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The Heliosphere

2 July 2005, edited by Tinu

The following is a slightly edited excerpt of the first chapter of my PhD-thesis. The full text is available as PDF file.

The Insterstellar Interaction

The solar system is immersed in a large cloud of low density gas, the Local Interstellar Medium (LISM). The solar wind moving supersonically at speeds between 300 and 800 km/s creates a cavity in the LISM, the heliosphere. Relative to the LISM, the Sun moves at about 26 km/s. At the outer edge of the solar system, at a distance of 100 to 300 astronomical units (AU) depending on model, the solar wind continually rams into the LISM, creating the ’interstellar interaction’. Three distinct boundaries are shown in the figure below; the termination shock where the solar wind slows down from a supersonic to a subsonic flow, the heliopause which separates the solar wind plasma from the LISM plasma, and possibly a bow shock or bow wave beyond which the LISM flow is unperturbed by the heliosphere.

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The Heliosphere

The sun moves to the right at about 26km/s. Interstellar neutral atoms enter the heliosphere whereas ions get deflected. Neutrals visible from Earth orbit are also produced by charge exchange in the inner heliosheath from solar wind protons, pickup protons, and energetic protons, providing a global image of the interaction region. Image: IBEX proposal.

The LISM is partly ionized but the heliospheric cavity is shielded from the inflow of interstellar ions because the solar wind plasma is highly magnetized compared to the LISM. A fraction of the inflowing neutral interstellar atoms becomes ionized by charge-exchange with ions that pile up at the so-called hydrogen wall. This pile-up occurs because the interstellar flow is slowed down, heated and deflected around the heliopause. The surviving interstellar neutral atoms and neutral atoms produced by charge exchange in the interaction region enter the the heliosphere. At solar distances of a few astronomical units, they become partly ionized by solar ultraviolet radiation or by charge-exchange with the solar wind. These newly created pickup ions gyrate about the interplanetary magnetic field that is frozen into the solar wind and are swept outward from the Sun. Acceleration at the termination shock transforms pickup ions into anomalous cosmic rays (ACR) with energies of 10—100 MeV per nucleon. The other part of the neutral atom population with energies between a few eV to a few keV per atom, can be detected from as close as 1 AU to the Sun as energetic neutral atoms (ENAs), e.g. from an observer in Earth’s orbit or on an interplanetary trajectory in the inner solar system. It is possible therefore to infer the properties of the neutral interstellar gas and the interstellar interaction region by measuring the properties of these ENAs. Such observations of the inflow of neutral helium were done by the neutral interstellar gas instrument (GAS) on the ULYSSES spacecraft. However, only limited data are available for other important species such as hydrogen or oxygen.

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Charge exchange process

A fast ion (black, +) bound to the magnetic field B hits a neutral atom (white, n). The charge is exchanged between the two reaction partners but their respective velocity vectors at the time of interaction are preserved. The neutralized atom (black, n) is no longer bound to magnetic fields an travels away as energetic neutral atom (ENA) on a ballistic trajectory whereas the newly created ion (white, +) may be accelerated and starting to gyrate around the magnetic field lines.

Detection of Energetic Neutral Particles

Detection of low-energy neutral atoms is much more complicated than detecting ions. Available particle detectors, e.g. micro channel plates (MCP) or solid state detectors (SSD), exhibit an energy threshold below which the detection efficiency drops very quickly to very small values. Furthermore, neutral particles are not subject to electric fields, thus standard ion optical elements such as focusing or energy analyzing elements can not be used directly. In a low-energy neutral particle instrument the neutrals would first need to be ionized and then analyzed using conventional mass spectrometry used in space research.

Surface ionization has been identified as the only viable ionization technique to meet the requirements concerning ionization efficiency for the energy range of 10 eV to 1 keV within the limitations imposed by the resources (space, weight, power, etc.) available on a spacecraft. A neutral particle hitting a suitable surface at grazing incidence will undergo charge exchange with this surface and emerge with a certain probability, depending on surface material and species, as negatively or positively charged ion. These ions are then collected, accelerated, and their energy and mass analyzed using conventional ion optical means. For optimal performance, the conversion surfaces used should exhibit a large ionization yield combined with mirror-like reflection properties (i.e., be flat on an atomic scale), be chemically stable, and should not require special treatments as reconditioning or heating. If, in addition to the detection the neutral atom, also the direction where it came from is registered, two dimensional maps or images of the ENA flow can be recorded. ENA imaging has made it possible to remotely image space plasmas as done by the IMAGE satellite for the Earth’s magnetosphere and by ASPERA-3 (an instrument on ESA’s Mars Express spacecraft) for the Martian magnetosphere. ENA imaging will now be used by the IBEX-mission to image the interstellar interactions and interstellar boundaries at the edge of the heliosphere.

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updated 2 July 2005