The Juno Ring and Normal Modes

The juno ring is an elongated halo that surrounds Jupiter. The ring extends from the inner edge of the main Jovian ring, near its boundary with the solar wind, to about 10000 km over its rim plane.

Several models have been proposed to explain the ring’s structure and origin. Most of them involve Kelvin-Helmholtz instabilities, in which plasma from the solar wind interacts with the Jovian magnetosphere. Other models suggest that dayside magnetic reconnection events produce the ring’s emissions, in which oppositely directed Jovian and interplanetary magnetic fields converge and rearrange, reconnecting.

However, no model is consistent with the occurrence of a high-latitude halo. It is also difficult to detect normal modes inside a gas giant like Jupiter, which has a complex and dynamic interior. The detection of normal modes has important implications for the exploration of the interior of gas giants, and may help to understand their dynamical structure in general.

This paper investigates the amplitudes of normal modes in the planet’s surface using Juno’s Doppler observations. We fit the data to a model based on a multi-arc least square estimation filter. For each arc, we solve for local parameters that represent the spin axis initial position and velocity, the polar moment of inertia factor, and the normal modes amplitudes. The amplitudes of the modes are then transformed into a corresponding GM (gravitational perturbation).

We find that the gravity acceleration produced by f-modes is larger than that produced by p-modes at any surface amplitude, which implies that f-modes penetrate deeper into the planet’s structure. They are thus more likely to produce the ring’s anomalous gravity acceleration.

A key feature of this amplitude is that it has a nonlinear dependence on the velocity and surface amplitude of the ring. This property is reminiscent of the behavior of the Sun’s juno ring system, in which modes have a larger amplitude at lower velocities. We also find that a significant fraction of the mass displaced by f-modes is absorbed by the ring. The resulting energy is dissipated in the ring, which produces an increase in its size and density.

Another feature of the ring’s amplitude is that it has a relatively small diameter. This is in line with the radial structure of the Jovian ring, which is shaped by its tesseral harmonics. We also observe that the amplitude of normal modes in the ring is much larger than it would be without the tesseral modulation.

Our results imply that the presence of normal modes inside Jupiter is a real possibility. This is a crucial step for the future exploration of the interior of this class of bodies, as well as the discipline of seismology.

NASA’s Juno mission has been exploring Jupiter since July 2016. This summer, the agency’s most distant planetary orbiter will undergo an extended mission to become an explorer of the full Jovian system, with multiple flybys of three of its most intriguing Galilean moons: Ganymede (2), Europa (3) and Io (11). In addition, the spacecraft will continue its exploration of the planet’s atmosphere, deep environment and magnetic field, as well as Jupiter’s polar cyclones and aurora.