Research Review #1

Particle-in-cell simulations of the solar wind interaction with lunar crustal magnetic anomalies: Magnetic cusp regions
A. R. Poppe et al.

Journal of Geophysical Research, VOL. 117, A09105, doi:10.1029/2012JA017844, 2012
http://www.agu.org/pubs/crossref/2012/2012JA017844.shtml

The Moon spends a major part of its orbit in the solar wind, and is usually used as an example of the interaction of the solar wind with an unmagnetized obstacle. However, the Moon does possess localized crustal remnant fields, first observed during the Apollo era. Further observations have been made by Lunar Prospector, Kaguya and the ARTEMIS mission. This paper presents results from an electrostatic PIC simulation of the interaction of the solar wind flow with the cusp region of a crustal magnetic anomaly. They term the simulation 1.5 D because electron perpendicular velocities are tracked, and treated as magnetized and conserving their magnetic moment, whereas ions are treated unmagnetized. The background magnetic field corresponds to the cusp of a dipole anomaly situated some distance below the surface of the Moon. The electrons respond self-consistently to both the electrostatic field and the cusp magnetic field (as described using the adiabatic invariant) while the solar wind protons are only affected by the electrostatic field. They find that a non-monotonic electrostatic potential builds up in front of the moon, within the cusp regions of lunar magnetic anomalies. The increasing potential from infinity acts to decelerate and for strong enough fields reflect a portion of the incoming supersonic solar wind protons, while accelerating solar wind electrons toward the Moon. This electric field is referred to as the ‘ambipolar’ field. Nearer the surface of the Moon (within 150m) the potential begins to decrease due to a sheath field associated with the lunar surface electrostatic charge. Observations of the electrostatic potential about crustal anomalies are very limited, but a comparison is made with one measurement from KAGUYA (Saito et al. 2012). Apart from the electrostatic potential, the simulations have implications for space weathering on other bodies with localized magnetic anomalies. If the potential is large enough to reflect the solar wind protons, ie reducing impact flux on surface, then there will be less space weathering; this may explain observations of some asteroids such as 4 Vesta.

Hybrid simulation of the shock wave trailing the Moon
P. Israelevich and L. Ofman

Journal of Geophysical Research. 117, A08223, doi:10.1029/2011JA017358, 2012
http://www.agu.org/pubs/crossref/2012/2011JA017358.shtml

A standing shock wave in the solar wind behind the moon was predicted by Michel (1967) but was never observed or simulated. This paper presents results from a 1D hybrid simulation of the formation of the plasma cavity behind the moon, and the resulting shockwave, for typical solar wind parameters. This 1D model moves in a reference frame of the solar wind, with initial conditions having a plasma cavity the size of the obstacle, in this case, the moon. The plasma expands into this cavity, and converting time back to x-coordinates, the simulation draws out the shape of the plasma flow behind the moon. The results focus on two cases where the local magnetic field aligns in the Y direction with the expanding plasma, and where the magnetic field is in the Z direction, and the plasma expansion is across the field.

They find that a negatively charge potential builds up in the cavity behind the moon, due to the faster moving electrons that reach and fill the cavity first. The results show a shockwave is produced in agreement with theory. The shock is produced by the interaction of oppositely directed proton beams in the plane containing solar wind velocity and interplanetary magnetic field vectors. In the direction across the magnetic field and the solar wind velocity, the shock results from the interaction of the plasma flow with the region of the enhanced magnetic field inside the cavity
Predictions are made for the approximate locations of the shock and wake. A weaker shock is formed from a solar wind with cooler electron temperatures. The paper provides predictions for future observations of these features of the moon, suggesting that ARTEMIS spacecrafts places at L1 and L2 points of the Earth-moon systems make be able to make observations of these features.

Electromagnetic ion cyclotron wave generation by planetary pickup ions: One-dimensional hybrid simulations at sub-Alfvénic pickup velocities
M. M. Cowee and S. P. Gary

Journal of Geophysical Research, VOL. 117, A06215, doi:10.1029/2012JA017568, 2012
http://www.agu.org/pubs/crossref/pip/2012JA017568.shtml

Enhanced electromagnetic plasma waves generated by unstable pickup ion distributions have been detected by spacecraft in several planetary environments including those of Venus, Earth, Mars, comets, Jupiter and Saturn. Interpretation of these waves relies on understanding the source of free energy and how that free energy is reduced through instability growth. This paper presents results from a 1D hybrid simulation where the pickup velocity is sub-Alfvenic, and illustrates the differences in instability behaviour.

They find that several instabilities are possible but the dominant (observed and according to linear theory) instability is the ion cyclotron ring instability, so analysis focuses on these. The simulations are carried out in both the bulk plasma frame, and also in the planetary rest frame so spacecraft observational data can be inferred. A range of pickup angles alpha are simulated. The simulation results illustrate how the wave properties and pickup ion velocity space distributions are altered by varying alpha, providing a basic framework for interpreting observations of ion cyclotron waves generated by pickup ions. The paper then compares the simulated results to observations of ion cyclotron waves observed at Jupiters moons, Io and Europa, and again at Saturns moons, Enceladus and Titan.

Vlasov simulation of electrostatic solitary structures in multi-component plasmas
T. Umeda et al.

Journal of Geophysical Research, VOL. 117, A05223, doi:10.1029/2011JA017181, 2012
http://www.agu.org/pubs/crossref/pip/2011JA017181.shtml

Recent theoretical work has suggested that solitary structures can exist in the solar wind as electron acoustic solitary waves, and can be modelled by an interaction of four components of a plasma consisting of core electrons, two counter-streaming electron beams, and one species of background ions. This paper presents results from a 1D Vlasov simulation that attempts to replicate this phenomenon and compare the results to observations from the Cluster spacecraft. The simulation initially shows that a four component description only produces solitary structures in a positive or negative direction, given the input velocity distribution functions as measured from the cluster spacecraft. The simulated structures have longer duration times (or slower propagation speeds) and higher wave amplitudes than the solitary structures observed in the magneto-sheath and lead the authors to add a fifth component of the plasma, a weak high energy beam in order to reproduce the wave.

The final simulation run shown creates solitary structures more similar to observations, and the argument is provided that the small weak electron beam could have been present in the Cluster data, but might not have been measured or resolved by the spacecrafts instruments. This paper provides an alternative mechanism to how solitary structures can be formed in the solar wind.

Investigation of the viscous potential using an MHD simulation
R. Bruntz et al.

Journal of Geophysical Research, VOL. 117, A03214, doi:10.1029/2011JA017022, 2012
http://www.agu.org/pubs/crossref/2012/2011JA017022.shtml

The viscous interaction is a “viscous-like” interaction between the between the solar wind and plasma inside the Earth’s magnetopause, which induces ionospheric plasma circulation. A major driver of this is magnetic reconnection between the Earths magneto-sphere and the IMF.
This paper presents results from a global 3D Lyon-Fedder-Mobarry (LFM) magneto-hydrodynamic simulation that simulates this affect. The outer boundary of the model can be formed from solar wind data, but here the 8 solar wind parameters are used. The simulation calculates plasma parameters into a radius of 2.8Re around the Earth. This inner boundary maps to either an empirical 2-D ionospheric or a simple 2-D uniform conductivity ionospheric simulation. The authors find that the viscous potential increases with increasing solar wind density, and increasing solar wind velocity. These results are consistent with previous studies but the actual values of the scaling factors are not identical. The results can be used in combination with measurements of polar cap potential to produce an equation for predicting the viscous potential in LFM, based on solar parameters.

[Written by David Burgess and Chris Haynes]