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**INTAS Joint
Research Project**

*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*

**WORK
PROGRAMME:**

__OBJECTIVES__

** **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:

- backscattered solar Lyman-alpha radiation measurements (SWAN/SOHO, GHRS/HST and UVS/Voyager;
- interstellar pick-up ions and time- and space-resolved interstellar neutral fluxes derived therefrom (SWICS on Ulysses and on ACE);
- recently discovered direct observation of energetic neutral atoms from the interaction region between heliosphere and interstellar medium (SOHO/CELIAS/HSTOF).

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

__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**

**interface.**

*Description:*

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

**Izmodenov, Geiss, Lallement, Gloeckler, Baranov,Malama,**Filtration of interstellar hydrogen in the two-shock heliospheric interface: inferences on the LIC electron density, submitted to JGR**Izmodenov, Lallement, Geiss**, Interstellar oxygen in the heliospheric interface: influence of electron impact ionization, submitted to A & A.

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

*Description:*

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

**I.3
Calculation of fluxes of energetic neutrals.**

*Description:*

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

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

*Description:*

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:__

__Co-investigators: __* *

__ Status: __

__Results__**
**are reported in the papers:

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

*Description:*

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

complementary.

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:__

__Co-investigators: __* *

__ Status: __

__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.

Subtasks:

*
**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