RECENT MAGNETIC-FIELD RESULTS FROM THE GALILEO AND ULYSSES SPACECRAFT

Both Galileo and Ulysses spacecraft have made important exploratory me asurements before either start the main phase of their missions. Galil eo has flown by several objects in the inner Solar System before its r eaching Jupiter. The most notable results from the spacecraft; magneto meter 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 sub stantial dipole moment, a result that raises substantial questions abo ut how and where the object cooled. Ulysses not only sent continuous d ata back from its flight out to 5 AU in the ecliptic plane but also fl ew past Jupiter as a prelude to its climb out of the ecliptic in polar solar orbit. Despite being the fifth spacecraft to visit Jupiter, Uly sses in 1992 has produced some surprising new information. For example , the null field regions first identified by Ulysses and then discover ed not only in the earlier data sets but also in the Voyager data at S aturn, indicate that the current sheet appears to sporadically shed ma terial at its outer edge. The contrast between the field and plasma en vironment detected on the inbound (morning sector) and outbound (dusk) pass of Ulysses raises challenging questions about how much accelerat ion occurs as material rotates around the dayside of Jupiter.