A. M. Stickle

Multiscale/multifluid simulations of flux ropes at the magnetopause within a global magnetospheric model

[1] The magnetopause current sheet is known to have a thickness comparable to an ion gyroradius/skin depth where the magnetic and electric field can differ markedly from that assumed in MHD treatments. Multifluid/multiscale simulations are used to provide the first investigation of these processes in a global simulation that includes high-resolution (200 km) gridding around the magnetopause. It is shown that the model is able to capture the quadrupole core magnetic field and the fast (tens of ion cyclotron periods) reconnection seen in idealized studies reconnection for a Harris current sheet. Within a global magnetosphere, multiple X-line reconnection occurs for southward IMF due to localized pinching of the magnetopause current sheet via the convection of the magnetosheath plasma against a three-dimensional magnetopause. Localized flux ropes with a thickness of a few hundred to a few thousand kilometers develop and can expand laterally due to current sheet acceleration of ions that have a gyroradius comparable to the current sheet thickness. These flux ropes are shown to have essentially the same properties as flux transfer events (FTEs), including being quasi-periodic with a curvature greater on the magnetosheath side than on the magnetospheric side, a strong core magnetic field, and a mixture of magnetospheric and magnetosheath plasma. The speeds of the plasma flows associated with flux ropes are also similar to those observed with FTEs. The presence of multiple X-line reconnection is shown to produce the rippling of the magnetopause and gives a nature explanation to the multiple magnetopause encounters that typically occur for slow moving spacecraft. These small-scale processes are shown to have global effects with a reduction of the cross-polar cap by as much as 20% seen between simulations with and without high resolution about the magnetopause.

Discrete shear failure planes resulting from oblique hypervelocity impacts

A combination of laboratory and numerical experiments examines the role of shear localization in subsurface damage following very oblique (15–30°) hypervelocity impacts. Laboratory experiments reveal subsurface damage planes (“blades”) parallel to the impact trajectory for highly oblique impacts (15–30°), which are characterized by unique surface textures relative to other failure regions. Observations of growth rate and surface texture of the damage planes combined with three-dimensional CTH simulations indicate that the blades are the result of frictional processes during localized shear deformation. Laboratory experiments also reveal that impact angle and projectile failure play a role in the development of these blades: aluminum projectiles result in distinct along-trajectory blades for both 15° and 30° impacts, whereas the blades are weakly developed for Pyrex projectiles and nonexistent for planar polymethylmethacrylate projectiles. The blades form early in the cratering process and are signatures of the projectile momentum being transferred into the target. Based on the growth rate, and melting seen along the surface of these damage planes, the blades may provide an analog for the generation of pseudotachylytes during the early stages of impact crater formation.

The Surface Roughness of Large Craters on Mercury: SURFACE ROUGHNESS OF CRATERS

This study investigates how individual large craters on Mercury (diameters of 25–200 km) can produce surface roughness over a range of baselines (the spatial horizontal scale) from 0.5 to 250 km. Surface roughness is a statistical measure of change in surface height over a baseline usually after topography has been detrended. We use root mean square deviation as our measure of surface roughness. Observations of large craters on Mercury at baselines of 0.5–10 km found higher surface roughness values at the central uplifts, rims, and exteriors of craters, while the crater floors exhibit the lowest roughness values. At baselines <10 km, the regions exterior to large craters with diameters >80 km have the highest surface roughness values. These regions, which include the ejecta and secondary fields, are the main contributors to the increased surface roughness observed in high-crater density regions. For baselines larger than 10 km, the crater cavity itself is the main contributor to surface roughness. We used a suite of numerical models, utilizing the measured surface roughness obtained in the study, to model the cumulative effect of adding large craters to a surface. The results indicate that not all of the surface roughness on Mercury is due to fresh large craters but that impact craters likely contribute to the Hurst exponent from baselines of 0.5–1.5 km and the shape of the deviogram. The simulations show that the surface roughness varied around an asymptote at the baselines studied before the surface was covered in impact craters. Plain Language Summary Impact cratering is the main process by which many planetary bodies are roughened, where an increase in the number of craters is related to higher surface roughness. In this study, we use observations and artificial data to explore how individual complex craters on Mercury can change the surface topography and produce surface roughness. Observations of the surface roughness of complex craters on Mercury found surface roughness related to several different geologic features of the craters. We found that impact craters are the main source of surface roughness on Mercury. We modeled how impact crater density affects surface roughness and found that it is difficult to relate surface age to surface roughness.