ALBERT

Description: As the neutral components of the interstellar gas flow through the heliosphere their spatial distribution is modified by charge exchange with the solar plasma, photoionization, and radiation pressure. The deep space probes Pioneers 10/11 and Voyagers 1/2 have provided an opportunity to investigate this distribution through VUV observations of the heliospheric glow. Since the interactions of the inflowing neutrals with the heliosphere depend on both space and time it is particularly useful to have multiple spacecraft observations. Pioneer 10 is in the downstream region of the inflowing interstellar breeze at 50 AU while Pioneer 11 and Voyager 2 are upstream at about 30 AU. Voyager 1 is also upstream at about 40 AU but at a heliographic latitude of + 30 deg. Both temporal and spatial effects are expected to be quite different for the upstream and downstream regions. Some of the recent VUV data and its implications with respect to the heliospheric structure will be presented.

Description: Pioneer 10 UV photometric data are used to study the nature of the heliospheric boundary interactions between solar wind and inflowing partially ionized interstellar wind and the interstellar magnetic field. The radial intensity dependence of solar Ly-alpha photons backscattered by inflowing interstellar hydrogen atoms is observed. The data in the downstream direction at distances greater than about 37 AU show a distortion of the incoming hydrogen atom density profile. It is suggested that this is characteristic of a nearby solar wind shock probably located about 50 AU from the sun in January 1987.

Description: The effect of a postulated nearby solar wind shock on the radial density profile of the cold inflowing neutral hydrogen atoms is studied. It is found that a nearby solar wind shock strongly affects the neutral hydrogen breeze. Model calculation reveals that the strongly perturbed hydrogen atom distribution beyond a solar wind shock can be remotely detected by observation of the radial dependence of the backscattered UV glow from the deep space probes Pioneer 10/11 and Voyager 1/2.

Description: The location of the termination shock is a problem of great interest in astrophysics and the current availability of deep space probes with UV detectors has made it possible in principle to remotely sense the termination shock. The method depends upon a measurement of the neutral hydrogen distribution which results from the presence of the heliospheric plasma structure. The charge exchange interaction between the inflowing neutral hydrogen and the plasma structure leads to an observable modification of the inflowing hydrogen distribution. Hence, both the radial dependence of the backscattered Lyman-alpha glow and the absolute difference in the upstream and downstream glow brightness are modified. Recent Voyager 1 and 2 and Pioneer 10 spacecraft data are consistent with the presence of a termination shock. The observational evidence and theoretical models from which a termination shock is inferred will be presented.

Description: Two primary loss mechanisms of interstellar neutral hydrogen in interplanetary space are resonance charge exchange ionization with solar wind protons and photoionization by solar EUV radiation. The later process has often been neglected since the average photoionization rate has been estimated to be as much as 5 to 10 times smaller than the charge exchange rate. These factors are based on ionization rates from early measurements of solar EUV and solar wind fluxes. Using revised solar EUV and solar wind fluxes measured near the ecliptic plane we have reinvestigated the ionization rates of interplanetary hydrogen. The result of our analysis indicates that indeed the photoionization rate during solar minimum can be smaller than charge exchange by a factor of 5; however, during solar maximum conditions when solar EUV fluxes are high, and solar wind fluxes are low, photoionization can be over 60% of the charge exchange rate at Earth orbit. To obtain an accurate estimate of the importance of photoionization relative to charge exchange, we have included photoionization from both the ground and metastable states of hydrogen. We find, however, that the photoionization from the metastable state does not contribute significantly to the overall photoionization rate.

Description: Heliospheric neutral hydrogen scatters solar Lyman-alpha radiation from the Sun with '27-day' intensity modulations observed near Earth due to the Sun's rotation combined with Earth's orbital motion. These modulations are increasingly damped in amplitude at larger distances from the Sun due to multiple scattering in the heliosphere, providing a diagnostic of the interplanetary neutral hydrogen density independent of instrument calibration. This paper presents Cassini data from 2003-2004 obtained downwind near Saturn at approximately 10 AU that at times show undamped '27-day' waves in good agreement with the single-scattering models of Pryor et al., 1992. Simultaneous Voyager 1 data from 2003- 2004 obtained upwind at a distance of 88.8-92.6 AU from the Sun show waves damped by a factor of -0.21. The observed degree of damping is interpreted in terms of Monte Carlo multiple-scattering calculations (e.g., Keller et al., 1981) applied to two heliospheric hydrogen two-shock density distributions (discussed in Gangopadhyay et al., 2006) calculated in the frame of the Baranov-Malama model of the solar wind interaction with the two-component (neutral hydrogen and plasma) interstellar wind (Baranov and Malama 1993, Izmodenov et al., 2001, Baranov and Izmodenov, 2006). We conclude that multiple scattering is definitely occurring in the outer heliosphere. Both models compare favorably to the data, using heliospheric neutral H densities at the termination shock of 0.085 cm(exp -3) and 0.095 cm(exp -3). This work generally agrees with earlier discussions of Voyager data in Quemerais et al., 1996 showing the importance of multiple scattering but is based on Voyager data obtained at larger distances from the Sun (with larger damping) simultaneously with Cassini data obtained closer to the Sun.

Description: The Pioneer 10 ultraviolet measurements obtained during the Jupiter encounter in 1973 have been further examined by using improved data handling and analysis techniques. The Pioneer 10 observations of Jupiter and its satellites during the encounter have been carefully reviewed in order to improve our understanding of the morphology of the Io plasma torus and Jupiter's upper atmosphere and to investigate the possible existence of other emission source such as Europa. In addition, the morphology of Io's bimodal torus observed during the Pioneer 10 encounter has been compared with the Voyager observations obtained approximately 6 years after the Pioneer 10 flyby and significant differences in the torus characteristics are found. The Io torus in 1973 was more similar to the 1992 Ulysses observations of a longitudinally asymmetric ring than to the complete ring observed by Voyager. Pioneer 10 observed a significantly dimmer Io torus and Jupiter upper atmosphere in the EUV compared to the Voyager observations. Apart from the torus and Jupiter, Pioneer 10 observed additional emissions which have been attributed to Io itself. Three distinct possibilities have been discussed to explain these additional emissions. The most likely is that Pioneer 10 observed volcanism on Io. There is also evidence of Pioneer 10 observing emissions from Europa. The present analysis clearly shows that the Jovian system in 1973 was significantly different from that observed in 1979.

Description: The Voyager and Pioneer 10 spacecraft are moving upstream and downstream into the local interstellar flow, monitoring H Lyman alpha radiation resonantly scattered from heliospheric hydrogen. Voyager Cruise Maneuver observations obtained between 15 and 35 AU reveal that H Lyman alpha intensities in the upstream direction fall as r exp -0.75+/-0.05. Beyond 15 AU downstream, Pioneer 10 intensities fall as r exp -1.07+/-0.1. These trends cannot be simultaneously reproduced using a hot H distribution model that does not include termination shock structure. The Voyager H Lyman alpha intensities also show a distinctive trend to decrease less rapidly with increasing heliocentric distance. Between 15 and 20 AU, Voyager intensities fall as r exp -1, whereas between 30 and 35 AU they fall as r exp -0.35. This flattening trend implies that the upstream H density is increasing rapidly with heliocentric distance beyond about 25 AU. This steepening trend is significant because similar H density gradients are predicted in models which include the effects of the termination shock. Taken together, the Voyager and Pioneer 10 H Lyman alpha observations beyond 15 AU imply the existence of a solar wind termination shock, suggesting that it lies between 70 and 105 AU in the upstream direction.

Description: Pioneer 10 Lyman-alpha data obtained beyond the heliocentric distance of 30 AU are used here to determine the local ISM hydrogen density and downwind Galactic glow. The resulting asymptotic interstellar hydrogen density obtained using a conventional interplanetary model is 0.06/cu cm, and the Galactic Lyman-alpha glow is found to be negligible in the downwind direction. Discrepancies are found between observations and the conventional theoretical predictions of the glow dependence on radial distance. Based solely on data obtained between 30 and 39 AU, a better fit is obtained with a constant density distribution beyond 30 AU. In this case, a density of 0.05/cu cm and Galactic glow of 1.5 R is obtained. These results suggest that a complex density distribution may be more appropriate than either the conventional exponential distribution or the constant distribution.

Description: Our knowledge of the various heliospheric phenomena (location of the solar wind termination shock, heliopause configuration and very local interstellar medium parameters) is limited by uncertainties in the available heliospheric plasma models and by calibration uncertainties in the observing instruments. There is, thus, a strong motivation to develop model insensitive and calibration independent methods to reduce the uncertainties in the relevant heliospheric parameters. We have developed such a method to constrain the downstream neutral hydrogen density inside the heliospheric tail. In our approach we have taken advantage of the relative insensitivity of the downstream neutral hydrogen density profile to the specific plasma model adopted. We have also used the fact that the presence of an asymmetric neutral hydrogen cavity surrounding the sun, characteristic of all neutral densities models, results in a higher multiple scattering contribution to the observed glow in the downstream region than in the upstream region. This allows us to approximate the actual density profile with one which is spatially uniform for the purpose of calculating the downstream backscattered glow. Using different spatially constant density profiles, radiative transfer calculations are performed, and the radial dependence of the predicted glow is compared with the observed I/R dependence of Pioneer 10 UV data. Such a comparison bounds the large distance heliospheric neutral hydrogen density in the downstream direction to a value between 0.05 and 0.1/cc.