ESA Science & Technology30-Jun-2005 16:32:01

Background Science

Interstellar Gas and Dust

Schematic showing the characteristic distances in the heliosphere and
local interstellar medium on a logarithmic scale, measured in
Astronomical Units (AU). 1 AU = 150 million km.

As the strength and level of solar activity varies in time, so does the size and behaviour of the heliosphere. It would probably extend outwards from the Sun indefinitely if the solar wind did not come up against a barrier, a conflicting stream of charged particles coming from interstellar space. The solar system is at the edge of an interstellar cloud that is moving independently of the rotation of the Milky Way galaxy. The solar wind is brought to a halt when the pressure of its outward flow is matched by that exerted by the local interstellar medium (LISM).

Because the solar wind is magnetized, its presence is felt by all charged particles, making it difficult for them to enter the heliosphere. To the neutral atoms that make up a significant part of the local interstellar cloud, however, the solar wind is largely invisible. Affected only by gravity and the pressure of sunlight, many of these neutrals are able to penetrate far into the heliosphere before finally interacting with a charged particle in the solar wind and becoming ionized by losing an electron. They then start a new life as so-called "pick-up ions", becoming embedded in the expanding solar wind. Measurements of interstellar gas, either directly as neutral atoms, or indirectly as pick-up ions, can provide information about the LISM surrounding the heliosphere, as well as conditions at the boundary itself.

Once they have been converted into pick-up ions, interstellar gas atoms can take on a third identity, masquerading as cosmic rays. The pick-up ions interact with the termination shock, which acts as a huge accelerator. They then travel back towards the Sun, mimicking galactic cosmic rays.

They have been given the name "anomalous" cosmic rays. Ulysses has made the first ever direct measurement of neutral helium and extensive measurements of a wide range of previously undetected pick-up ions. With these observations, scientists have been able to identify the particular local interstellar cloud in which the Sun resides.


The following instruments on Ulysses make measurements of interstellar gas and dust:  

  • GAS (Interstellar Neutral Helium experiment)  
  • DUST (Cosmic Dust experiment)  
  • SWICS (Solar Wind Ion Composition Spectrometer)

The following instruments on Ulysses measure Anomalous Cosmic Rays:  

  • HI-SCALE (Heliosphere Instrument for Spectra, Composition and Anisotropies at Low Energies)  
  • EPAC (Energetic Particle Composition experiment)  
  • COSPIN (Cosmic Ray and Solar Particle Investigation)

GAS (Interstellar Neutral Helium GAS experiment)
GAS is a new instrument, flown for the first time in space on board Ulysses. It detects interstellar neutral helium atoms that penetrates the heliosphere, and measures the flow speed, the temperature, and density of the gas.

DUST (Cosmic DUST experiment)
The DUST experiment measures the mass and arrival direction of dust particles of interplanetary and interstellar origin. DUST discovered the presence of interstellar dust grains in the inner heliosphere.

SWICS (Solar Wind Ion Composition Spectrometer)
In addition to solar wind ions, SWICS measures the elemental and ionic-charge composition, temperature, and mean speed of interstellar pick-up ions. The pick-up ion data are being used to infer the composition of interstellar gas elements that are otherwise inaccessible.

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