Pressure anisotropy-driven instabilities regulate the jovian magnetodisk

Jupiter's magnetosphere is often presented as a template for fast-rotating magnetospheres. Distinct from Earth-like, solar wind-driven magnetospheres, it contains an extended magnetodisk encircling the planet. Although the magnetodisk has been studied since the 1970s, its stability and non-equilibrium dynamics remain poorly understood. Here, we present observational evidence for the role of plasma pressure anisotropy-driven instabilities, including the mirror, cyclotron, and firehose instabilities, in these processes. Data from the Juno mission, supported by theoretical analysis, indicate that these instabilities determine the marginal equilibrium states towards which the magnetodisk plasma tends to evolve after being disturbed. Detailed analyses particularly highlight the role of firehose instability, which acts as a key mechanism to dissipate free energy produced by Fermi acceleration during magnetic dipolarizations. Our observations thus suggest that pressure anisotropy-driven instabilities govern the non-equilibrium evolution of the Jovian magnetodisk following disturbances, offering insights into the physics of Jupiter's magnetodisk and magnetosphere.