Per una missione STEREO che comincia a dare i primi risultati, è tempo di pensionamento per la sonda Ulysses, lanciata 17 anni fa da ESA e NASA proprio per studiare il sole e la sua eliosfera. C'è uno splendido articolo sul sito dell'ESA da cui ricavo i principali risultati ottenuti dalla missione dichiarata ufficialmente chiusa il primo di luglio. Altro materiale, qui.
Ulysses has shown that the heliospheric magnetic field is more complicated than had been thought: its field lines follow a chaotic pattern rather than a more regular Archimedean spiral and are also more spread out in latitude than was expected. The behaviour of the Sun's magnetic dipole was by contrast found to be more simple than expected: it simply rotates through 180° from sunspot minimum through sunspot maximum and back to sunspot minimum again in order to achieve the polarity reversal that has been observed.
Ulysses observed the solar wind in 4-dimensions and demonstrated the bimodal nature of the wind: it discovered that a steady fast wind (~750 km/s) is present throughout most of the solar cycle and only vanishes at solar maximum when the polar coronal holes disappear. The slow wind (~350 km/s) emerges from the Sun's equatorial zone. The transition from fast to slow is relatively abrupt.
Prior to Ulysses energetic particles had been studied in great detail near the equator, but not at higher latitudes. Ulysses has shown that such particles are present at all phases of the solar cycle. They can be found at all latitudes at solar maximum and are shown to be confined to the inner heliosphere in 'reservoirs' from which they slowly escape. During solar minimum energetic particles can reach into the polar caps - contrary to what was expected.
The distribution of galactic cosmic rays was shown by Ulysses to be essentially spherically symmetric with the flux being the same at the equator and in the polar regions.
Ulysses measured the abundances of many radioactive cosmic-ray isotopes, and showed that these cosmic rays spend much of their lifetime (typically ~20 million years) in the galactic halo before reaching us.
Ulysses directly measured the properties of interstellar gas (neutral helium) for the first time. It also showed that the interstellar wind is fast enough to create a bow shock upstream of the heliosphere.
Dust particles of interstellar origin were detected by Ulysses proving that they can enter the heliosphere. Large dust grains, with an average mass 30 times greater than expected, were also detected. These contribute ~2% to the total mass of the interstellar medium.
Measurements by Ulysses of interstellar helium isotope abundances at the current epoch are consistent with Big Bang theory of light element formation and with a matter density supporting an 'open' Universe.
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