1. 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. 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/r 2 ) primarily affects the less massive hydrogen atoms and acts to effectively “weaken” the gravitational force. 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@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 Sun’s 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 instrument’s 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 Launched on March 25, 2000, the IMAGE spacecraft was designed to take global images of the Earth’s magnetosphere by monitoring terrestrial ultraviolet, radio, and neutral atom emissions. IMAGE is in an elliptical polar orbit, with its perigee near the Earth’s south pole. The spacecraft’s altitude varies between 1000 km at perigee to 7 Earth radii at apogee. 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 instrument’s 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. 3. First LENA Observations of Interstellar Neutral Atoms Shown here is a LENA spectrogram for January 20, 2001. The three panels show start counts, stop counts, and coincidences for the LENA imager’s 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 detector’s “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 Earth’s motion is in the “upwind” direction relative to the interstellar neutral atom orbits. 5. Numerical Models (Cont’d) 2. Interstellar Neutral Atoms 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/r 2, 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. Conclusions 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 Sun’s 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 . 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. 4. Numerical Models. 6. Density and Flux Models Computer models of the orbits of interstellar neutral atoms about the Sun have been created; they indicate that the signal seen in the LENA imager’s 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 imager’s data during the early part of the year, when the Earth’s velocity vector is nearly anti- parallel to the interstellar neutral atoms’ velocity vectors, thus giving a maximum relative velocity. At any given time, the interstellar neutral signal seen in the LENA imager’s 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. 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 Earth’s orbit. Note the density secondary maximum in the downwind (  =0) direction due to gravitational focusing of the particles behind the Sun. 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.