Horst Scherer

HST/GHRS Observations of the Velocity Structure of Interplanetary Hydrogen

We present high-resolution spectra of the emission-line profile of inflowing interplanetary hydrogen atoms along lines of sight with the Earth orbital motion upwind (into the flow), downwind, and across the flow to Doppler-shift the line from the geocoronal emission. The line-center positions, in comparison with hot-model profiles, confirm that the inflow speed of H atoms far from the Sun (~50 AU) is in the range 18-21 km s-1, which implies a decrease in the velocity distribution of 5-8 km s-1 for hydrogen within the solar system, relative to the He flow and to the local interstellar medium. Best-fit values are derived for the speed and effective solar gravity along the three lines of sight by comparison with model profiles convolved with the instrument line-spread function. For the assumed inflow direction, the cross-flow line profile requires that the μ-value be slightly less than unity near solar minimum, and a technique is presented for determining the exact inflow direction and μ-value independently of the other parameters. The line widths indicate a broadening along the flow direction in addition to the dynamical effects near the Sun expected from two different hot models, whereas the cross-flow line width is similar to the hot-model profiles. The altered velocity distribution in the inflow direction appears likely to be related to the crossing of the interstellar/interplanetary medium interface structure, although questions remain about the cumulative effects of changing solar activity on the timescale of the H atom flow through the solar system.

Recalculation of H-Lα sky surveys: No need for anisotropic solar wind mass outflows?

The variation with position and view direction of heliospheric H-Lα scattered intensity seen by a Lα detector in the heliosphere is re-examined. Here, a frequency- and angle-dependent multiscattering calculation (Scherer, 1994; Scherer and Fahr, 1995) is used that takes into account the local thermodynamical conditions of the scattering agent, such as temperature, density and bulk velocity of the neutral interplanetary hydrogen, as given in a recent model by Osterbart and Fahr (1992). The calculated intensity patterns show strongly pronounced dependencies of the direction of the line of sight which are explained by effects of the Doppler shift in the resonance absorption and the dipole-phase function used in the multiscattering calculation. The theoretical results obtained with these computations nicely fit the intensities measured byPrognoz 5/6 probe (Lallement, Bertaux, and Dalaudier, 1985; Bertauxet al., 1985) without, however, assuming a latitude dependence of the solar wind mass outflow. This expresses the fact that, using an adequate radiation transport calculation, it is possible to explain thePrognoz andGalileo data without the need to specify anisotropic solar wind mass outflow from the corona preferred by several authors (Lallement, Bertaux, and Dalaudier, 1985; Lallement, 1989; Broadfoot and Kumar, 1978; Bertauxet al., 1985; Ajelloet al., 1994). In view of forthcomingUlysses solar wind measurements at polar latitudes this might be of importance to know.

Diagnostics of the solar corona using EUV radiation backscattered by pick-up ions close to the Sun

Abstract Recent investigations of pick-up ion data obtained with the AMPTE and Ulysses spacecraft led to the discovery that, in contrast to previous expectations, pick-up ion velocity distributions can be anisotropic. Such anisotropy implies a pitch-angle scattering being less efficient than is usually assumed and translates into scattering mean free paths longer than those previously attributed to these particles. Dedicated observations could give valuable insights into the details of the scattering process. Here we demonstrate that existing and forthcoming in-situ observations can be supplemented with Earth-bound observations of the extreme-ultraviolet resonance glow of pick-up ions in the vicinity of the Sun. The proposed observations, combined with a theory of ions being picked-up by the heliospheric magnetic field close to the Sun, will not only allow us to obtain information about the anisotropy of the pick-up ion velocity distributions, but also about the coronal magnetic field topology as well as the accelerating solar wind. Therefore, such observations will contributed to an improvement of our understanding of the physics of the solar corona in general.