Location and shape of the Jovian magnetopause and bow shock

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 bullet‐like) 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 north‐south 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.