Consolidation of the Physical Interstellar Medium Parameters and Neutral Gas Filtration
– Coordinated Effort at ISSI
E. Möbius1, M. Bzowski2, H.-J. Fahr3, P. Frisch4, P. Gangopadhyay5, G. Gloeckler6,7,
V. Izmodenov8, R. Lallement9, H.-R. Müller10, W. Pryor11, J. Raymond12,
J. Richardson13, K. Scherer14, J. Slavin12, M. Witte15
1
Dept. of Physics and EOS, University of New Hampshire, Durham, NH, USA, eberhard.moebius@unh.edu
2
Space Research Centre, Warsaw, Poland
3
Institut für extraterrestrische Forschung, Universität Bonn, Bonn, Germany
4
Dept. of Physics and Astronomy, University of Chicago, Chicago, IL, USA
5
Space Science Center, University of Southern California, Los Angeles, CA, USA
6
Dept. of Physics and IPST, University of Maryland, College Park, MD, USA
7
Dept. of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI, USA
8
Moscow State University, Moscow, Russia
9
Service d'Aéronomie du CRNS, Verrieres-le-Buisson, France
10
Dept. of Physics and Astronomy, Dartmouth College, Hanover, NH USA
and IGPP, University of California, Riverside, CA USA
11
Central Arizona College, Coolidge, AZ, USA
12
Harvard Smithsonian Center for Astrophysics, Cambridge, MA, USA
13
Space Science Center, Massachusetts Institute of Technology, Cambridge, MA, USA
14
Dat-Hex, Kaltenburg-Lindau, Germany
15
Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany
ABSTRACT
The neutral gas component of the local interstellar
cloud (LIC) flows through the inner heliosphere due to
the relative motion of the Sun and the surrounding
medium. Interstellar ions are diverted around the
heliosphere, which leads to filtration of many species,
in particular H and O. A recent coordinated analysis
hosted by the International Space Science Institute
(ISSI) has produced consolidated physical He
parameters from in-situ observations of the neutral He
flow. Starting with the assumption that He represents
kinetic LIC parameters for all species, a similar effort is
underway for interstellar H, which is substantially
filtered. Using pickup ion, solar wind slowdown, and
UV scattering observations at various solar distances,
and a full chain of modeling from the pristine LIC to
the inner heliosphere, the filtering of H and its density
in the LIC, as well as ionization fractions and the
radiation environment in the LIC are inferred.
1.
INTRODUCTION
In-situ diagnostics of the local interstellar neutral gas
inside the heliosphere is now almost routinely
performed using three of its products and their visible
effects: neutrals atoms themselves, pickup ions, and
scattering of solar UV at the local interstellar gas. These
in-situ measurements return information on the spatial
distribution and the kinetic characteristics of the
interstellar gas flow through the inner heliosphere.
Extrapolation of these observations into the local
interstellar cloud (LIC) requires modeling because of
the sun’s effects on the neutral gas flow and filtering in
the heliospheric interface. Helium is not filtered and
thus provides an unbiased account of the LIC
conditions. It can also be reasonably assumed that the
kinetic parameters, such as bulk flow velocity vector
and temperature are the same for all species in the LIC.
On the other H and O are heavily filtered, meaning that
their density is reduced inside the heliosphere and that a
slowed down and heated component of neutral gas is
added to the flow in the interface (Ripken & Fahr, 1983;
Baranov & Malama, 1993). These effects depend on the
inaccessible plasma component of the LIC.
Therefore, the determination of the physical parameters
and the composition in the LIC is performed in several
steps. First, the kinetic parameters and densities of the
accessible neutral species are deduced from in-situ
observations in the inner heliosphere, followed by
modeling, which deduces the filtering in the boundary
regions from the observational constraints (Izmodenov
et al., 2003; Zank & Müller, 2003). Finally, these
observations are built into the detailed modeling of the
Table 1: Consensus parameters for He in the LIC
Parameter
He Flow Direction:
He Flow Speed:
He Temperature:
He Density:
Observed Value
Longitude  = 74.68±0.56o
Latitude  = - 5.31±0.28o
vHe∞ = 26.24±0.45 km/s
THe∞ = 6300±390 K
nHe∞ = 0.0148±0.002 cm-3
ionization and radiation environment outside the
heliosphere (Slavin & Frisch, 2002).
The effort of this ISSI Team will add a consolidated
neutral H density (nH) at the termination shock, better
constraining parameters in the LIC, and it coordinates
the different modeling approaches.
2.
COORDINATED ANALYSIS OF THE
INTERSTELLAR HE PARAMETERS
Because of its unchanged access to the inner heliosphere
and the availability of three observation methods the
first comprehensive physical parameter set in the LIC
has been obtained for He. Simultaneous observations of
interstellar He as neutral gas, pickup ions, and of its UV
glow with Ulysses, ACE, EUVE, and SOHO were
recently used to derive a consensus set of He parameters
for the LIC (Möbius et al., 2004). The results are
compiled in Table 1. Direct observation of the neutrals
provides the most accurate account of the kinetic He
parameters (Witte, 2004), whereby Doppler-dimming
observations are fully consistent with the neutral gas
results (Vallerga et al., 2004). The density is obtained
most accurately from He2+ pickup ion observations,
which do not require an absolute calibration (Gloeckler
et al., 2004). The neutral gas result is identical with the
pickup ion result, but carries somewhat larger
uncertainty. To achieve consistent results with all
methods required the assumption of additional
ionization through electron impact and of a latitudinal
variation of the ionization rates.
3.
CONSTRAINTS
FOR
THE
PARAMETERS AND IONIZATION
LIC
Making the reasonable assumption that the kinetic
parameters of all species in the LIC are represented by
those obtained for He, the only missing parameter for H,
the majority species of the neutral interstellar gas, is its
density. As a stepping stone before talking into account
any filtration the H density in the heliosphere at large
distance must be determined. Current values for the
neutral H density nHTS at the termination shock,
obtained from pickup ion (Gloeckler and Geiss, 2004)
and solar wind slowdown observations (Wang &
Richardson, 2003), and the recent encounter of Voyager
1 with the termination shock (TS) provide already
increasingly tighter constraints for the neutral and
ionized hydrogen (nH and nH+) in the LIC. Izmodenov et
Methods
Neutrals, UV, Pickup
Neutrals, UV
Neutrals, UV
Neutrals, UV
Pickup, Neutrals, UV
al. (2003) combined nHTS = 0.095 cm-3 with nH = 0.18
cm-3 and nH+ = 0.06 cm-3 and found a TS distance in the
Voyager direction of 95 AU, very close to the actual
encounter. Since this evaluation of the parameters is
based on a model of the filtration of neutral atoms on
their way into the heliosphere it also constrains the
filtration process.
The composition of the neutral interstellar gas as
obtained from pickup ion and anomalous cosmic ray
observations constrains models for the ionization
fraction of H and He and their spatial structure in the
local interstellar medium (LISM), which includes the
LIC that embeds the heliosphere (Slavin & Frisch,
2002). Adding to these constraints the consolidated He
parameters have now considerably narrowed the
available parameter space. Current best fits of the He
parameters to the model, which take into consideration
the composition of the neutral interstellar gas from
pickup ion and anomalous cosmic ray observations, are
consistent with a total H density (neutral + ions) nHTot =
0.22 or = 0.24 cm-3 and a magnetic field B = 0.24 -3.7
µG in the LISM (Slavin & Frisch, 2005). Narrowing the
uncertainty for nH and consolidation as well as coupling
of the TS distance to the heliosphere dynamics will
further improve these results and will allow a better
deduction of B.
4.
EFFORT TO CONSOLIDATE THE H
DENSITY AT THE TERMINATION SHOCK
After compiling a benchmark set for the interstellar He
parameters a key quantity that constrains the physical
state in the LIC and the filtering is the neutral H density
at large distances in the heliosphere, i.e. at the
termination shock. It can be determined with several
different in-situ methods. From pickup ion observations
a neutral H Density of nHTS = 0.095±0.01 cm-3 has been
derived (Gloeckler and Geiss, 2004). This result is
sensitive to the actual ionization rates and radiation
pressure in the inner heliosphere. Observations with
Ulysses at 1.3-5.4 AU will be used to adapt
simultaneously the local density and the gradient in the
modeling to improve this result.
Solar wind slowdown observations on Voyager 2 have
put the neutral H density at nHTS = 0.09±0.02 cm-3
(Richardson et al., 1995; Wang and Richardson, 2003).
This result is sensitive to the interstellar neutral gas and
solar wind modeling. Improvements can be achieved by
adapting the two-components of interstellar H and
fitting simultaneously to the absolute value and the
gradient of the slowdown. The same parameters will be
used in a consolidated modeling for both in-situ
methods, pickup ion and slowdown observations.
From UV backscattering on Voyager a density nHTS =
0.075 cm-3 has been derived (Gangopadhyay et al.,
2004). This result is strongly sensitive to the absolute
calibration of the UV sensors as well as to the modeling
of the radiation transport and of the heliospheric
interface. Within the ISSI Team a new approach is
being used that does not require absolute calibration.
We will make use of the availability of Ly  backscatter
observations from 1 AU (SOHO), via 1.3 - 5.4 AU
(Ulysses), 5 AU (Galileo at Jupiter), 9 AU (Cassini at
Saturn), and 39 - 55 AU (Voyager) to track the damping
of the Ly  modulation due to the sun’s activity regions
and its rotation, as shown schematically in Fig. 1. The
damping of this modulation with distance is a measure
of the local neutral H density. Comparing all three
results with the same model assumptions that include
the appropriate variations of ionization rates and
radiation pressure with their anisotropy should provide a
consensus value for nHTS.
5.
EFFORT TO CONSOLIDATE
HELIOSPHERIC MODELING
Figure 1. Schematic view of the attempt to use the
modulation of the sun’s UV radiation due to the motion
of activity regions with the rotation of the sun. Because
of scattering at the interstellar H atoms intensity of the
illuminating active region fades out with distance from
the sun, like the beacon of a lighthouse in fog. The
decrease of the modulation with distance from the sun is
a measure of the interstellar H density nH, which can be
obtained without the need of absolute calibration
through modeling and observations of backscattered Ly
 radiation from SOHO, Ulysses, Galileo, Cassini, and
Voyager. An example of the modulation of Ly 
radiation at 1 AU, based on the SOLAR 2000 model
(Tobiska, 2004) is shown in the inset near Earth. The
decrease of the modulation is sketched qualitatively in
the insets at larger distances.
6.
THE
All results from these observations are model
dependent, in particular, the extrapolation to the LISM.
Therefore, the ISSI Team has also mounted an attempt
to a) use the same model assumptions for all analysis
steps and b) evaluate how different model approaches
return the same critical heliospheric parameters, such as
the distance of TS, HP, and BS, for the same parameter
set. Models that include the behavior of neutral gas are
either fully kinetic for the neutral component (e.g.
Baranov and Malama, 1993; Izmodenov et al., 2005),
treat the neutrals as multiple fluid components (e.g.
Zank and Müller, 2003), or keep the neutral
distributions after the last interaction in a general multifluid approach (e.g. Fahr et al., 2000). A direct
comparison shows that the difference for the distances
of the boundaries between the two model families can
be kept to approx. 10% or below, if at least three
different neutral fluids are used, but noticeable
differences remain (Alexashov & Izmodenov, 2005). In
addition, pure fluid implementations do not include the
influx of neutrals from all directions out of the H wall
and the heliosheath, because neutral fluids follow only
flow lines (as sketched in Fig. 2). Thus a fully kinetic
approach is needed definitely to simulate energetic
neutral atom observations, as planned with the
Interstellar Boundary Explorer (IBEX) (McComas et al.,
2005).
In addition, it will be assessed through comparison of
stationary and time-dependent modeling how strongly
temporal variations of the solar wind dynamics as a
function of the solar activity cycle influence the H
inflow, the ENA distributions, and the model dependent
results for the interstellar parameters (e.g. Scherer &
Fahr, 2002; Fahr & Scherer, 2004).
FUTURRE DEVELOPMENTS
A consolidation of the LIC parameters, including the H
density, will allow a re-calibration of past UV
backscattering observations that have the longest
history. In this way potential small scale variations in
the LIC parameters and their temporal history can be
constrained. The next step with substantial
improvements in the knowledge of the interstellar
parameter set and in the understanding of the filtering
will become possible with the launch of IBEX, which
will provide the first direct measurement of the
interstellar O flow velocity vector inside the heliosphere
(McComas et al., 2005). The recent observation that the
flow direction of interstellar H inside the heliopshere is
different from that of He, points to an inclined
interstellar magnetic field (Lallement et al., 2005), This
effect has recently been modeled quantitatively by
Izmodenov et al. (2005). Based on this conclusion,
improved observations of this diverted flow and of a
related asymmetry of the heliosphere in the light of
ENAs will allow a much better deduction of magnetic
field strength and direction, whose knowledge is still
relatively uncertain.
7.
ACKNOWLEDGEMENTS
Figure 2. Schematic view of the heliosphere and the
incoming ISM neutral gas flow. The upper half indicates
the behavior of the gas flow, as seen through full kinetic
modeling of the neutrals, while the lower half shows
their behavior as seen through multi-fluid modeling. The
key difference between the two models is that kinetic
models describe the full velocity distributions, which
allows to see the arrival of secondary neutrals from all
sides out of the interface layer, while strict multi-fluid
models represent the gas by the bulk flow of its
important components along their flow lines. In this
picture secondary neutrals would flow mostly along the
boundaries and reach 1 AU only from the nose, unless
the velocity distribution of the neutrals is maintained
after the last interaction between neutrals and ions.
The authors would like to thank the International Space
Science Institute (ISSI) for the support of this activity.
E. M. would like to thank the staff at ISSI and at the
Physics Institute of the University of Bern for their
hospitality during a sabbatical visit and thankfully
acknowledges support by the Hans-Sigrist Stiftung. This
work was supported under NASA grants NAG5-12929,
NAG5-10890, NAG5-12879, NAG5-13611 and
NNG05GD69G, under the Polish SCSR grant
1P03D00927, Russian Grant RFBR 04-02-16559,
INTAS Grant 2001-0270. …….
8.
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