Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

The GHRS and the Heliosphere

R. Lallement, J.T. Clarke, Y. Malama, E. Quémerais, V.B. Baranov, J.L. Bertaux, H.J. Fahr, F. Paresce

Service d' Aéronomie du CNRS, BP3, Verrieres le Buisson, 91371, France AOSS Department, University of Michigan, Ann Arbor, MI 48109 USA Institute for Problems in Mechanics, Prospect Vernadskogo 101, Moscow 199899, Russia IAEF, Bonn University, Auf dem Hugel 71, D5300 Bonn, Germany STScI, 3700 San Martin Drive, Baltimore, MD 21218 USA



The Sun is moving through the Local Interstellar Cloud (whose relative motion is precisely measured, due to the LIC identification in nearby star ground-based and GHRS spectra (Lallement et al. this issue). A fraction of the neutral content of this gas succeeds in penetrating within the heliosphere through the solar wind/interstellar medium interface and invades the interplanetary space. This allows a comparative study of the interstellar neutral atoms properties inside and outside the heliospheric interface, to infer the selective filtration at this interface. In particular, neutral hydrogen penetrating the heliosphere and approaching the Sun backscatters the solar Ly-alpha radiation and produces an emission glow. High resolution GHRS spectra of this diffuse emission were recorded in different directions, in configurations allowing a large enough Doppler shift between the geocoronal and interstellar signals. They show that, at variance with the interstellar neutral helium, measured ``in situ'' by Ulysses, which keeps its initial heliocentric velocity of 26 (or the LIC velocity), the bulk speed of the interstellar neutral hydrogen appears to have decreased by about 6 during its approach to the heliospheric interface. This is in qualitative agreement with models including charge-exchange coupling with the decelerated plasma, itself excluded from the heliosphere. According to a preliminary analysis the observed line profiles suggest a significant velocity dispersion when the viewing direction is parallel to the bulk flow, in agreement with the models. These observations bring constraints on the interstellar proton density (estimated to be of the order of 0.1 cm) and, in turn, on the size of the heliosphere (about 100 A.U.), of prime importance for the future crossing of the interface by the two Voyager spacecraft. A self-consistency check with the electron density deduced from GHRS interstellar lines ratios towards the nearby stars is then obtained.

Keywords: heliosphere, solar wind, local interstellar medium

The Lyman-alpha Interstellar Glow

Thanks to its relative motion with respect to the Sun, neutral gas from the Local Interstellar Cloud is invading the interplanetary space. Neutral hydrogen atoms having succeeded in penetrating close to the Sun scatter the solar Ly-alpha (121.6 nm) radiation, creating the interplanetary-interstellar Lyman-alpha glow. Depending on their closest distance to the Sun, atoms are or are not ionized by the solar EUV flux, and by the solar wind through charge-exchange reactions with the protons. As a result, a region void of visible neutral H (the ionization cavity) is created in the so-called ``downwind'' region. In the supersonic solar wind, the charge-exchange reaction (H + H + H) between an incident (slow) interstellar H atom and a fast solar wind proton acts as a loss process for the glow because it replaces the slow atom by a fast neutral atom which, moving radially with the solar wind velocity, is shifted out from the solar line. The Ly-alpha emission, when observed from the earth's orbit, is almost entirely produced within the first 50 astronomical units, essentially due to the solar flux decrease with R, although multiple scattering effects are present.

Classical models of this emission do assume a homogeneous flow ``at infinity'', characterized by a velocity V and a temperature T . For these models, infinity means at about 50 A.U., i.e., outside the ionization cavity, before any perturbation due to the Sun and the supersonic solar wind.

Spectra of the Ly-alpha emission (Figure 1a) recorded by the GHRS (Echelle A gratings and L S A), provide the best quality spectra ever obtained. Copernicus and IUE had already recorded a spectrum of the Ly-alpha glow, which have been analyzed by Adams & Frisch (1977), and Clarke et al. (1984,1995). Here, however, the observations combine high sensitivity, high resolution, and a good separation of the earth and interplanetary emissions. For each direction the date of observation is calculated in order to maximize the Doppler shift between the earth emission (the geocorona), and the interplanetary background. The geocorona has been used to calculate the point-spread function, and is then modeled and removed.

Figure: a: A GHRS spectrum of the diffuse Ly-alpha background: the strong and narrow geocoronal emission is very well separated from the Ly-alpha background. b: Schematic view of the solar wind-interstellar medium interface

The Solar Wind-interstellar Plasma Interface

Such a simple view ignores the existence of the interstellar ionized fraction. Charged species of the inflowing interstellar medium are excluded from the heliosphere (the solar wind domain), and forced to flow around the heliopause, the discontinuity surface between the two media (Figure 1b). On the inner side of the interface, the supersonic solar wind, before reaching a pressure equilibrium with the ISM at the heliopause, is expected to be decelerated at the ``termination shock'', while on the outer side, the ISM plasma itself may (or not) be decelerated by a weak shock, depending on the actual values of the ambient ISM plasma density and magnetic field. The clue parameters for the interface are the interstellar plasma density and magnetic field. The magnetic field is known within a factor of 5 only. The interstellar plasma density has been estimated from the magnesium ionization equilibrium in the Local Cloud, itself measured from nearby stars absorption lines. It has been found to be of the same order or larger than the neutral density (Lallement et al. 1995, and this issue).

A Link Between the Local Cloud and the Heliosphere

Interstellar neutral helium atoms are extremely weakly interacting with the plasma, and are supposed to flow through the ISM/Solar wind interface without perturbations. Indeed, He atoms have been detected inside the heliosphere by the Ulysses probe, and found to be characterized by V= 25.7 and T =6700 K (Witte et al. 1993).

These values are in excellent agreement with the Local Cloud heliocentric velocity and temperature found from absorption lines in nearby stars spectra (Lallement et al. 1993, Linsky et al. 1993), providing a link between the heliosphere and the Local Cloud. Thanks to this agreement, the temperature and the relative velocity of all interstellar species before the entrance within the heliosphere are known accurately, allowing inside-outside comparisons for species interacting with the plasma.

Expected Perturbations of the Neutral H Due to Coupling with the Decelerated and Heated Plasma

Neutral H atoms suffer charge exchange reactions (H + H + H) with the protons, and for this reason, they are significantly coupled to the plasma. This means that in the region where the interstellar plasma is decelerated and deviated, to flow around the heliosphere, a fraction of these perturbations is transferred to the neutrals. Sophisticated self-consistent neutrals/plasma models, taking into account the charge-exchange processes, allow to estimate the transfer of plasma momentum to the neutral flow (Baranov & Malama 1993) . The neutral population approaching the Sun is made from two components:

- atoms having not charge-exchanged : the primary component

- atoms resulting from charge-exchange reactions : the secondary component The proportions of these populations strongly depend on the plasma density in the ISM. The higher the plasma density, the larger the proportions of secondary atoms.

Figure: Secondary interstellar H atoms distribution in the heliosphere from a self-consistent neutral-plasma model. The interstellar shock, the heliopause, and the solar wind shock are the white lines. The interstellar plasma density is 0.1 cm

Figure 2 illustrates the effect of the interface by showing secondary atoms iso-density contours for an interstellar proton density of 0.1 cm and a vanishing magnetic field. White lines are the terminations hock, heliopause and interstellar shock surfaces. There is a marked concentration of secondaries between the bow shock and the heliopause (the ``hydrogen wall''). Figure 3a shows the neutral H primary and secondary densities along the sun/wind axis for the same model parameters, as well as the total density, and Figure 3b displays the mean velocity when combining the two populations. It is clear from these two figures that for an observer located close to the Sun, the apparent density ``at infinity'' (at, say, 50--70 AU), is smaller than the true interstellar density, due to the exclusion from the heliosphere of a fraction of the H atoms. At the same time the apparent bulk velocity ``at infinity'', is also smaller than the initial ISM velocity, because the newly created H atoms are moving less rapidly.

Figure: a) Primary and Secondary H atoms densities along the Sun-Wind axis on the upwind side. b) Combined primary and secondary H atoms bulk velocity along the Sun-Wind axis on the upwind side. c) Primary and secondary H atoms bulk velocities along the Sun-Wind axis on the up wind side. The difference between the two velocities implies a large dispersion in addition to the thermal dispersion, and a subsequent broadening of the emission line. d) Same as c) for a crosswind direction (perpendicular to the mean flow). The dispersion is very small in this direction, and the emission line is not expected to be broadened

Figures 3c and 3d display the radial velocities of the two populations along the upwind (from where the wind blows), and the crosswind (or side wind= at 90 from the wind axis) directions. Evidently, there is a strong velocity dispersion for the upwind L-O-S, because the secondary atoms have been decelerated, while the surviving primary atoms have on the contrary a velocity larger than the initial one, due to selection effects (fast particles spend less time in the interface and suffer less charge-exchange). Along the crosswind direction, both components have velocities around zero, and there is little dispersion. The velocity distributions are reflected by the Doppler shifts and the widths of the Ly-alpha emission. This is why these effects can be searched for in UV spectra.

GHRS Spectra: Deceleration and Broadening along the Wind Axis

Spectra were recorded for the upwind and crosswind (at right angle with the flow) directions. In the case of the crosswind spectrum, it has been possible to find a reasonable fit to the data with a classical model of an initial homogeneous flow (i.e., without any coupling with the plasma) at a temperature of about 8000K. This is in agreement with Figure 3d and the absence of velocity dispersion in this direction.

Figure: An upwind spectrum corrected for the geocorona. A classical model for an initial velocity of 26 has been superimposed. The observed spectrum is shifted towards lower heliocentric velocities. The adjustment requires an initial bulk velocity of 19--20 . The observed line width is also apparently broader than the model predicts.

The results were different for the upwind spectra. An upwind spectrum is shown in Figure 4. It appears impossible to fit the data with an homogeneous flow at an initial velocity of 26 (identical to the LIC and helium velocity). The data implies a lower velocity of about 19--20 . This is in agreement with Figure 3b, and suggests that here has been a significant deceleration of the neutral flow. The interstellar plasma density is then estimated to be of the order of 0.1 cm. The modeled line profile is also found to be narrower than the observed line (Fig. 4b), suggesting a significant velocity dispersion, in agreement with the model velocities shown in Fig. 3c.

Conclusions and Perspectives: A Crossing of the Termination Shock by the Voyager?

The GHRS Ly-alpha glow data show a deceleration and a dispersion of the interstellar neutral H flow in the inner heliosphere, as theoretically expected from coupling with the plasma at the heliospheric interface. An ambient interstellar proton density of the order of 0.1 cm is derived from a first rough comparison with the neutral/plasma models. This density is of the same order than the electron density derived for the Local Cloud, also with the GHRS, from nearby stars spectra showing absorptions. This value implies a distance to the heliopause of the order of 150 A.U., and to the solar wind termination shock of the order of 90 A.U. This is close enough to the Sun for the Voyager spacecraft to be still ``alive'' when crossing the shock.


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