The observed direction of the interstellar neutral atoms will be in the direction of their velocity vectors (tangent to the orbit); the magnitude of the velocity vectors can be found from conservation of energy. After computing solar and spacecraft

1
SH22C-05 Direct Observations of Interstellar
Neutral Atoms from IMAGE
D.G. Simpson, M.C. Collier, T.E. Moore, M-C. H.
Fok (NASA/GSFC) S.A. Fuselier (Lockheed Martin)
P. Wurz (University of Bern) Contact David G.
Simpson, Code 692, NASA Goddard Space Flight
Center, Greenbelt, Maryland 20771
(David.G.Simpson.1_at_gsfc.nasa.gov) URL
http//lena.gsfc.nasa.gov
Abstract The Imager for Magnetosphere-to-Aurora
Global Exploration (IMAGE) spacecraft was
launched in early 2000. Its Low-Energy Neutral
Atom (LENA) imager, designed primarily for remote
sensing of terrestrial neutral atom emissions
generated by plasma heating, has an energy range
of 10 to 1000 eV. We report direct LENA
observations of interstellar neutral atoms
detected by the LENA imager during certain times
of the year when the IMAGE orbit geometry is
favorable. This is when the spacecraft is
downwind of the Sun, within the Suns
gravitational focusing cone for interstellar
neutral atoms, and beginning to turn into the
upwind direction. Comparison of LENA
observations with numerical models indicates that
the LENA instrument has detected primarily
interstellar neutral helium, which can be
observed by sputtering of adsorbed atoms from the
LENA instruments conversion surface. A summary
of the LENA observations will be presented, along
with a description of the numerical models with
which they have been compared.
1. The IMAGE/LENA Instrument
3. First LENA Observations of Interstellar
Neutral Atoms
2. Interstellar Neutral Atoms
Launched on March 25, 2000, the IMAGE spacecraft
was designed to take global images of the Earths
magnetosphere by monitoring terrestrial
ultraviolet, radio, and neutral atom emissions.
IMAGE is in an elliptical polar orbit, with its
perigee near the Earths south pole. The
spacecrafts altitude varies between 1000 km at
perigee to 7 Earth radii at apogee.
As the Solar System moves through interstellar
space, interstellar neutral hydrogen and helium
atoms flow past the Sun, forming a wind of
interstellar neutral particles. These
interstellar neutral (ISN) atoms execute
hyperbolic orbits around the Sun, as shown here.
Solar radiation pressure (? 1/r2) primarily
affects the less massive hydrogen atoms and acts
to effectively weaken the gravitational force.

Shown here is a LENA spectrogram for January 20,
2001. The three panels show start counts, stop
counts, and coincidences for the LENA imagers
time-of-flight detector. A vertical section
through the spectrogram at any time shows a
360-degree panoramic neutral atom image in the
spacecraft orbit plane at that time. The thick
red band at the center of each panel is due to
penetrating radiation at perigee. The
time-of-flight detectors stop counts are most
sensitive to small signals (center panel). Note
the appearance of interstellar neutral signals
(labeled ISN). This signal was observed in
December 2000 and January 2001, during the time
of year when the Earths motion is in the
upwind direction relative to the interstellar
neutral atom orbits.
IMAGE includes among its six instruments a
low-energy neutral atom (LENA) imager, designed
to detect neutral atoms with energies in the
range of 10 to 1000 eV. Shown here is a diagram
of the LENA instruments ion optics. Neutral
atoms first pass through a collimator with
electrically charged plates to eliminate charged
particles (upper left of figure). The remaining
neutral particles then strike a tungsten
conversion surface, where they become ionized by
charge exchange with adsorbates on the conversion
surface. The ions are then accelerated through a
hemispherical electrostatic analyzer (bottom) and
into a time-of-flight detector, where the mass,
incident direction, and energy can be recorded.
Species observed by the LENA imager are
typically H- and O-, since these have stable
negative ion states. Although helium is not
readily ionized and thus not observed directly,
it can be detected indirectly through sputtering
of adsorbates from the conversion surface.
As they approach the Sun, interstellar
neutral atoms may become ionized. The dominant
ionization mechanism for hydrogen is charge
exchange with solar wind protons helium atoms
will tend to be photoionized by solar ultraviolet
radiation. In both cases, the ionization
probability is inversely proportional to the
square of the distance from the Sun.
Interstellar neutral atoms pass through the Solar
System with initial speeds of 20-25 km/s (before
being accelerated by the Suns gravity). They
are believed to originate from a point near the
direction of the solar apex the ecliptic
coordinates of the upwind direction are ?252?,
?7?.
5. Numerical Models (Contd)
4. Numerical Models
6. Density and Flux Models
.
A computer model may be used to calculate the
orbits in individual particles around the Sun as
a means of finding expected number densities and
flux densities of interstellar neutral atoms in
the inner Solar System. In this model, we begin
with a one-dimensional grid of particles
perpendicular to the flow velocity at some large
(50 AU) distance from the Sun. The orbit of
each particle is then propagated through the
Solar System using an orbit integrator that
includes the effects of gravity, radiation
pressure, and losses due to photoionization and
charge exchange. As each particle is
propagated in its orbit, its position is recorded
at regular time intervals in a two-dimensional
10-AU square grid. The result yields a contour
plot of the density of interstellar neutral
particles, as shown in the upper figure for
helium. The Sun is at the center of the figure,
the orbit of the Earth is indicated by 1-AU white
circle, and the interstellar neutral flow is in
the x direction. The minimum near the center
(dark blue) is due to modeled photoionization and
charge exchange losses. In the lower figure,
the calculated helium number density is plotted
along the Earths orbit. Note the density
secondary maximum in the downwind (?0) direction
due to gravitational focusing of the particles
behind the Sun.
A numerical model of the orbits of interstellar
neutral atoms around the Sun can be constructed,
and the results compared with the observed
position of the LENA signal. In the model used
here, the equations of the hyperbolic LENA orbits
are found analytically the initial energy gives
the semi-transverse axis, and the semi-conjugate
axis (impact parameter) is found by solving a
fourth-order polynomial involving the
semi-transverse axis and the position of the
Earth. The polynomial solution then yields the
impact parameters for those hyperbolae that will
intersect the Earth on a given date.
Allowance for solar radiation pressure is made
through a parameter ? (the ratio of the solar
radiation force to gravitational force). Since
the radiation pressure is, like gravity,
proportional to 1/r2, it can be modeled by
weakening the gravitational force by a factor
of (1- ?). Typical values of the parameter ? are
0.8 - 1.0 for hydrogen, and 0.0 for helium.
The numerical models may be run over extended
periods of time to investigate the time
dependence of parameters important to the
observation of interstellar neutral atoms. Shown
here is a plot of the predicted velocity of
interstellar neutral helium atoms with respect to
the spacecraft over a period of six months. The
thick band-like nature of the plot is due to the
motion of the spacecraft in its elliptical orbit
about the Earth, which leads to large
short-period variations in the spacecraft
velocity for each 14-hour orbit. Removing the
spacecraft motion from the calculation leads to
the white dashed line shown, so that this line
includes only the motions of the neutral atoms
and the Earth. Note that the maximum
relative velocity occurs in the early part of the
year (mid-December to mid-February), when the
Earth is moving in the interstellar upwind
direction. This is the time of year during which
interstellar neutral atoms are visible by the
LENA imager.
The observed direction of the interstellar
neutral atoms will be in the direction of their
velocity vectors (tangent to the orbit) the
magnitude of the velocity vectors can be found
from conservation of energy. After computing
solar and spacecraft ephemerides, we can find the
velocity of the IMAGE spacecraft with respect to
the interstellar neutral atoms by adding the
relative velocities of the neutral atoms, Earth,
and spacecraft When the resulting velocity
vector is transformed into the spacecraft
reference frame, its direction may be overlaid on
a spectrogram from the LENA instrument, as shown
here by the black line marked ISN LENA. This
calculation was done for interstellar helium
atoms for January 25, 2001. Note the close
agreement of the predicted ISN position with the
observed signal (light green). The predictions
for helium are found to match the observed signal
more closely than those of hydrogen.
At any given time, the interstellar neutral
signal seen in the LENA imagers data may be fit
to a Gaussian curve to find the peak observed
intensity at that time. By plotting these peak
intensities as a function of time over a
two-month period (as shown here), we find a
maximum in the observed intensity of the
interstellar neutral signal in mid-January 2001.
Error bars are due to statistical uncertainties
in the Gaussian fits.
Current work is being directed toward modeling
the flow of interstellar neutral atoms through
the inner Solar System in an effort to calculate
expected flux densities and compare them with
these observations, as described in the following
section.
Conclusions
Computer models of the orbits of interstellar
neutral atoms about the Sun have been created
they indicate that the signal seen in the LENA
imagers spectrograms in early 2000 is consistent
with the expected position of interstellar
neutral helium. Because of the geometry of the
orbits, interstellar neutral atoms are visible in
the LENA imagers data during the early part of
the year, when the Earths velocity vector is
nearly anti-parallel to the interstellar neutral
atoms velocity vectors, thus giving a maximum
relative velocity.