Recent Magnetic Field Results from the Galileo and Ulysses Spacecraft [and Discussion]

Both Galileo and Ulysses spacecraft have made important exploratory measurements before either start the main phase of their missions. Galileo has flown by several objects in the inner Solar System before its reaching Jupiter. The most notable results from the spacecraft magnetometer are the detection of magnetic field deflections in the vicinity of the two asteroids, Gaspra and Ida, that the spacecraft has flown by. The signatures are not the result of a direct sensing of an internal asteroid field. The asteroid disrupts the solar wind flow by emitting low-frequency waves and these form the signature that the spacecraft detects. The size of the disrupted region set up by Gaspra has led the Galileo magnetometer team to propose that the asteroid may have a substantial dipole moment, a result that raises substantial questions about how and where the object cooled. Ulysses not only sent continuous data back from its flight out to 5 AU in the ecliptic plane but also flew past Jupiter as a prelude to its climb out of the ecliptic in polar solar orbit. Despite being the fifth spacecraft to visit Jupiter, Ulysses in 1992 has produced some surprising new information. For example, the null field regions first identified by Ulysses and then discovered not only in the earlier data sets but also in the Voyager data at Saturn, indicate that the current sheet appears to sporadically shed material at its outer edge. The contrast between the field and plasma environment detected on the inbound (morning sector) and outbound (dusk) pass of Ulysses raises challenging questions about how much acceleration occurs as material rotates around the dayside of Jupiter.