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.

After 3 years and 31 close flybys of Titan by the Cassini Orbiter, Titan was finally observed in the shocked solar wind, outside of Saturn's magnetosphere. These observations revealed that Titan's flow-induced magnetosphere was populated by "fossil" fields originating from Saturn, to which the satellite was exposed before its excursion through the magnetopause. In addition, strong magnetic shear observed at the edge of Titan's induced magnetosphere suggests that reconnection may have been involved in the replacement of the fossil fields by the interplanetary magnetic field.

Collisionless shocks are loosely defined as shocks where the transition between pre-and post-shock states happens on a length scale much shorter than the 2 collisional mean free path. In the absence of collision to enforce thermal equilibrium post-shock, electrons and ions need not have the same temperatures. While the acceleration of electrons for injection into shock acceleration processes to produce cosmic rays has received considerable attention, the related problem of the shock heating of quasi-thermal electrons has been relatively neglected.In this paper we review that state of our knowledge of electron heating in astrophysical shocks, mainly associated with supernova remnants (SNRs), shocks in the solar wind associated with the terrestrial and Saturnian bowshocks, and galaxy cluster shocks. The solar wind and SNR samples indicate that the ratio of electron temperature, (Te) to ion temperature (Tp) declines with increasing shock speed or Alfvén Mach number. We discuss the extent to which such behavior can be understood via cosmic ray-generated waves in a shock precursor, which then subsequently damp by heating electrons. Finally, we speculate that a similar mechanism may be at work for both solar wind and SNR shocks.

Jupiter's auroral emissions reveal energy transport and dissipation through the planet's giant magnetosphere. While the main auroral emission is internally driven by planetary rotation in the steady state, transient brightenings are generally thought to be triggered by compression by the external solar wind. Here we present evidence provided by the new Hisaki spacecraft and the Hubble Space Telescope that shows that such brightening of Jupiter's aurora can in fact be internally driven. The brightening has an excess power up to ~550 GW. Intense emission appears from the polar cap region down to latitudes around Io's footprint aurora, suggesting a rapid energy input into the polar region by the internal plasma circulation process.

Saturn's magnetic field acts as an obstacle to solar wind flow, deflecting plasma around the planet and forming a cavity known as the magnetosphere. The magnetopause defines the boundary between the planetary and solar dominated regimes, and so is strongly influenced by the variable nature of pressure sources both outside and within. Following from Pilkington et al. (2014), crossings of the magnetopause are identified using 7 years of magnetic field and particle data from the Cassini spacecraft and providing unprecedented spatial coverage of the magnetopause boundary. These observations reveal a dynamical interaction where, in addition to the external influence of the solar wind dynamic pressure, internal drivers, and hot plasma dynamics in particular can take almost complete control of the system's dayside shape and size, essentially defying the solar wind conditions. The magnetopause can move by up to 10–15 planetary radii at constant solar wind dynamic pressure, corresponding to relatively “plasma‐loaded” or “plasma‐depleted” states, defined in terms of the internal suprathermal plasma pressure.

Data from the Cassini Electron Spectrometer are used to investigate the location of magnetic reconnection at Saturn's magnetopause. Heated, streaming electron distributions in the boundary layer on the magnetosheath side of the magnetopause are evidence of reconnection and an open magnetopause. A model for the location of reconnection is used to compare the modeled and observed streaming direction of the heated electron distributions. Magnetic reconnection at Saturn's magnetopause is predicted and observed to occur at locations similar to those at Earth's magnetopause. Although not conclusive, the results here are consistent with the expected importance of X-line drifts in suppressing low-shear reconnection. Because of different conditions at Saturn's magnetopause, this suppression is predicted to be more severe at Saturn than at Earth.