Sonda Ulysses, 17 anni nella eliosfera

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.

Ai confini del vento solare

La missione STEREO, la coppia di satelliti lanciata per creare immagini stereoscopiche della superficie solare e misurare il flusso di ioni associato alle esplosioni del nostro astro, è riuscita a "fotografare" la zona non visibile con osservazioni tradizionali costituita dal punto in cui il vento solare, caldo e a velocità ultrasoniche, impatta con la materia interstellare. E' la cosiddetta elioguaina (heliosheath) la zona intermedia tra la bolla del cosiddetto termination shock, la soglia raggiunta la quale il vento cominia a decelerare, e l'eliopausa, il guscio più esterno della eliosfera (una turbolenta bolla dai confini molto variabili), dove il vento solare si placa definitivamente.
Le "osservazioni" della coppia di satelliti STEREO sono avvenute con strumenti che misurano il flusso di atomi resi ormai neutri dalla cessione di energia dovuta alle interazioni con la materia interstellare. Questi atomi rimbalzano allontanandosi dalla eliopausa e ritornano verso le strumentazioni STEREO. Sembra che la nuova "immagine" ottenuta sia servita a chiarire il perché le misurazioni effettuate nella elioguaina dalle sonde Voyager presentavano importanti discrepanze a livello di bilancio energetico.

STEREO CREATES FIRST IMAGES OF THE SOLAR SYSTEM'S INVISIBLE FRONTIER

GREENBELT, Md. -- NASA's sun-focused Solar Terrestrial Relations Observatory, or STEREO, twin spacecraft unexpectedly detected particles from the edge of the solar system last year. This helped scientists map the energized particles where the hot solar wind slams into the cold interstellar medium.
The two STEREO spacecraft were launched in 2006 into Earth's orbit around the sun to obtain stereo pictures of the sun's surface and measure magnetic fields and ion fluxes associated with solar explosions. From June to October 2007, sensors aboard both STEREO spacecraft detected energetic neutral atoms originating from the same spot in the sky, where the sun plunges through the interstellar medium.
Mapping the region by means of neutral, or uncharged, atoms instead of light "heralds a new kind of astronomy using neutral atoms," said Dr. Robert Lin, professor of physics at the University of California, Berkeley and lead for the suprathermal electron sensor aboard the STEREO spacecraft. "You can't get a global picture of this region, one of the last unexplored regions of the heliosphere, through normal telescopes," Lin said. The heliosphere is a bubble in space produced by the solar wind. It stretches from the sun to beyond the orbit of Pluto. The solar wind streams off the Sun in all directions at great speeds. Once beyond the orbit of Pluto, this supersonic wind must slow down to meet the gases in the interstellar medium. As the solar wind slows, it changes direction to form a comet-like tail behind the sun. This subsonic flow region is called the heliosheath.
The results, reported in the July 3 issue of the journal Nature, clear up a discrepancy in the amount of energy dumped into space by the decelerating solar wind. The solar wind was detected when Voyager 2 entered the heliosheath.
Researchers determined that the newly discovered population of ions in
the heliosheath contains about 70 percent of the dissipated energy from the solar wind, exactly the amount unaccounted for by Voyager 2's instruments. The Voyager 2 results also are reported in the July 3 issue of Nature. The Berkeley team concluded that these energetic neutral atoms were originally ions heated up in the termination shock area that lost their charge to cold atoms in the interstellar medium and, no longer hindered by magnetic fields, flowed back toward the sun and into the sensors aboard STEREO. "This is the first mapping of energetic neutral particles from the edge of the heliosphere," Lin said. According to Lin, the neutral atoms are probably hydrogen, which comprise most of the particles in the local interstellar medium.
The charge exchange between hot ions and neutral atoms to generate energetic neutral atoms is well known around the sun and planets, including Earth and Jupiter. Spacecraft have used this as a means of remotely measuring the energy in ion plasmas since neutral atoms travel much farther than ions. NASA plans to launch the Interstellar Boundary Explorer, or IBEX, later this year to more thoroughly map the boundary of the solar system.

For more information about NASA's STEREO mission, visit:

http://www.nasa.gov/stereo