J. Paral

Dawn–dusk asymmetry in the Kelvin–Helmholtz instability at Mercury

The NASA MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft entered orbital phase around Mercury on 18 March 2011. A surprising consistent feature in the data returned is large-scale vortices that form exclusively on the dusk side of the magnetosphere. Here we present global kinetic hybrid simulations that explain these observations. It is shown that vortices are excited by a Kelvin-Helmholtz instability near the subsolar point, which grows convectively along the dusk-side magnetopause. Virtual time series along a track approximating a flyby of the MESSENGER show correspondence with the satellite data; the data contain sawtooth oscillations in plasma density, flow and magnetic field, and exhibit the observed dawn-dusk asymmetry. It is shown that asymmetry between dawn and dusk at Mercury is controlled by the finite gyroradius of ions and by convection electric fields. Mercurys magnetosphere offers a natural laboratory for studying plasma regimes not present in other planetary magnetospheres or the laboratory.

Properties of the lunar wake predicted by analytic models and hybrid‐kinetic simulations

An approximate model derived by Hutchinson (2008a), describing the interaction between the solar wind and the Moon, is applied to describe plasma in the lunar wake. The model accounts for plasma entering the wake region from two tangent points around the Moon. Cylindrical geometry is assumed, along with a strong constant magnetic field, and fixed transverse velocity and temperature. Under these approximations two angle-dependent equations for fluid flow are obtained, which can be solved using the method of characteristics to provide the density inside the wake region. It is demonstrated that the model valid under these assumptions provides excellent agreement with observations from the ARTEMIS mission and with large-scale hybrid-kinetic plasma simulations. The model provides a practical alternative to kinetic simulations and is generally useful for determining properties of the lunar wake under different solar wind conditions. It will be useful as well for predicting properties of the plasma environment around unmagnetized bodies that have not yet been visited by spacecraft.