The fluctuating magnetic field in the middle jovian magnetosphere: initial Galileo observations

The region of the jovian magnetosphere from 10–24 planetary radii is a region of strong, nearly dipolar, magnetic field through which the plasma added to the Io torus must pass as it convects and diffuses outward to the magnetodisk region and ultimately to be lost down the tail. Magnetic fluctuations can be used to diagnose the processes present in this plasma. Herein we examine magnetic fluctuations at frequencies below 0.04 Hz observed in the middle magnetosphere on a single pass of the Galileo spacecraft. These frequencies are generally well below the ion gyro-frequencies in this portion of the magnetosphere. Near perijove, about 10.7 RJ on this orbit, the equatorial region is not more disturbed than off the equator, but it is quite disturbed at the outer edge of this dipolar region near 24 RJ where the magnetic field becomes weak. These fluctuations are quasi-periodic, strongly compressional, and incoherent resembling somewhat the mirror mode. Conversely, the region off the equator is quiet in this outer portion of the middle magnetosphere. In the quasi-dipolar region there are irregular transverse fluctuations everywhere Galileo travels. Compressional transient events are also observed that may mark flux tube interchange. An unusual structure that could be due to the outward convection of Europas wake is also seen. In toto, these fluctuations indicate that the middle magnetosphere is a very dynamic region.