The structure of Mercury's dayside magnetosphere is investigated during three extreme solar wind dynamic pressure events. Two were the result of coronal mass ejections (CMEs), and one was from a high-speed stream (HSS). The inferred pressures for these events are~45 to 65 nPa. The CME events produced thick, low-β (where β is the ratio of plasma thermal to magnetic pressure) plasma depletion layers and high reconnection rates of 0.1-0.2, despite small magnetic shear angles across the magnetopause of only 27 to 60°. For one of the CME events, brief,~1-2 s long diamagnetic decreases, which we term cusp plasma filaments, were observed within and adjacent to the cusp. These filaments may map magnetically to flux transfer events at the magnetopause. The HSS event produced a high-β magnetosheath with no plasma depletion layer and large magnetic shear angles of 148 to 166°, but low reconnection rates of 0.03 to 0.1. These results confirm that magnetic reconnection at Mercury is very intense, and its rate is primarily controlled by plasma β in the adjacent magnetosheath. The distance to the subsolar magnetopause is reduced during these events from its mean of 1.45 Mercury radii (R M ) from the planetary magnetic dipole to between 1.03 and 1.12 R M . The shielding provided by induction currents in Mercury's interior, which temporarily increase Mercury's magnetic moment, was negated by reconnection-driven magnetic flux erosion.

Magnetic reconnection is an important process that occurs at the magnetopause boundary of Earth's magnetosphere because it leads to transport of solar wind energy into the system, driving magnetospheric dynamics. However, the nature of magnetopause reconnection in the case of Saturn's magnetosphere is unclear. Based on a combination of Cassini spacecraft observations and simulations we propose that plasma βconditions adjacent to Saturn's magnetopause largely restrict reconnection to regions of the boundary where the adjacent magnetic fields are close to anti‐parallel, severely limiting the fraction of the magnetopause surface that can become open. Under relatively low magnetosheathβconditions we suggest that this restriction becomes less severe. Our results imply that the nature of solar wind‐magnetosphere coupling via reconnection can vary between planets, and we should not assume that the nature of this coupling is always Earth‐like. Studies of reconnection signatures at Saturn's magnetopause will test this hypothesis.

[1] We present Cassini observations of a plasma vortex in Saturn's dayside outer magnetosphere. The vortex encounter took place on 13 December 2004 as Cassini was travelling toward the planet. The spacecraft crossed the magnetopause 3 times, before being immersed in the low-latitude boundary layer. During the transition between the boundary layer and the magnetosphere proper, the spacecraft observed deflected boundary layer plasma, a twisted magnetic field topology, and high-energy (>20 keV) directional electron fluxes. These observations are consistent with an encounter with a vortex on the inner edge of the boundary layer, an interface that is expected to be susceptible to the growth of the Kelvin-Helmholtz (K-H) instability due to its low magnetic shear. The size of the vortex is determined to be at least 0.55 R S , and a simple model of the current system resulting from the formation of the vortex is proposed. The possible acceleration mechanisms responsible for the high-energy electrons are discussed. The identification of the structure provides compelling evidence of the operation of the nonlinear K-H instability at Saturn's morning magnetospheric boundaries and has implications for our understanding of the transfer of energy and momentum between the solar wind and Saturn's magnetosphere.