Lyman α observations as a possible means for the detection of the heliospheric interface
Résumé
We first summarize our current knowledge of the flow of interstellar H and He gas through the solar system. Both come from the same direction, but the H velocity is 20 km/s, whereas the He velocity is 26 km/s as recently determined by Ulysses, identical to the velocity found for the local interstellar cloud (LIC) as recently determined from absorption lines detected in the spectrum of nearby stars. This velocity difference may be assigned to the deceleration of H atoms at the heliospheric interface through coupling with the partially ionized interstellar plasma. Seen from the inner solar system, the Lyman α emission pattern of H atoms (resonance scattering of solar photons) is quite compatible with a standard model including no interaction for H at the heliopause and therefore cannot be used to characterize such an interaction. We investigate three other types of Lyman α observations which could bear the signature of the heliopause. First, it is shown from Monte Carlo modeling of the interface perturbation that the Lyman α line profile (accessible through high resolution spectroscopy from Earth orbit) varies in a different way from upwind to downwind direction, whether there is a perturbation or not at the heliopause. Indeed, some Prognoz data show such a behaviour, and the potential of planned future observations is discussed (HST and SOHO). Second, since some interaction models predict a decrease of H density at heliopause crossing along the wind (or H increase when cruising upwind), the slope of the radial antisolar Lyman α intensity between 15 and 50 AU, characterized by γ, such as I ≈ k rγ, is computed for various models of H density. It is shown that (1) the value of γ recorded along downwind trajectories depends too much on solar parameters (radiation pressure and ionization) to be very useful to detect a departure from a standard model; (2) the value of γ along upwind trajectories is much less sensitive to solar parameters but radiative transfer of Lyman α in the interplanetary medium must absolutely be taken into account for a correct interpretation; (3) if taken at face value, the value γ = − 0.78 reported for Voyager data between 15 and 50 astronomical units cannot be explained with any standard model, and calls for an increase of density somewhere further upwind. A good fit is obtained with a doubling of the density at 54 astronomical units (hydrogen wall), but this is not a unique solution. Finally, the use of Lyman α intensity maps recorded at large distances (30 ‐ 50 AU) is considered. The shape of the map is different when a hydrogen wall is present, and this method is less prone to instrumental drift and solar Lyman α changes than the preceding one. The first method works whatever is the distance of the heliopause, whereas the two others require observations from upwind region, not too far from the heliopause (a few tens of astronomical units at most).
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