A) General picture of the heliosphere

During the past two decades the results of space experiments have led to a general understanding of the structure of
the heliosphere and of some of the physical processes in action: The Sun is travelling through an interstellar cloud
(The Local Interstellar Cloud or LIC). Due to this relative motion a flow of interstellar matter continuously
interacts with the Sun. The Solar Wind, which is expanding radially from our star, establishes a cavity of solar
matter inside the interstellar medium, which is called the heliosphere. The LIC is believed to be partly ionised. Its
plasma component interacts with the solar wind plasma, and a tangential discontinuity, the heliopause (HP) , is
assumed to form separating the solar wind plasma from the plasma component of the interstellar medium. Two
shocks decelerate the two media before they reach the heliopause. The supersonic solar wind is decelerated by the
termination shock (TS), while the plasma component of the LIC is decelerated by the interstellar bow shock (BS).
The region between the bow shock and the termination shock is the heliospheric interface.

Only the neutral fraction of the LIC can enter the heliosphere. After the interstellar neutral gas has penetrated
through the heliospheric interface into the heliosphere, some of its atoms are ionised and picked up by the magnetic
field frozen in the expanding solar wind to become the so-called "pick-up ions". These particles are partly
accelerated to cosmic ray energies near or at the TS by means of first-order Fermi or by shock drift processes. They
can be detected by spacecraft instruments as the so-called anomalous cosmic rays (ACR). In case of
charge-exchange with solar ions fast ( energetic) neutral atoms (ENA) are also created. Neutrals or their
derivatives are directly or indirectly detectable by particle instruments or through resonant scattering of solar
photons, respectively. Some interstellar neutrals (H, O) interact with the plasma through charge exchange
reactions while crossing the heliospheric interface. This interaction changes both atom and plasma parameters. At
the same time, an additional pressure due to the coupling between the plasma and the neutral component pushes
the heliospheric interface closer to the Sun.

The plasma component of the LIC, which exerts the pressure to confine the solar wind, is not directly observable yet
because it is diverted around the heliosphere and therefore excluded from it. This makes predictions on the size and
structure of the heliosphere very difficult.

B) Recent observational advances

None of the discontinuities has been observed until now by the probes moving in the outward direction of the solar
system. However, there are now many signatures or new observations that constrain the location of the interface.

Voyager plasma experiments just found signs for the solar wind slowing down, while they now cruise at about 60 AU
from the Sun. This deceleration, a precursor of the solar wind subsonic transition and stopping by the interstellar
plasma flow, is due to constant loading of interstellar atoms. Due to charge exchange with post-shocked protons
slow and hot population of interstellar H atoms is created in the heliospheric interface. This leads to changes in the
bulk velocity, the temperature and the density of interstellar neutrals into the region. Indeed, an increase in the
number density of the H atoms in the heliospheric interface due to the charge exchange (the "hydrogen wall"), is
detected by the UV instruments on board Voyager, and the presence of hot and decelerated H has been detected in
absorption with the Hubble Space Telescope (HST) towards the star alpha Centauri. Also, there are indications
that the flow of neutral H reaching the Sun is hotter and slower than the corresponding flow of helium. This is due
to the fact that helium is not coupled to the plasma, as confirmed by the perfect agreement between its properties
outside (in the LIC) and inside the heliosphere, while H is coupled and modified. There are evidences that the flow
of H is also weaker than it should be without the interface, as a consequence of filtration at the interface. Other
types of diagnostics for the interface are the ACR's gradients and some radio signals detected by Voyager which are
assumed to be emitted near the heliopause, or even maybe near the bow shock in the interstellar medium itself.

C) Current state of the model results

At present there are still contradictions between the descriptions deduced from the different kinds of measurements
as well as some gaps in the theoretical understanding of the problem. The two major problems are the disagreement
on the value of the interstellar proton number density, which directly governs the size of the heliosphere, and the
poor knowledge about intensity and direction of the interstellar magnetic field. From solar Lyman-alpha radiation
backscattered by the neutral H and from interstellar measurements the LIC plasma density is believed to be larger
than 0.07 cm-3, while the radio emission ?cut-off? plays in favour of a lower value of about 0.04 cm-3.

D) New data

Observations from space with unprecedented quality or that are completely new which have direct impact on the
problem area are now currently gathered and the analysis is under way:

    extensive photometric and for the first time fully spectroscopic measurements of backscattered solar
    Lyman-alpha with SWAN on SOHO

    measurements of pick-up ions with SWICS on board Ulysses (1-5 AU) or ACE;

    detection of energetic neutral atoms (CELIAS/SOHO and in the future MIMI/Cassini)

E) Interactions between the groups

The combination of the six teams interested in the proposed research corresponds to an optimisation of the abilities
to fulfill the requirements for the most realistic representation of the heliosphere, accounting for all observational
aspects, from the solar wind birth place and its large scale anisotropy as a function of activity, to the most recent
constraints on the interstellar medium properties and abundances, and for all the theoretical aspects.

We propose to use all types of data, namely the new data from SWAN/SOHO provided by the groups from France
and Finland, as well as the UVS/Voyager data provided by a collaboration with the Voyager UVS team, the pick up
ions and ENA's data from SWICS/Ulysses, SWICS/ACE and CELIAS/SOHO provided by the group at ISSI in
Switzerland, combined with published data of other experiments. Our aim is to explain all the data with a unique
model. This includes the implementation of some physical effects not yet taken into account. The theoretical
modelling of the interface will be provided by the Russian groups. The Moscow State University (MSU) and the
Institute of the Problem in Mechanics (IPM) have created a model, which has already been successful in predicting
some effects later observed by space experiments as the neutral hydrogen perturbations (Prognoz-5 and 6,
Voyager), the existence of the "H wall" in the vicinity of the heliopause, and the filtering during the penetration of
neutral species as hydrogen and oxygen (HST and Ulysses) into the heliosphere. However, a better understanding of
the production mechanisms of the ACR particles as well as of the influence of the pick-up ions on the heliospheric
interface structure are now required. We propose to include the influence of these particles into the above described
heliospheric interface model.

Another important improvement is the development of a time-dependent model. At present, a time-dependant
model of the interstellar atom flow in the vicinity of the Sun has been developed by the Finnish group. However,
only the neutrals are taken into account. We propose to combine the existing models and to create a fully
time-dependant heliospheric interface model. We expect to be able to explain some already measured effects with
this model such as the 2 kHz radio emission and time-variations of the ACRs. For the construction of this
time-dependant model, a new mathematical technique, provided by the team at Sobolev Institute of Mathematics
in Novosibirsk (Russia), will be applied in collaboration with the MSU group. In this respect the proposed project
can be considered as an interdisciplinar study with new mathematical methods being applied to the interpretation
of new experimental data.