Evidence for reformation of the Uranian bow shock: Hybrid simulations and comparisons with Voyager data

The cyclic reformation of shock structures is still a major unresolved issue for collisionless shock physics. We investigate the Voyager 2 spacecraft's encounter with the Uranian bow shock and present the first detailed quantitative evidence that the bow shock was reforming. This evidence is based on finding very good agreement between Voyager observations of the Uranian bow shock and results from a standard 1‐D hybrid simulation code run with similar plasma parameters. Specifically, the multiple large localized magnetic field enhancements B / B 0 ∼ 16 observed downstream by Voyager 2, where B 0 = 0.19 nT is the upstream magnetic field, are quantitatively consistent with the reforming shock found in the simulation. Moreover, the variability and large enhancement factors for B and the number density, as well as the variations of the plasma velocity, and ion temperature over an almost 2‐h period including and after the shock crossing, are consistent with Voyager being very close to the reforming bow shock and with the Uranian bow shock receding planetward with the spacecraft. Additional simulations show the magnetic overshoot factor to increase approximately linearly with Alfvén Mach number and to be robust against changes in magnetic field orientation (from quasi‐parallel to quasi‐perpendicular). These large overshoots may be important in applications involving particle acceleration and type II solar radio bursts. In addition, we estimate the effective numerical resistivity of our code by comparing simulations with varying applied resistivity. The estimated numerical resistivity found is consistent with estimates inferred from observations of Earth's bow shock by Scudder et al. (1986).