These pages are now located on ISSI server


INTAS Joint Research Project

The heliosphere in the local interstellar medium

in cooperation between:

Service d'Aeronomie, CNRS, FRANCE;
International Space Science Institute, Bern, Switzerland;
Finnish Meteorological Institute, Helsinski
Moscow State University, Russia
Institute for Problems in Mechanics, Russian Academy of Science, Russia
Sobolev Institute of Mathematics, Novosibirsk, Russia



 The heliosphere is the volume of solar plasma established by the Sun inside the Local Interstellar Cloud (LIC) in hich our star is travelling. We propose the development of a quantitative structural model of the heliosphere and
of the interface between the solar wind and the surrounding interstellar medium. New data accessible to us have
recently been (or are currently) gathered from space which provide different types of diagnostics of this interface
and need to be synthesized:

    For each of these experiments at least one participating team has a direct access to the data. Combined with previous data from the Voyager, Pioneer and Ulysses spacecraft these new data should lead to a
quantitative description of the heliosphere if they are compared with a variety of appropriate models. We propose
to develop these models and to apply them to the combined sets of data. The theoretical work will be focussed on
the implementation of a new method for the solution of the 3D time-dependent kinetic Boltzmann equation for
the interstellar neutral gas flow.
    This should provide the possibility to determine the characteristics of the interstellar medium in the vicinity of the Sun and to discuss them in the context of recent interstellar observations. As a practical consequence of this study the distance can be estimated that a spacecraft needs to travel to reach the interstellar medium.

Background & Justification for Undertaking the Project

Research Programme

Group Homepage

Detail description of individual tasks during each phase of the proposed study:
Sumarized table

I.1 Interpretation of previous data on the basis of the existing axisymmetric model of the heliospheric

It is impossible to measure the parameters of the local interstellar cloud directly.
Perhaps the Voyager spacecraft can probe it in situ if still operating after crossing the interface, but until now it still  cruises in the supersonic solar wind. However, an indirect measurement of the proton number density as well as of the temperature and velocity of the circumsolar interstellar medium can be deduced comparing the experimental data with the model results.

The self-consistent axisymmetric model of the solar wind interaction with the local interstellar medium taking
into account neutral and plasma components has been created by the MSU team. These numerical codes are
unique in that they use a kinetic description of the interstellar neutrals. A very efficient numerical code based on
Monte-Carlo methods has been developed to solve the kinetic equations.

Analyses of different observations on the basis of the existing models will provide us preliminary parameters of the
LIC and a better estimate of which additional physical effect must be included in the model.

Responsible persone for the individual task:  Izmodenov
Co-investigators:    Lallement, Geiss, Baranov, Malama
 Status:     DONE
Results are reported in the papers:


I.2. Stochastic acceleration of interstellar pick-up ions in the solar wind.

During their propagation to the outer parts of the heliosphere the pick-up ions are not only scattered in
pitch-angle but also accelerated by Alfvenic turbulence, magnetosonic turbulence, and by interplanetary shock
waves associated with corotating and merged interaction regions. Thus, upstream of the termination shock, the
actual energy distribution of the pick-up ions is different from the initial KeV-shell distribution function, and has
already developed a high-energy tail. At the solar wind termination shock particles from this tail are efficiently
injected into a strong acceleration process up to ACR energies.
    We intend to calculate the spectra of these energized pick-up ions in the whole region extending from the Earth's  orbit up to the termination shock. The results of this study will be compared with the observations of charged energetic particles on Ulysses, Voyager, and Pioneer spacecraft.
    In order to quantify the pre-acceleration of pick-up hydrogen, helium and oxygen in the heliosphere, we intend to use our numerical two-dimensional model with the symmetry axis oriented along the bulk velocity of the local
interstellar medium.
    The model describes the production of pick-up ions in each volume element, their convection, adiabatic
deceleration in the expanding solar wind and their acceleration by interaction with all kinds of solar wind
turbulence. To solve the governing Fokker-Planck equation in two spatial dimensions we make use of the
mathematical equivalence of this type of a partial differential equation with a coupled system of stochastic
ordinary differential equations which describe individual trajectories of particles in phase space.

Responsible persone for the individual task:  Chalov
Results are reported in the papers:

I.3 Calculation of fluxes of energetic neutrals.

The energized pick-up ions can exchange charge with neutral atoms from the LISM at any location in the outer
heliosphere. A fraction of the energetic neutralised particles can reach the Earth's orbit. Recently, these particles
have been observed by the CELIAS/HSTOF instrument on board SOHO.
    Using our computed spectra of the energized pick-up ions we intend to derive the fluxes of energetic neutral atoms (ENA's) originating from the outer parts of the heliosphere and to interpret the observations in the light of these  results.
    We want to calculate the ENA?s fluxes combining simplified analytical models of neutral atom transport and
numerical Monte Carlo models.

 Responsible persone for the individual task:  Chalov
Results are reported in the papers:

I.4 Interpretation of new SWAN/SOHO Lyman-alpha measurements

The SWAN instrument on board SOHO has collected a considerable amount of Lyman-alpha data. They will
enable extremely precise measurements of the line-of-sight temperatures and bulk velocities of atomic H in all
directions as well as their variations year after year. The analyses of the data from different directions on the basis
of the axisymmetric heliospheric interface model will provide us with the best set of interstellar parameters.

Responsible persone for the individual task:
Results are reported in the papers:

I.5 Interpretation of new SWICS (Ulysses) pick-up ion measurements

    With pick-up ion measurements we can determine the neutral H fluxes, which depend on the solar ionisation
processes only. The situation is even more favorable because the solar wind, which is mainly responsible for the
destruction of the interstellar flux, is measured at the same time as each set of pick-up ions. On the other hand, the
measurements of pick-up ions cannot give us information about velocity and temperature of the interstellar
neutral atoms. The backscattered Lyman-alpha glow as a diagnostic tool for the neutral H density (I.4) suffers
from uncertainties on photon radiative transfer effects, and on radiation pressure and ionisation rate
measurements, but provides temperature and velocities. It is clear that the two types of data are then perfectly
    Until now the interpretation of pick-up ion data have been performed on the basis of classical ?hot? models. It is
assumed that the value of the interstellar atom density which is derived from this model corresponds to the
interstellar atom density at the TS (i.e. very far from the Sun, but still inside the heliosphere) because the ?hot?
model does not take into account any filtering at the heliospheric interface. However, due to neutral-plasma
coupling at the interface there are spatial variations of the interstellar atom parameters even at large distances
from the Sun (e.g. at the TS), where the hot model assumes that the density is constant. Thus the 'hot' model is not
perfectly adapted to the interpretation of pick-up ion measurements, especially for high interstellar plasma
densities. This is why a thorough analysis of the new precise SWICS/Ulysses pick-up data requires a model
including heliospheric interface perturbations of the neutrals.

 Responsible persone for the individual task:
Results are reported in the papers:

I.6 Influence of non-stationary effects induced by the 11 year solar activity cycle.

The interaction of the solar wind with the local interstellar medium is described by a stationary model (see I.1).
However, solar activity (for example, solar flares or simply the 11-year solar cycle) can give rise to non-stationary
processes in the heliosphere (the heliosphere ?breathes?). The consequences of these variations occur at two levels:
at the interface itself , where H atoms are filtrated because the balance between the solar wind and the interstellar
medium changes, and close to the Sun, where H atoms are directly ionised by a variable solar wind.

In a first approach we will test the amplitude of the effects (a quasi-stationary model), and then we intend to
construct a real non-stationary model responding to solar activity temporal variations.


  1.6.1 Quasi-Stationary Monte-Carlo technique applied to the variations of the interstellar atom distribution at the heliospheric interface caused by the solar activity cycle.

The kinetic gas-dynamic axisymmetric model (I.1) can not be applied to the time-dependant problems in a
straightforward manner. We propose to perform stationary calculations for all phases of the solar cycle. This study
should be considered as a preliminary step to 1.6.3. On this step we will analyse the quasi-stationary solutions and
find a 'median' interstellar atom distribution.


I.6.2 Time-dependent model without heliospheric interface.

A time-dependent ?hot? model (no interface) of the neutral flow has been developed by the FMI team and will be
applied to the new data from the SWAN/SOHO and SWICS/Ulysses instruments. This model will be helpful
because it will show how variations of the flow characteristics originating from the interface are propagating into
the heliosphere up to the first AU where the Lyman-alpha and pick-up ion data are collected. Of course, the
validity of this model is limited because the response of the interface itself to the solar cycle variations is not
considered. This is why we have to go one further step.


I.6.3 Development of a new statistical method applied to the solution of a time-dependent Boltzmann equation.

Traditional Monte-Carlo methods are not efficient for heliospheric models. Therefore, an improved method with
a 'splitting' of the trajectories ("weighted" Monte-Carlo method) has been developed [10]. However, while this
scheme works efficiently in the 2D stationary case, a 3-dimensional time-dependent model requires an even more
powerful tool. We propose to create a Monte-Carlo algorithm using the method of ?non-simulation estimators?
developed by Prof. Khissamoutdinov [11-15]. The principle of "Weighted" Monte-Carlo methods is the use of
transition probabilities for all the underlying Markov processes (e.g. each charge-exchange, each
photo-ionization, each momentum change, etc.). In simulating real astrophysical problems one has to consider the
Markov processes for each physical value separately. At variance with this approach, ?non-simulation estimator?
methods keep the information on some physical parameters only in ?estimators? of these parameters, which makes
the code more efficient. This method was already applied successfully to different physical problems in nuclear
physics and we intend to apply this new mathematical technique for the first time to an astrophysical problem.


I.7 Influence of the interstellar and interplanetary magnetic fields on the heliospheric interface structure

A magnetic field of solar origin is "frozen" in the expanding solar wind. It is also known that the Alfven Mach
number in the pre-shock region of the solar wind termination shock is much larger than unity, i.e. the
interplanetary magnetic field can be neglected in this region. However, the situation in the post-shock region of the
termination shock (compressed and decelerated solar wind) is different and the magnetic field in this region has an
influence on the heliospheric structure.

As for the interstellar magnetic field, at present neither its magnitude nor its direction in the vicinity of the solar
system are known. Nevertheless, the LIC magnetic field could influence significantly the heliospheric structure and
the interpretation of the experiments.

We plan to include the interstellar as well as the interplanetary magnetic fields into our model. The first step will be
the introduction of the interstellar magnetic field in the axisymmetric approximation (magnetic field parallel to
the velocity vector) using numerical methods suggested by the IPM team. Then we will consider the more general
case of an oblique field.


I.8. Back reaction of ACRs and galactic cosmic rays (GCRs) on the solar wind flow and on the structure of
the interaction region.

Cosmic rays are coupled to the solar wind and the LIC plasma through the random fluctuations of the magnetic
fields frozen in the flows. One expects that the plasma structure of the heliosphere is affected by the pressure
gradient of cosmic rays (CRs). Preliminary calculations have shown that CRs can considerably modify the structure
of the termination and the bow shock.

We intend to calculate the dynamical influence of ACRs and GCRs on the global structure of the heliosphere and
interface region in the pure plasma case.

This task should be considered (together with I.7) as a preliminary step to include these effects in the
plasma/neutrals self-consistent model.


Phase II: Further developments and synthesis


II.1. Time-dependent 3D model including neutrals, interstellar and solar magnetic fields as well as
anomalous and galactic cosmic rays influences.

It is planned to join the efforts of the different participants in the creation of a full time-dependent 3D model of
the interaction of the solar wind with the interstellar medium including interstellar neutrals, interstellar and solar
wind plasma, interstellar and solar magnetic fields as well as influences from anomalous and galactic cosmic rays.
The development of this model is a difficult task and the major goal of this proposal. For this purpose, we will
combine the developments described in sections I.6, I.7, I.8 with the interpretation of data (I.1, I.3, I.4).

This goal can be achieved if the numerical codes for the previous steps (I.6, I.7, I.8) are compatible one with each
other. This is why a concertation between the different groups prior to these tasks is mandatory.


II.2 Origin of anomalous cosmic rays (ACRs).

The last theoretical development that we plan is an improvement of the model for the pick-up ion acceleration
(I.2) processes. The model of pick-up ion acceleration at the termination shock will be extended so that it also
describes particles at the highest energies, i.e. at energies of the ACRs. The results of our investigations of the
pre-acceleration of pick-up ions in the solar wind will be used to model the "injection" of particles into the ACR
regime. One of our goals in doing so is the search for an explanation of the well-known deficit of anomalous
hydrogen in the ACRs.


II.3 Analyses of the new data on the basis of the unified model

After a parametric study with the code developed on II.1, we plan to apply the results of this study to the
interpretation of SWAN/SOHO, SWICS/Ulysses, and also Voyager data (practically to repeat the study I.1, I.4, I.5
on the basis of the new time-dependent 3D model II.1). This study requires extensive computations because there
are some more free parameters than in the existing model. However, the 3D model can be applied to the
interpretation of full sky observations of Lyman-alpha photons and of all the pick-up ion data while the previous
model could only be applied to measurements near the ecliptic plane.



Created by Vlad Izmodenov. Updated on October 13, 1998