The solar system is immersed in a large cloud of low density gas, the Local Interstellar Medium (LISM). The interaction between the solar wind and this plasma manifests itself in the buildup of the heliosphere. The heliospheric cavity is shielded from the inflow of interstellar ions, since the solar wind plasma is highly magnetized compared to the LISM. Only neutral particles can enter the heliosphere. Due to the motion of the solar system relative to the interstellar medium these neutral particles penetrate deeply into the solar system . The interstellar neutrals have speeds of about 25km/s. In an neutral particle instrument these neutrals would first be ionized and then measured using conventional mass spectrometry used in space research. Surface ionization was identified as the only viable ionization technique to meet the requirements concerning ionization efficiency for the energy range of 10eV to 1keV within the limitations imposed by the resources (space, weight, power, etc.) available on a satellite [2,3,4,5].
To get information about the composition of the neutral particles an ionization surface is combined with a time-of-flight spectrometer. The prototype of the Neutral Interstellar Composition Experiment prototype (NICE) was built to demonstrate the performance of this approach (for details see Chapter 6). Figure 1.1 depicts the schematic diagram of the sensor.
March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland