Solar TErrestrial RElations Observatory (STEREO)

STEREO is a NASA space mission that consists of two satellites with nearly identical instrumentation. One satellite leads ahead of Earth on its orbit around the Sun, and the other one lags behind. This configuration enables a three-dimensional picture of the solar atmosphere and the interplanetary space between Sun and Earth.

This way, it is possible to investigate spatial structures in the solar corona. Of special interest are coronal mass ejections (CMEs). CMEs are associated with enhanced emission of energetic particles (protons and electrons). If a CME impacts on the terrestrial magnetosphere, it can trigger a geomagnetic storm. Energetic particles can damage satellites, and endanger astronauts who are outside of Earth's magnetosphere, e.g. on their way to the Moon or Mars. Geomagnetic storms can disturb navigation systems, and induce strong electric currents in power lines and pipelines. These solar-terrestrial relations are usually called "space weather".

STEREO's main scientific objectives are:

  • Understand the cause and mechanism of CME formation
  • Describe the propagation of CMEs through the heliosphere
  • Discover mechanisms and locations of particle acceleration in the low corona and in interplanetary space
  • Improve our understanding of the solar wind

STEREO's Instruments

In order to better understand CME release and propagation, and to reach the abovementioned objectives, the STEREO satellites are equipped with diverse instruments that observe the Sun in visible and ultraviolet light, receive radio emission, register energetic particles, and analyse the composition of CME plasma. The instruments are combined in the following groups:

  • SWAVES (STEREO/WAVES) consists of radio receivers that enable to trace solar radio bursts from the corona up to Earth's orbit. For this purpose, SWAVES covers the frequency range of 16 MHz - 40 kHz, and allows for the determination of intensity, direction, and spatial extension of the radio source. Additionally, it contains receivers for electromagnetic waves around the local plasma frequency, i.e. 10 kHz - 40 kHz, that study the plasma environment of the satellites. Furthermore, there is a receiver at 50 MHz that supports ground-based observations, and Time Domain Samplers that determine the electric field vector.
  • SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation) observes the Sun and the heliosphere with telescopes for visible and ultraviolet light. SECCHI consists of two white light coronographs, a heliospheric imager, and a EUV (extreme ultraviolett light) imager. These instruments investigate the three-dimensional development of CMEs from their release in the solar corona, their journey through interplanetary space, up to their potential impact on Earth.
  • IMPACT (In-situ Measurements of Particles and CME Transients) studies the three-dimensional velocity distribution function of electrons in the solar wind, energetic electrons and ions, and the magnetic field vector. Thus, IMPACT enables observations of CME plasmas as the CME passes the STEREO satellites.
  • PLASTIC (PLAsma and SupraThermal Ion Composition) investigates the properties of protons, helium ions (alpha particles), and heavy ions. This way, it is possible to study the distribution of ion masses and charge states in CMEs, and to draw conclusions on their origin.

AIP's contribution

The AIP's participation in the project refers to the instrument SWAVES. Since radio waves with frequencies below 10 MHz cannot pass the Earth's ionosphere, SWAVES data are an ideal supplement to ground-based observations. Since the frequency of solar radio emissions decreases with increasing height of the source in the solar atmosphere, SWAVES observations enable studies of the upper corona and its transition into interplanetary space.     

For a better understanding of the physical processes of CME release and propagation, it is very helpful to combine the radio observations with other instrument data. An example for such complementary data are ground-based radio data at higher frequencies, like they are gained by the AIP's Observatory for Solar Radioastronomy at Tremsdorf. They enable an investigation of the early phase of the CME in the lower corona. Even lower solar atmospheric layers are accessible to optical observationis, e.g. the chromosphere in H-alpha. Another example are data from STEREO's particle detectors. Since CME speeds exceed local sound- and Alfven velocities, CMEs are associated with shock waves that accelerate electrons and protons. These electrons are responsible for radio wave emission, but a direct detection of the particles provides a deeper understanding of CME structure.

The task of our working group is the development of data analysis tools for the joint interpretation of SWAVES data together with other sources like those listed above. Such tools will facilitate the analysis and interpretation of the observations considerably.