Location and shape of the Jovian magnetopause and bow shock

Abstract

Abstract. Following Galileo's arrival at Jupiter in fall 1995, a total of six spacecraft have now sampled the Jovian magnetosphere. Using these data sets to investigate the average location and shape of the Jovian boundaries, we fit ellipse profiles to the observations, allowing for the disk-like shape of the magnetosphere and taking account of variable solar wind pressure. We find that the subsolar magnetopause location varies with solar wind dynamic pressure to power between -1/5 and -1/4, in contrast to the terrestrial -1/6 power; this is a well-known difference attributed to the presence of hot plasma and centrifugal stretching in the Jovian magnetodisk that lessens the pressure gradients in the outer magnetosphere, resulting in its unusual responsiveness to compression. The magnetopause is less flared than the bow shock as expected, and the magnetopause shape is especially streamlined (least flared and more bulletlike) during the higher solar wind dynamic pressure conditions encountered. The average subsolar shock-to-magnetopause standoff ratio is approximately 6/5, while at low incident solar wind dynamic pressure the ratio rises to around 4/3 suggesting a blunter Earth-type magnetopause shape under these conditions. In particular, our analysis confirms that the magnetopause boundary shape is influenced by the radially inflated magnetodisk, as has been previously inferred from the stretched magnetic field lines seen within the magnetosphere. Our fits to the observations reveal that the average magnetopause boundary is indeed contracted on the northsouth axis about the magnetic equator. The bow shock is not found to be so asymmetric in shape, suggesting that there is little effect of external magnetic field direction, and supporting our conclusion that the internal magnetodisk shape is the cause of the magnetopause polar flattening.

Polar confinement of Saturn's magnetosphere revealed by in situ Cassini observations

“…This contrasts with the study by Huddleston et al [1998] who found the Jovian magnetosphere to be more streamlined at high D P .…”

“…This allowed us to normalize the crossings to a fixed pressure so that we could fit models to the data to determine if the magnetosphere of Saturn exhibits polar flattening as has been observed at Jupiter by Huddleston et al [1998]. Even so, a considerable amount of scatter is present in the data, and further measures were taken to reduce this by comparing two different pressure estimates for each crossing and removing those where these estimates deviate by more than a factor of 3.…”

“…Crossings within 1 h of each other were averaged together, as it is likely that these were caused by boundary waves in the magnetopause surface. Temporal averaging of magnetopause crossings has been carried out in various forms by, e.g., Slavin and Holzer [1981], Slavin et al [1983Slavin et al [ , 1985, and Huddleston et al [1998]. Arridge et al [2006] decided instead to average the crossings spatially to account for the different spacecraft velocities of Cassini and Voyager.…”

“…Polar flattening of the Jovian magnetosphere was observed by Huddleston et al [1998] using data from the Galileo, Ulysses, Voyagers 1 and 2 and Pioneers 10 and 11. They concluded that the disk-like shape of the obstacle was the cause of the apparent flattening along the north-south axis.…”

Magnetopause reconnection rate estimates for Jupiter's magnetosphere based on interplanetary measurements at ~5AU

“…Here we therefore take L o to be half the subsolar radius of Jupiter's magnetosphere R mp , a value which is dependent on the dynamic pressure of the solar wind. Huddleston et al (1998) give an empirical formula for this dependence, that is…”