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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:
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
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.
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
Co-investigators:
Status:
Results
are reported in the papers:
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
Co-investigators:
Status:
Results
are reported in the papers:
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