The slipperiness of ice is an everyday-life phenomenon which, surprisingly, remains controversial despite a long scientific history. The very small friction measured on ice is classically attributed to the presence of a thin self-lubricating film of meltwater between the slider and the ice. But while the macroscale friction behavior of ice and snow has been widely investigated, very little is known about the interfacial water film and its mechanical properties. In this work, we develop a stroke-probe force measurement technique to uncover the microscopic mechanisms underlying ice lubrication. We simultaneously measure the shear friction of a bead on ice and quantify the in-situ mechanical properties of the interfacial film, as well as its thickness, under various regimes of speed and temperature. In contrast with standard views, meltwater is found to exhibit a complex viscoelastic rheology, with a viscosity up to two orders of magnitude larger than pristine water. The non-conventional rheology of meltwater provides a new, consistent, rationale for ice slipperiness. Hydrophobic coatings are furthermore shown to strongly reduce friction due to a surprising change in the local viscosity, providing an unexpected explanation for waxing effects in winter sports. Beyond ice friction, our results suggest new avenues towards self-healing lubricants to achieve ultralow friction.

Measurements with the ACE, STEREO, and GOES spacecraft during the first 5.8 years of solar cycle 24 show that the number of large Solar Energetic Particle (SEP) events is reduced by ~32% compared to this point of cycle 23, while the fluences of >10 MeV/nucleon ions from H to Ni are reduced by factors ranging from 4 to ~10. A comparison of H, O, and Fe energy spectra from the ten largest events of the two cycles shows that the spectral breaks that are typically observed in SEP energy spectra are occurring ~3 times lower in energy/nucleon than in cycle 23. We investigate the origin of these cycle-to-cycle fluence, spectral and composition differences by evaluating possible factors that include: 1) the properties of the associated CMEs; 2) the interplanetary magnetic field strength; and 3) the density of suprathermal seed particles. These properties are evaluated in the context of existing SEP acceleration models. We conclude that both the reduced magnetic field strength and the reduced seed particle densities are contributing to the reduction in SEP output during cycle 24. In particular, we point out that in the standard model for SEP shock acceleration the maximum energy achieved is a strong function of the rate at which protons are injected into the shock acceleration process.