: Comets do emit X--rays. This emission is now understood as the result of charge exchange interactions between heavy, highly charged ions in the solar wind and cometary neutrals. A similar process might occur in the exospheres of Venus and Mars, which can be considered as the closest planetary analogs to comets, due to the presence of an atmosphere, the absence of a strong magnetic field, and their proximity to the Sun. Indeed, X--ray radiation was discovered from both planets in 2001 with Chandra. However, the main source of this radiation was fluorescent scattering of solar X--rays in their upper atmospheres, in agreement with the expectation. While no evidence of a charge--exchange induced X--ray halo was found at Venus, the presence of such a halo was indicated at Mars, although at a very low statistical significance. Mars was observed again in November 2003 with XMM--Newton. This observation, characterized by a considerably higher sensitivity, confirms the presence of the X--ray halo and makes further investigations of its spectral, spatial, and temporal properties possible. The results of this observation will be presented together with those of recent X--ray observations of comets, which allow us now to study the interaction between the solar wind and the cometary coma in unprecedented detail.
P42A-02 10:50h
EUVE Spectroscopy of Mars and Venus: Capture of the Solar-Wind Alpha-Particles and Comparison with the CXO Observations
Long-exposure spectroscopy of Mars and Venus with the Extreme Ultraviolet Explorer (EUVE) has revealed emissions of He 584 Ã… on both planets and He 537 Ã…/O+ 539 Ã… and He+ 304 Ã… on Venus. Using the recent data on the solar emission at 584 Ã… and eddy diffusion in the upper atmospheres of both planets, the derived helium mixing ratios are 10 ± 6 and 9 ± 6 ppm in the lower atmospheres of Mars and Venus. Collisions with hot oxygen atoms, some ion reactions, sputtering by O+ pickup ions, and photo- and electron impact ionization followed by sweeping out of He+ by the solar wind result in a total loss of He from Mars at 8× 1023 s-1. There is no current volcanism on Mars, and seepage of volcanic gases is also very low. Therefore the current outgassing of He formed by radiactive decay of U and Th in the interior is negligible, and the loss is compensated by the capture of solar wind α-particles with an efficiency of 0.3. A similar analysis of the helium loss processes on Venus results in a total loss of 4.4× 1024 s-1. The corresponding lifetime of He on Venus is very long, 0.5 Byr, and helium released from the interior in the last resurfacing episode 0.6 Byr ago covers 0.3 of the loss. The most of the loss is compensated by the capture of solar wind α-particles with an efficiency of 0.1. We compare our derived α-particle capture efficiencies for Mars and Venus with observed x-ray emissions resulting from the charge exchange of solar wind heavy ions with the extended atmospheres on both planets (Dennerl et al. 2002, Dennerl 2002). (The most of the observed emissions are from scattering and fluorescence of the solar x-rays and not relevant to this discussion.) The emissions from both disk and halo on Mars agree with our calculated values; however, we do not see a reasonable explanation for the x-ray halo emission on Venus. An efficiency of the x-ray excitation in the martian halo is essentially similar to those in comets McNaught-Hartley and LINEAR S4. The ratio of the charge exchange efficiencies derived from the disk x-ray emissions of Mars and Venus is similar to the ratio of the capture efficiencies for these planets. Capture of the solar wind α-particles as well as charge exchange of the heavy ions impacting the atmospheres of Mars and Venus are kinetic effects that are proportional to ratios of gyroradii to the planet radii. This ratio is greater on Mars than on Venus by a factor of 3.8, in accord with the derived capture efficiencies and the measured x-ray emissions. The surprisingly bright emission of He+ at 304 Ã… observed by EUVE and Veneras 11 and 12 suggests that charge exchange in the flow of the solar wind α-particles around the ionopause is much stronger than in the flow of α-particles into the ionosphere.
P42A-03 INVITED 11:05h
Laboratory Charge Exchange Cross Sections and X-Ray Astrophysics
The exciting and unexpected observation that hot X-rays could be produced from cold comets has spurred considerable work in the area of collisions of highly-charged ions (to simulate the solar wind) with gases such as H2, CO, H2O, CO2, and CH4 (to simulate the comet-evolved gases). The spectral energies and intensities suggest that the X-rays arise from solar wind highly-charged ions (HCIs) that have charge-exchanged with the neutral species, leaving excited HCIs that stabilize via X-ray emission [1,2]. We will review recent work in the area of X-ray emissions and charge exchange, and present recent results of absolute charge-exchange cross sections and X-ray spectra. The work carried out at JPL/Caltech was supported through agreement with NASA. [1] V. Kharchenko et al., Ap. J. 585, L73 (2003). [2] J. B. Greenwood et al., Phys. Scripta T110, 358 (2004).
P42A-04 11:25h
Laboratory Simulation of the X-Ray Emission from Comet C/1999 T1 (McNaught-Hartley)
We have performed a series of laboratory experiments to simulate the X-ray spectrum of comet C/1999 T1 (McNaught-Hartley) observed with the Chandra X-ray Observatory. Our experiments utilize the University of California Lawrence Livermore National Laboratory EBIT-I electron beam ion trap, in which the relevant ions are generated and made to collide with the appropriate neutral gases in order to produce X-ray emission by charge exchange. The X-ray emission is recorded with a high-resolution microcalorimeter detector built at the Goddard Space Flight Center. In addition to the K-shell X-ray emission of highly charged ions of C, N, and O, we have also recorded the emission of heliumlike Ne. Unlike in an earlier fit of comet C/1999 S4 (LINEAR), neon emission was found to be necessary to simulate the McNaught-Hartley data. Excellent agreement between the laboratory simulation and the McNaught-Hartley spectrum is obtained. The ion fractions inferred from the simulation agree very well with direct solar wind measurements. This work was performed under the auspices of the Department of Energy by UC-LLNL under contract W-7405-ENG-48 and supported by NASA's Planetary Atmospheres Program under Work Order W-19,938.
P42A-05 11:40h
New Chandra Observations of Comets, 2003 - 2005.
The highly favorable perigee passage of the well studied comet 2P/Encke in late 2003 and the very active comet 2001 Q4 (NEAT) in early 2004 provided an excellent opportunity to use Chandra's high spatial, spectral, and temporal resolution to study cometary x-ray emission in the low neutral target density, low x-ray flux regime and the opposite, collisionally thick, high flux regime. X-ray emission from comet Encke was found only in a small, asymmetric region between 1500 km - 40,000 km from the nucleus. The Encke ACIS-S3 200 -- 1000 eV spectrum shows many of the same x-ray emission lines previously observed from comets (C+5, O+6,O+7), including confirmation of several emission lines in the 800 to 1000 eV range. However, the Encke spectrum shows very different line ratios in the 200 - 700 eV range than any previous comet. A lightcurve with peak-to-peak amplitude of 20% and period 11.7 hours was found over the 15 hour observing period, similar to that reported by Belton et al. (2004) and Fernandez et al. (2004) for the comet. Comparing the observations to contemporaneous measurements of the coma and solar wind made by other means, we find the combination of a low density, collisionally thin (to charge exchange) coma and a post-massive X-flare, high temperature, moderate density solar wind can explain our unusual Encke x-ray observations. X-ray emission from 2001 Q4 was highly extended and very bright. The image morphology was similar to the hemispherical shape seen for other bright comets, as was the spectrum. New to the study of comets was the contemporaneous observations of the comet by the CHIPS spacecraft in the 47 - 134 eV energy range, sensitive to emissions from Fe, Mg, Si, and S. We present our preliminary results on the analysis of the Q4 observations. The Deep Impact spacecraft. launched in January 2005, is set to rendezvous with comet 9P/Tempel 1 on July 4, 2005. An 380 kg impactor will hit the comet at 10.2 km sec-1 at a set time with known kinetic energy, creating a hypervelocity explosion with impact flash, ejected surface material, and non-equilibrium plasma from the vaporized impactor. We have obtained 300 ksec of Chandra time to observe the comet before, during, and after thee impact. Here we describe our planned observations and expectations for the encounter.