A. P. Dimmock

The link between shocks, turbulence, and magnetic reconnection in collisionless plasmas

Global hybrid (electron fluid, kinetic ions) and fully kinetic simulations of the magnetosphere have been used to show surprising interconnection between shocks, turbulence, and magnetic reconnection. In particular, collisionless shocks with their reflected ions that can get upstream before retransmission can generate previously unforeseen phenomena in the post shocked flows: (i) formation of reconnecting current sheets and magnetic islands with sizes up to tens of ion inertial length. (ii) Generation of large scale low frequency electromagnetic waves that are compressed and amplified as they cross the shock. These “wavefronts” maintain their integrity for tens of ion cyclotron times but eventually disrupt and dissipate their energy. (iii) Rippling of the shock front, which can in turn lead to formation of fast collimated jets extending to hundreds of ion inertial lengths downstream of the shock. The jets, which have high dynamical pressure, “stir” the downstream region, creating large scale disturbances ...

A statistical study into the spatial distribution and dawn‐dusk asymmetry of dayside magnetosheath ion temperatures as a function of upstream solar wind conditions

The magnetosheath contains the shocked solar wind and behaves as a natural filter to the solar wind plasma before it reaches the magnetosphere. The redistribution of kinetic energy at the bow shock results in significant thermalization of the solar wind plasma, resulting in a magnetosheath temperature profile which is highly nonhomogeneous and nonisotropic and differs between the dawn and dusk flanks. The present study attempts to study the spatial distribution of magnetosheath ion temperature as a function of upstream solar wind conditions. We pay particular attention to the dawn/dusk asymmetry in which we attempt to quantify using experimental data collected over a 7 year period. We also compare these data to simulated data from both the Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme (BATS-R-US) MHD code and a kinetic hybrid model. We present evidence that the dawn flank is consistently hotter than the dusk flank for a variety of upstream conditions. Our statistical data also suggest a dependency on solar wind speed such that the level of asymmetry increases with faster speeds. We conclude that the dawn-favored asymmetry of the magnetosheath seed population is insufficient to explain the dawn asymmetry (30–40%) of cold component ions in the cold, dense plasma sheet, and therefore, other mechanisms are likely required.

Solar wind energy input to the magnetosheath and at the magnetopause

Using Time History of Events and Macroscale Interactions During Substorms observations, we show that the efficiency of the energy entry through the magnetopause as measured by the Poynting vector normal component depends on the combination of the solar wind speed and the southward component of the interplanetary magnetic field (IMF): Most efficient energy transfer occurs when the IMF BZ is only moderately negative, and the solar wind speed is high. This means that for the same level of solar wind driver parameters (electric field, epsilon, or other), different combinations of V and BZ will produce different driving at the magnetopause. The effect is strongest for low to moderate driving conditions, while the influence is smaller for the intense space weather events.

Magnetosheath control of solar wind-magnetosphere coupling efficiency

We examine the role of the magnetosheath in solar wind-magnetosphere-ionosphere coupling using the Time History of Events and Macroscale Interactions during Substorms plasma and magnetic field observations in the magnetosheath together with OMNI solar wind data and auroral electrojet recordings from the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer chain. We demonstrate that the electric field and Poynting flux reaching the magnetopause are not linear functions of the electric field and Poynting flux observed in the solar wind: the electric field and Poynting flux at the magnetopause during higher driving conditions are lower than those predicted from a linear function. We also show that the Poynting flux normal to the magnetopause is linearly correlated with the directly driven part of the auroral electrojets in the ionosphere. This indicates that the energy entering the magnetosphere in the form of the Poynting flux is directly responsible for driving the electrojets. Furthermore, we argue that the polar cap potential saturation discussed in the literature is associated with the way solar wind plasma gets processed during the bow shock crossing and motion within the magnetosheath.

Universal properties of mirror mode turbulence in the Earth's magnetosheath

We use more than 7 years of Time History of Events and Macroscale Interactions during Substorms observations to quantify the non-Gaussian properties associated with mirror mode turbulence in the Earths magnetosheath. We find that non-Gaussian statistics of mirror modes lead to the parabolic collapse of kurtosis as the square of the skewness (K = aS2+B with a ∼ 1.3 and b ∼− 1). The parabolic scaling is a global constraint for the magnetosheath and is dictated by kinetic processes. This parabolic scaling is qualitatively independent of the distance to the magnetopause or bow shock, which implies that, even though the bow shock is driving the mirror mode instability, the dynamical evolution of mirror structures is independent of the source region and due to local processes. The parabolic relationship and coefficients between kurtosis and skewness for mirror modes in the Earths magnetosheath are also similar to those found in a wide range of geophysical and laboratory turbulent environments, providing further evidence that turbulent systems dominated by non-Gaussian fluctuations hold universal statistical properties.

Ion-Scale Wave Properties and Enhanced Ion Heating Across the Low-Latitude Boundary Layer During Kelvin-Helmholtz Instability

In the Earths magnetosphere, the magnetotail plasma sheet ions are much hotter than in the shocked solar wind. On the dawn-sector, the cold-component ions are more abundant and hotter by 30-40 percent when compared to the dusk sector. Recent statistical studies of the flank magnetopause and magnetosheath have shown that the level of temperature asymmetry of the magnetosheath is unable to account for this, so additional physical mechanisms must be at play, either at the magnetopause or plasma sheet that contribute to this asymmetry. In this study, we perform a statistical analysis on the ion-scale wave properties in the three main plasma regimes common to flank magnetopause boundary crossings when the boundary is unstable to KHI: hot and tenuous magnetospheric, cold and dense magnetosheath and mixed [Hasegawa et al., 2004]. These statistics of ion-scale wave properties are compared to observations of fast magnetosonic wave modes that have recently been linked to Kelvin-Helmholtz (KH) vortex centered ion heating [Moore et al., 2016]. The statistical analysis shows that during KH events there is enhanced non-adiabatic heating calculated during ion scale wave intervals when compared to non-KH events. This suggests that during KH events there is more free energy for ion-scale wave generation, which in turn can heat ions more effectively when compared to cases when KH waves are absent. This may contribute to the dawn favored temperature asymmetry of the plasma sheet, recent studies suggest KH waves favor the dawn flank during Parker-Spiral (PS) interplanetary magnetic field (IMF).

On the Dawn-Dusk Asymmetry of the Kelvin-Helmholtz Instability Between 2007 and 2013

Using data from Time History of Events and Macroscale Interactions during Substorms (THEMIS), a statistical study was performed to determine whether a dawn-dusk asymmetry exists in the occurrence rates of the Kelvin-Helmholtz instability during Parker-Spiral (PS) and Ortho-Parker-Spiral (OPS) orientations of the interplanetary magnetic field (IMF). It is determined from the data that there is a strong preference toward the dawnside during PS orientation, and although a preference to the duskside during OPS is suggested, this requires further study for an unambiguous confirmation. The uncertainty in the OPS result is due to a low number of events, which satisfied our selection criteria. Because IMF is statistically in PS orientation, the Kelvin-Helmholtz instability (KHI) preference for the dawn flank during this orientation may help explain the origin of the plasma sheet asymmetry of cold component ions because it has been shown that KHI can drive kinetic-scale wave activity capable of ion heating.

The response of the Venusian plasma environment to the passage of an ICME: hybrid simulation results and Venus Express observations

Owing to the heritage of previous missions such as the Pioneer Venus Orbiter and Venus Express (VEX), the typical global plasma environment of Venus is relatively well understood. On the other hand, this is not true for more extreme driving conditions such as during passages of Interplanetary Coronal Mass Ejections (ICMEs). Some of the outstanding questions are how do ICMEs, either the ejecta or sheath portions, impact: 1) the Venusian magnetic topology, and 2) escape rates of planetary ions? One of the main issues encountered when addressing these problems is the difficulty of inferring global dynamics from single spacecraft obits; this is where the benefits of simulations become apparent. In the present study, we present a detailed case study of an ICME interaction with Venus on 05 November 2011 in which the magnetic barrier reached over 250 nT. We use both VEX observations and hybrid simulation runs to study the impact on the field draping pattern and the escape rates of planetary O+ ions. The simulation showed that the magnetic field line draping pattern around Venus during the ICME is similar to that during typical solar wind conditions and that O+ ion escape rates are increased by approximately 30% due to the ICME. Moreover, the atypically large magnetic barrier appears to manifest from a number of factors such as the flux pile up, day-side compression, and the driving time from the ICME ejecta.

The Cross-Polar Cap Saturation in GUMICS-4 During High Solar Wind Driving

It is well known that the Earths ionospheric cross‐polar cap potential (CPCP) saturates as a response to the solar wind driver especially when the level of driving is high and the interplanetary magnetic field (IMF) is oriented southward. Moreover, previous studies have shown that the upstream Alfven Mach number may be an important factor in the saturation effect. While the CPCP is often viewed as a measure of the solar wind ‐ magnetosphere ‐ ionosphere ‐coupling the processes associated with the nonlinearity of the coupling remains an open issue. We use the Grand Unified Magnetosphere‐Ionosphere Coupling Simulation (GUMICS‐4) and artificial solar wind data to mimic weak and strong driving in order to study the CPCP response to a wide range of IMF magnitudes (3.5 nT – 30 nT) and upstream Alfven Mach number values (1.2 – 22). The results provide the first overview of the CPCP saturation in GUMICS‐4 and show that the onset of saturation is strongly dependent on the upstream Alfven Mach number and the physical processes responsible for the saturation effect might take place both in the Earths magnetosheath and in the upstream solar wind.