- Press Release
- August 19, 2022
The Flow of Interstellar Helium in the Solar System
‘Consensus on conditions in the cloud of interstellar gas surrounding
the Sun from several in-situ observation methods’
Through coordinated observations with instruments on several ESA and
NASA spacecraft and a collaborative analysis effort hosted by the
International Space Science Institute (ISSI) an international team of
scientists has compiled for the first time a consistent set of the
physical parameters of helium in the very local interstellar gas cloud
the surrounds the solar system.
Careful analysis of data from three complementary observation methods
produced a reliable set of physical parameters for the local cloud which
can be used to model the interaction between the Sun and the surrounding
interstellar gas and establish dimensions of the solar system.
The three independent observational methods use:
1. the direct analysis of interstellar neutral helium atoms that
penetrates close to the Sun
2. the production of ions from this gas and their transport with the
3. and scattering of solar UV light from the penetrating gas, which,
until recently, was the only method for estimating these parameters.
Knowledge of physical characteristics of extrasolar material, obtained
with these local space measurements, represents an important step in our
understanding the Sun’s interaction with its immediate interstellar
The Sun is located in the outskirts of the Milky Way, about 30 000 light
years from its centre, embedded in a fairly dilute and warm cloud of
interstellar gas, which consists of a mixture of neutral and ionized gas
or plasma. This cloud material represents a sample of today’s
interstellar matter in our Milky Way Galaxy from which stars and
planetary systems form. In the case of the Sun and its planets this
occurred about 4.5 billion years ago. Differences in the composition of
these two samples of galactic matter tell the story of the evolution of
matter in the galaxy as heavy elements are added by dying stars.
The solar wind, which expands radially from the Sun at supersonic speed,
blows a cavity – the heliosphere – into the surrounding interstellar
cloud, filling it with solar material and magnetic field. The plasma
component of the interstellar gas is kept outside the heliosphere.
By balancing the solar wind ram pressure, which is easily measured,
against the pressure of the surrounding cloud, the size and shape of the
heliosphere is determined. With the new consensus set of interstellar
helium parameters, its density, temperature, flow direction and speed
relative to the Sun, the interstellar pressure can now be computed more
reliably and the roughly 100 AU size of the heliosphere more accurately
Because the Sun’s motion relative to the surrounding gas, an
interstellar breeze of neutral atoms blows through the heliosphere, very
much like the wind felt when driving an open car. Only very close to
the Sun is the neutral gas ionized by the Sun’s UV light and the by the
solar wind, which leads to a small cavity in the neutral gas, roughly of
several AU in size. Except for hydrogen, which is affected by radiation
pressure, the Sun’s gravity deflects the neutral gas flow, leading to a
concentration of neutral gas density in the direction opposite to inflow
direction of the gas.
The resulting flow pattern is shown in Figure 1 for helium. It is this
flow pattern that is analyzed to derive the flow speed, its direction,
and temperature. Helium, the second most abundant element after
hydrogen, distinguishes itself by infiltrating closest to the Sun, to
distances even inside the Earth’s orbit. Furthermore, because its
density, temperature, and speed are not affected by processes at the
heliospheric boundary, analysis of the properties of the helium gas
inside the heliosphere allows one to establish the state of the pristine
Since the late 1990’s various instruments on a fleet of spacecraft have
provided observations of interstellar helium. These observations are
depicted schematically in Figure 1:
* The GAS instrument on the ESA/NASA Ulysses spacecraft collects
images of the He gas flow (coloured insert), whose observed
deflection by the Sun’s gravity is translated into the flow
conditions outside the heliosphere.
* NASA’s EUVE has repeatedly scanned the focusing cone in the light
of the He line at 58.4 nm as illuminated by the Sun (yellow insert).
* The SWICS instrument on NASA’s ACE spacecraft and on Ulysses
collects He ions that are created inside the spacecraft orbit from
the interstellar gas and then transported outward with the solar
wind. The blue insert shows the pickup ion flux as the Earth
passed the He cone in December 2000.
* UVCS on the ESA/NASA SOHO spacecraft observes the cone intensity
at 0.2 ? 0.5 AU in the He 58.4 nm line, as the first coronagraph
that ever observed the interstellar gas. The green insert shows
how the cone intensity decreases from 1996 (solar activity
minimum) to 2000 (solar maximum) as the ionization, monitored with
the CELIAS SEM sensor on SOHO, increases.
Figure 2 shows a compilation of the observations along with weighted
mean values from all available observations. The upper panel shows the
flow direction, and the lower panel contains the flow speed and
temperature. It is evident that the direct observation of the neutral
atoms provides the flow characteristics with the least uncertainty.
A comparison with speed and temperature obtained through absorption of
star light averaged over several light years indicates that these values
appear to be typical for the entire cloud, for which other observations
show that the Sun is located close to its edge.
The He density has been established to be 0.0151±0.015 cm-3 (Ulysses
SWICS), 0.015±0.03 cm-3 (Ulysses GAS), 0.013±0.03 cm-3 (EUVE), with the
pickup ions providing the smallest uncertainty.
Together with the recent observations by Voyager 1, indicating that this
pioneering spacecraft is close to the termination shock or may have even
crossed it temporarily (the termination shock is the boundary where the
solar wind is quickly decelerated to subsonic speed) this consistent and
accurate set of parameters is now beginning to provide strong
constraints on models that describe the size and structure of the
A new team hosted by ISSI has started to extend this effort to hydrogen,
a more complex task because about half of the hydrogen atoms do not
penetrate the outer boundary of our solar system, which strongly affects
Science Team hosted by the International Space Science Institute (ISSI)
E. Mˆbius (Lead),1 M. Bzowski,2 S. Chalov,3 H.-J. Fahr,4 G.
Gloeckler,5,6 V. Izmodenov,7 R. Kallenbach,8 R. Lallement,9 D.
McMullin,10 H. Noda,11 M. Oka,12 A. Pauluhn,8 J. Raymond,13 D.
RuciÒski,2? T. Terasawa,12 W. Thompson,15 J. Vallerga,16 R. von
Steiger,8 M. Witte17
(1) Dept. of Physics and Space Science Center, University of New
Hampshire, Durham, NH 03824, U.S.A.
(2) Space Research Centre, Warsaw, Poland
(3) Institute for Problems in Mechanics, Russian Academy of Sciences,
(4) Institut f¸r extraterrestrische Forschung, Universit‰t Bonn, Bonn,
(5) Dept. of Physics and IPST, University of Maryland, College Park, MD
(6) Dept. of Atmospheric, Oceanic and Space Sciences, University of
Michigan, Ann Arbor, MI
(7) Department of Mechanics and Mathematics, Moscow State University,
(8) International Space Science Institute (ISSI), Bern, Switzerland
(9) Service d’AÈronomie du CRNS, VerriËres-le-Buisson, France
(10) Praxis, Inc., Alexandria, VA Space Science Center, University of
Southern California, Los Angeles, CA
(11) National Astronomical Observatory of Japan, Misuzawa, Japan
(12) University of Tokyo, Tokyo, Japan
(13) Harvard Smithsonian Center for Astrophysics, Cambridge, MA
(14) Goddard Space Flight Center, Greenbelt, MD
(15) Space Sciences Laboratory, University of California Berkeley,
(16) Max-Planck-Institut f¸r Aeronomie, Katlenburg-Lindau, Germany
Authors: Eberhard Mˆbius, George Goeckler and Rosine Lallement
The complete results of the team meetings held at the ISSI will be
presented in a series of 7 articles in an upcoming issue of the
Astronomy & Astrophysics journal. The articles are:
Synopsis of the interstellar He parameters from combined neutral gas,
pickup ion and UV scattering observations and related consequences, by
E. Mˆbius, M. Bzowski, S. Chalov, H.-J. Fahr, G. Gloeckler, V.
Izmodenov, R. Kallenbach, R. Lallement, D. McMullin, H. Noda, M. Oka, A.
Pauluhn, J. Raymond, D. Rucinski, R. Skoug, T. Terasawa, W. Thompson, J.
Vallerga, R. von Steiger, and M. Witte.
Kinetic parameters of interstellar neutral Helium: Review of results
obtained during one solar cycle with Ulysses GAS, by M. Witte
Observations of the Helium focusing cone with pickup ions, by G.
Gloeckler, E. Mˆbius, J. Geiss, M. Bzowski, H. Noda, T. Terasawa, M.
Oka, D. McMullin, S. Chalov, H. Fahr, D. Rucinski, R. von Steiger, A.
Yamazaki, and T. Zurbuchen
EUVE observations of the Helium glow: Interstellar and solar parameters,
by J. Vallerga, R. Lallement, J. Raymond, M. Lemoine, B. Flynn, F.
Dalaudier, and D. McMullin
Solar cycle dependence of the Helium focusing cone from SOHO/UVCS
observations, by R. Lallement, J. Raymond, J.-L. Bertaux, E. Quemarais,
Y.-K. Ko, M. Uzzo, D. McMullin, and D. Rucinski
Modelling the interstellar-interplanetary Helium 58.4 nm resonance glow:
Towards reconciliation with particle measurements, by R. Lallement, J.C.
Raymond, J. Vallerga, M. Lemoine, F. Dalaudier, and J.L. Bertaux
Heliospheric conditions that affect the interstellar gas inside the
Heliosphere, by D.R. McMullin, M. Bzowski, E. Mˆbius, A. Pauluhn, R.
Skoug, W. T. Thompson, M. Witte, R. von Steiger, D. Rucinski, M.
Banaszkiewicz and R. Lallement