Kelvin-Helmholtz Instability And Magnetic Reconnection At The Earth's Magnetospheric Boundary

Thesis (Ph.D.) University of Alaska Fairbanks, 2012Magnetic reconnection and Kelvin-Helmholtz (KH) instability are the two most important mechanisms for plasma transport across the Earth's magnetospheric boundary layer. Magnetic reconnection is considered as the dominant process for southward interplanetary magnetic field (IMF), and the KH instability is suggested to play an important role for northward IMF. It is interesting to note that this plasma entry is associated with a dramatic entropy increase, which indicates the existence of strong nonadiabatic heating during the entry process. Observations indicate a plasma entropy increase by two orders of magnitude during the transport from solar wind into the Earth's magnetosphere. Therefore, it is important to examine whether magnetic reconnection can provide sufficient nonadiabatic heating to explain the observed plasma properties and to identify plasma conditions that allow strong nonadiabatic heating. This thesis demonstrates that the entropy can indeed strongly increase during magnetic reconnection provided that the plasma beta, i.e., the ratio of thermal to magnetic energy density is small. A realistic three-dimensional configuration of the Earth's magnetopause for southward IMF conditions includes large anti-parallels magnetic components with a fast perpendicular shear flow. Thus, it is expected that KH modes and magnetic reconnection operate simultaneously and interact with each other. This thesis provides a systematic study on this interaction between reconnection and KH modes by means of three-dimensional MHD and Hall MHD numerical simulations. It is demonstrated that both reconnection and nonlinear KH waves change the other modes onset condition by changing the width of the transition layer. It is shown that dynamics of the system can be strongly modified by a guide field or Hall physics. In the presence of plasma flow, magnetic reconnection is also associated with the generation of field-aligned currents (FACs), which play a critical role in the coupling between the magnetosphere and ionosphere. This thesis also examines systematically the generation of FACs. It is demonstrated that such currents are generated either by a guide magnetic field, by shear flow, or by the inclusion of Hall physics already in two-dimensional magnetic reconnection

Estimation of the Kelvin–Helmholtz Unstable Boundary

The Kelvin–Helmholtz (KH) instability is one of the most important mechanisms of the viscous like interaction between the solar wind and the magnetosphere (MSP), which transport the mass, energy, momentum, and magnetic flux. Thus, it is important to examine whether the magnetopause boundary is KH unstable or not. Based on the KH onset conditions, this report proposes to use a matrix to identify the most KH unstable direction based on the in-situ measurements of the density, velocity, and magnetic field in the MSP and magneto sheath. The range of the KH unstable direction can be easily estimated based on the eigenvalues of the matrix. The eigenvectors of the matrix provide a new boundary normal coordinate system, which could be useful for 2-D KH instability simulation

Mechanisms of Field-Aligned Current Formation in Magnetic Reconnection

Satellite observations provide strong evidence for the generation of significant field-aligned currents (FACs) during magnetic reconnection. Reconnection of antiparallel magnetic field does not generate FACs in magnetohydrodynamics (MHD) due to coplanarity in MHD shocks. However, a guide magnetic field and a sheared velocity component are almost always present at the magnetopause and their absence is a singular case. It is illustrated that the presence of these noncoplanar fields requires FACs. Contrary to intuition, such currents are generated more efficiently for a small guide field and are more likely to be a result of the redistribution of already present FACs for large guide fields. It is demonstrated that moderate values of shear flow can generate significant ionospheric FACs. Similar to shear flow, the presence of Hall physics leads to significant FACs and we examine the scaling of these current with the ion inertia length

Categorizing FTE-like Boundary Layer Signatures Produced by the Kelvin-Helmholtz Instability Using Hall-MHD Simulations and Virtual Spacecraft

Magnetic reconnection and Kelvin-Helmholtz instability (KHI) are two fundamental processes at the planetary magnetospheres that can lead to plasma, momentum and energy transport over magnetospheric boundary. Flux Transfer Events (FTEs) are generally accepted to be produced by the magnetic reconnection at the dayside magnetopause. However, there are still other possible mechanisms which create FTE-like features in the boundary layer. Kelvin-Helmholtz instability can be one of the candidates. The deformed boundary driven by the KHI at the interface of two fluids usually leads to the bipolar signatures of the normal component of the magnetic field. By using two-dimensional Hall-MHD simulations, we study signatures observed by virtual satellites as they pass through KHI along different trajectories. For the same plasma parameters across the magnetosphere and magnetosheath, slightly adjusting the projection angle of the magnetic field will give us 12 combinations of in-plain components at both sides of the boundary. In addition, we assume 3 sets of spacecraft trajectories in each simulation, which totally bring 36 different KHI signatures. While the satellites encountered well-developed KH vortex and spine region, the signatures, when detected by a spacecraft in the magnetosphere, would be easily misidentified as FTEs. The presented analysis examines and categorizes these observed signatures that are clearly generated by the KHI. These results can be used as diagnostic when analyzing spacecraft data to help distinguish KHI created signatures from FTEs

Interaction of Magnetic Reconnection and Kelvin-Helmholtz Modes for Large Magnetic Shear: 2. Reconnection Trigger

A typical property of magnetopause reconnection is a significant perpendicular shear flow due to the fast streaming magnetosheath plasma. Therefore, the magnetopause represents a large magnetic and flow shear boundary during periods of southward interplanetary magnetic field, which can be unstable to Kelvin‐Helmholtz (KH) modes and to magnetic reconnection. A series of local three‐dimensional MHD and Hall MHD simulations is carried out to investigate the interaction of reconnection and nonlinear KH waves considering magnetic reconnection as the primary process. It is demonstrated that the onset reconnection causes a thinning of the shear flow layer, thereby generating small wavelength KH modes. In turn, the growing KH modes modify the current layer width, which modulate the diffusion regions, increase the local reconnection rates, and generate field‐aligned currents. The simulation results imply a limitation of total amount of open flux likely caused by nonlinear saturation of KH growth and the associated diffusion. It is also demonstrated that the reconnection rate maximizes for conditions that allow a strong nonlinear evolution of KH waves, i.e., fast shear flow and limited guide magnetic field. The presence of Hall physics increases the reconnection rate in the early stage; however, the maximum reconnection rate and the total amount of open flux at saturation are the same as in the MHD case

Characteristics of Kelvin-Helmholtz Waves as Observed by the MMS from September 2015 to June 2017

The Magnetospheric Multiscale (MMS) mission has presented a new opportunity to study the fine scale structures and phenomena of Earth’s magnetosphere, including cross scale processes associated with the Kelvin-Helmholtz Instability (KHI). We present an overview of 19 MMS observations of the KHI from September 2015 to June 2017. Unitless growth rates and unstable solid angles for each of the 19 events were calculated using 5 techniques to automatically detect plasma regions on either side of the magnetopause boundary. There was no apparent correlation between solar wind conditions during the KHI and its growth rate and unstable solid angle, though we note no KHI were observed for solar wind flow speeds less than 300 km/s or greater than 600 km/s, likely due to a filtering effect of the instability onset criteria and plasma compressibility. Two-dimensional Magnetohydrodynamic (2D MHD) simulations were compared with two of the observed MMS events. Comparison of the observations with the 2D MHD simulations indicates that velocity dependent methods are the most consistent when calculating growth rate and unstable solid angle, but a combination of the velocity dependent and independent methods can be used to select KHI events in which the vortex has rolled over. This may prove useful for future work studying secondary processes associated with the KHI

Unraveling the Multi-Scale Solar Wind Structure Between Lagrange 1-point, Lunar Orbit and Earth’s Bow Shock: Better Space Weather Prediction Through Information Theory

The space weather effects at the Earth’s magnetosphere are mostly driven by the solar wind that carries the Interplanetary Magnetic Field (IMF). The incoming solar wind properties at L1 are typically used for developing various space weather forecasts. In this presentation we use several years of data in the solar wind from lunar orbiting ARTEMIS spacecraft and MMS upstream Earth’s bow shock to study the multi-scale structure of the IMF as determined by the Wavelet analysis. We determine the lag times between different scales and their dependence on 1) solar wind plasma properties and 2) spacecraft positions. Many solar wind parameters are correlated and anticorrelated with one another. We test the concept of conditional mutual information to isolate the effect of a single parameter and the dependence of various solar wind parameters on the time lags to provide the best/worse correlations between different scales. This will aid in isolating solar wind conditions when single point measurements of the IMF at Lagrange 1 point will likely lead to compromised space weather prediction accuracy

Plasma Transport Driven by the Three-Dimensional Kelvin-Helmholtz Instability

It has been well demonstrated that the nonlinear Kelvin‐Helmholtz (KH) instability plays a critical role for the solar wind interaction with the Earth\u27s magnetosphere. Although the two‐dimensional KH instability has been fully explored during the past decades, more and more studies show the fundamental difference between the two‐ and three‐dimensional KH instability. For northward interplanetary magnetic field (IMF) conditions, the nonlinear KH wave that is localized in the vicinity of the equatorial plane can dramatically bend the magnetic field line, generating strong antiparallel magnetic field components at high latitudes in both North and South Hemispheres, which satisfy the onset condition for magnetic reconnection. This high‐latitude double reconnection process can exchange the portion of magnetosheath and magnetospheric flux tubes between those two reconnection sites. This study used a high‐resolution 3‐D magnetohydrodynamic simulation to demonstrate that nonlinear KH waves can generate a large amount of double‐reconnected flux during the northward IMF condition, which can efficiently transport the plasma with a high diffusion coefficient of 1 × 1010 m2 s−1 for typical magnetopause conditions at the Earth. The presence of the magnetic field component along the shear flow direction not only decreases the KH growth rate but also causes north‐south asymmetry, which generates more open flux and reduces the efficiency of the plasma transport process

Small Scale Plasma Waves and Heating within Kelvin-Helmholtz Instabilities at Earth’s Magnetopause

The Kelvin-Helmholtz Instability (KHI) is common at the magnetopause boundary enclosing Earth’s magnetosphere. The KHI drives several secondary processes which can transport plasma from the solar wind into Earth’s magnetosphere and convert kinetic energy in the plasma to thermal energy. Previous studies have shown the KHI and its associated secondary processes play an important role in the heating of ions and could help explain the observed asymmetry between ion populations in the dawn and dusk flanks of the magnetosphere. The contribution of the KHI to heating at the electron scale, however, is not well understood. Until the launch of the Magnetosphere Multiscale (MMS) mission in 2015, measurements of electron scale processes were not available. This study uses data collected by MMS between 2015 and 2020 to identify waves and potential sources of plasma heating between the ion and electron scales