STEREO - An early warning system in Space

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Image 1: The space probe with its instruments

STEREO – an early warning system in space

The earth is at the mercy of the sun’s moods: when violent gas explosions occur on the sun’s surface, an especially strong solar wind is thrown into the universe. If it hits the earth, it doesn’t only cause beautiful northern lights, but it also can black out a region’s communication or even the electricity grid. More than this, there is also a risk of satellite failure. And still more, small penetrating particles present a serious threat to our health – especially for astronauts working in space and people in high flying airplanes

  • NASA’s twofold mission: two eyes in space will trace coronal mass ejections in three dimensions from the sun to the arrival on earth. They will improve the weather forecast of space and warn about particle and magnetic currents
  • Space probe: built form the Applied Physics Laboratory/ The Johns Hopkins University, USA
  • Launch: summer 2006 from Kennedy Space Center, Cape Canaveral with a delta rocket. With the help of the moons pull, both of the probes will leave earth’s gravity and enter one orbit around the sun. One will move ahead of the earth and one will lag behind. The stereoscopic images of gas explosions on the sun and the directional information of the accompanying particle currents should already be possible after a few months.
  • Mission length: 2-year prime mission, 5-year extended mission

The scientific goals of STEREO

Coronal mass ejections on the sun’s surface are the most powerful manifestations of explosive physical processes in our solar system. The STEREO mission should contribute to a better understanding of these in the following areas:

  • Solar origin and development of coronal mass ejections
  • Dispersion of the ejections and interference of the sun on earth
  • Acceleration mechanics of the high energy solar particles
  • 3-dimentional structure and dynamics of the corona and the heliosphere

Kiel’s Contribution

  • SEPT (Solar Electron and Proton Telescope):
    16 detectors will collect the high energy material particles, electrons and protons, and differentiate them according type, energy, and direction of arrival for the energy ranges: protons 60 – 7000 keV, electrons 20 – 400 keV. The sophisticated experiment can separate the protons from the electrons, which makes up 90 percent of the particle radiation, through a complex arrangement of ultra-thin foils and ultra-strong permanent magnets. Because a proton is 1800 times heavier then an electron, the differences in their energies reveals information about the mechanics of their acceleration. Protons also cause more damage, and therefore the knowledge of the particle radiation’s composition is very important for a „storm warning“.
    Scientist hope to gain an important understanding from the instrument SEPT about:
    • Energy release and acceleration of particles in the sun’s atmosphere
    • Acceleration and dispersion of particles in interplanetary space
    • Correlation and measurement of optical and radio wavelengths
  • PLASTIC (Plasma and Suprathermal Ion Composition):
    The PLAsma and SupraThermal Ion Composition (PLASTIC) instrument is a linear time-of-flight mass spectrometer that will measure fundamental plasma parameters (density, speed and temperature of protons and alpha particles) and the charge conditions in the solar wind. Furthermore, it will determine the composition of the suprathermal particles. A large dynamic range (>106) is required for this which will be provided by three different entrances:
    • S-Channel: This small opening is for the measuring of the most abundant ions in the solar wind, protons (normally 95%) and alpha particles (normally 4%). In the solar wind, all particles have about the same speed and a different kinetic energy per charge, according the mass and charge. The greater the mass, the higher the energy per charge. Therefore, protons and alpha particles appear with low energies per charge. This allows them to be separated from much less common heavy ions directly after entering the instrument.
    • Main Channel: This large opening allows the composition of the less common (typically 1%) heavy ions of the solar wind to be determined with a good counting rate, and therefore with a large time interval. If the counting rate is too high, it will switch to the S-channel which will lower by several magnitudes (>106) with the gate electrodes the ions that entered through the main channel. .
    • Wide-Angle Partition (WAP): The suprathermal population in the solar wind can enter the instrument on all sides while the solar wind flies radially away from the sun. In this way, the extremely scarce particle populations can be clearly shown with the WAP.
    An ion is fully characterised for our use by three values: speed (and therefore kinetic energy), mass and charge. These can be measure by PLASTIC. It uses a combination of three measurements for this: time-of-flight and energy measurements and (Eq) is recorded. An ion enters into the entrance system and can only pass through if its energy per charge (Eq) is within the passband of the toroid-curved entrance plate (top hat) under high voltage (0 to ± 7 kV). Afterwards it is further accelerated through a potential difference of up to 25 kV and strikes a very thin carbon foil. As it passes through, the ion induces the emission of electrons that strike a start-detector (micro channel plates (MCPs)) and initiate the time-of-flight measurements. When it strikes a solid-state detector (SSD), secondary electrons are emitted which are measured by the stop-detector and end the time-of-flight measurement. In the SSDs, the ion energy is measured. The (vector) velocity can be determined by separate directional measurements.