Evolution of IMF By Induced Asymmetries: The Role of Tail Reconnection

North-south asymmetries arise in the magnetosphere-ionosphere system when a significant east-west (By) component is present in the interplanetary magnetic field (IMF). During such conditions, a By component with the same sign as the IMF By component is induced in the magnetosphere, and the locations of conjugate magnetic footpoints are displaced between the two hemispheres. It has been suggested that these asymmetries are introduced into the closed magnetosphere by tail reconnection. However, recent studies instead suggest that asymmetric lobe pressure induces the asymmetries, which are then reduced during periods of enhanced tail reconnection. To address this, we use the Lyon-Fedder-Mobarry (LFM) model and initiate a loading-unloading cycle in multiple runs by changing the IMF. Asymmetries are induced during the loading phase and reduced during the unloading phase. The model results thus suggest that asymmetries arise during periods with low tail reconnection and are reduced during periods with enhanced tail reconnection.publishedVersio

The Micro-Macro Coupling of Mass-Loading in Symmetric Magnetic Reconnection With Cold Ions

We investigate how magnetic reconnection is influenced by an inflow of a dense cold ion population. We compare two 2.5D Particle-In-Cell simulations, one containing the cold population and one without. We find that the cold population influences the reconnection process on both global and kinetic scales, and that the dominant contribution can be explained through mass-loading. We provide an analysis of how these multiscale changes are related through kinetic processes in the ion diffusion region, the so-called micro-macro coupling of mass-loading. The inertia of the cold ion population is found to be the significant link that connects the changes on different scales. The cold and warm populations exhibit counter streaming behavior when and after the ion diffusion region reorganizes itself in response to the arrival of the cold population. This signature of the cold population should be observable by spacecraft observatories such as MMS.publishedVersio

Electron Behavior Around the Onset of Magnetic Reconnection

We investigate the onset of magnetic reconnection, utilizing a fully kinetic Particle-In-Cell (PIC) simulation. Characteristic features of the electron phase-space distributions immediately before reconnection onset are identified. These include signatures of pressure non-gyrotropy in the velocity distributions, and lemon shaped distributions in the in-plane velocity directions. Further, we explain how these features form through particle energization by the out-of-plane electric field. Identification of these features in the distributions can aid in analysis of data where clear signatures of ongoing reconnection are not yet present.publishedVersio

The Role of Resistivity on the Efficiency of Magnetic Reconnection in MHD

Using a resistive MagnetoHydroDynamic (MHD) simulation, we study how the magnitude and shape of diffusion influence magnetic reconnection. Specifically, we investigate how and why the reconnection rate is influenced by variations in the diffusion distribution and magnitude. By running multiple MHD simulations where we vary the localized resistivity, we find that the properties of the diffusion region greatly influence the rate of reconnection. Increasing the magnitude of the imposed resistivity results in a higher reconnection rate, but the rate saturates at approximately 0.2. We show how a redistribution of the current density, leading to a bifurcated current sheet, play a major role in this limitation. In addition, we investigate the impact of different shapes of resistive region. The shape of the diffusion region also plays a major role in how efficient the reconnection energy conversion can operate. The highest reconnection rate, approximately 0.25, is achieved for an optimal opening angle. Our results imply that reconnection has a speed limit that may depend on properties outside the diffusion region.publishedVersio

Asymmetrically Varying Guide Field During Magnetic Reconnection: Particle-In-Cell Simulations

Using fully kinetic particle-in-cell modeling, we investigate how magnetic reconnection responds to a varying guide field in one of the inflow regions. We find that the reconnection rate varies significantly when the orientation of the magnetic field changes between being strictly antiparallel and having a guide field. These variations are fairly consistent with the scaling relation for asymmetric reconnection developed by Cassak and Shay (2007). However, the rate is also found to be nonlinearly modulated by changes in the ion inflow velocity. The spatio-temporal change in the inflow velocity arises as the magnetic forces reconfigure to regions of different magnetic field strengths. The variations in the inflow magnetic field configuration allow for different gradients in the magnetic field, leading to asymmetries in the magnetic tension force. By momentum conservation, this facilitates asymmetries in the inflow velocity, which in turn affects the flux transport into the reconnection site. The outflow is found to be less laminar when the inflow varies, and various signatures of the inflow variations are identified in the outflow.publishedVersio

Evolution of IMF By Induced Asymmetries: The Role of Tail Reconnection

North-south asymmetries arise in the magnetosphere-ionosphere system when a significant east-west (By) component is present in the interplanetary magnetic field (IMF). During such conditions, a By component with the same sign as the IMF By component is induced in the magnetosphere, and the locations of conjugate magnetic footpoints are displaced between the two hemispheres. It has been suggested that these asymmetries are introduced into the closed magnetosphere by tail reconnection. However, recent studies instead suggest that asymmetric lobe pressure induces the asymmetries, which are then reduced during periods of enhanced tail reconnection. To address this, we use the Lyon-Fedder-Mobarry (LFM) model and initiate a loading-unloading cycle in multiple runs by changing the IMF. Asymmetries are induced during the loading phase and reduced during the unloading phase. The model results thus suggest that asymmetries arise during periods with low tail reconnection and are reduced during periods with enhanced tail reconnection

On the Presence and Thermalization of Cold Ions in the Exhaust of Antiparallel Symmetric Reconnection

Using fully kinetic 2.5 dimensional particle-in-cell simulations of anti-parallel symmetric magnetic reconnection, we investigate how initially cold ions are captured by the reconnection process, and how they evolve and behave in the exhaust. We find that initially cold ions can remain cold deep inside the exhaust. Cold ions that enter the exhaust downstream of active separatrices, closer to the dipolarization front, appear as cold counter-streaming beams behind the front. In the off-equatorial region, these cold ions generate ion-acoustic waves that aid in the thermalization both of the incoming and outgoing populations. Closest to the front, due to the stronger magnetization, the ions can remain relatively cold during the neutral plane crossing. In the intermediate exhaust, the weaker magnetization leads to enhanced pitch angle scattering and reflection. Cold ions that enter the exhaust closer to the X line, at active separatrices, evolve into a thermalized exhaust. Here, the cold populations are heated through a combination of thermalization at the separatrices and pitch angle scattering in the curved magnetic field around the neutral plane. Depending on where the ions enter the exhaust, and how long time they have spent there, they are accelerated to different energies. The superposition of separately thermalized ion populations that have been accelerated to different energies form the hot exhaust population.publishedVersio

The Micro-Macro Coupling of Mass-Loading in Symmetric Magnetic Reconnection With Cold Ions

We investigate how magnetic reconnection is influenced by an inflow of a dense cold ion population. We compare two 2.5D Particle-In-Cell simulations, one containing the cold population and one without. We find that the cold population influences the reconnection process on both global and kinetic scales, and that the dominant contribution can be explained through mass-loading. We provide an analysis of how these multiscale changes are related through kinetic processes in the ion diffusion region, the so-called micro-macro coupling of mass-loading. The inertia of the cold ion population is found to be the significant link that connects the changes on different scales. The cold and warm populations exhibit counter streaming behavior when and after the ion diffusion region reorganizes itself in response to the arrival of the cold population. This signature of the cold population should be observable by spacecraft observatories such as MMS

Millisecond observations of nonlinear wave–electron interaction in electron phase space holes

Electron phase space holes (EHs) associated with electron trapping are commonly observed as bipolar electric field signatures in both space and laboratory plasma. Until recently, it has not been possible to resolve EHs in electron measurements. We report observations of EHs in the plasma sheet boundary layer, here identified as the separatrix region of magnetic reconnection in the magnetotail. The intense EHs are observed together with an electron beam moving toward the X line, showing signs of thermalization. Using the electron drift instrument onboard the satellites of the Magnetospheric Multiscale mission, we make direct millisecond measurements of the electron particle flux associated with individual electron phase space holes. The electron flux is measured at a millisecond cadence in a narrow parallel speed range within that of the trapped electrons. The flux modulations are of order unity and are direct evidence of the strong nonlinear wave–electron interaction that may effectively thermalize beams and contribute to transforming directed drift energy to thermal energy

On the Presence and Thermalization of Cold Ions in the Exhaust of Antiparallel Symmetric Reconnection

Using fully kinetic 2.5 dimensional particle-in-cell simulations of anti-parallel symmetric magnetic reconnection, we investigate how initially cold ions are captured by the reconnection process, and how they evolve and behave in the exhaust. We find that initially cold ions can remain cold deep inside the exhaust. Cold ions that enter the exhaust downstream of active separatrices, closer to the dipolarization front, appear as cold counter-streaming beams behind the front. In the off-equatorial region, these cold ions generate ion-acoustic waves that aid in the thermalization both of the incoming and outgoing populations. Closest to the front, due to the stronger magnetization, the ions can remain relatively cold during the neutral plane crossing. In the intermediate exhaust, the weaker magnetization leads to enhanced pitch angle scattering and reflection. Cold ions that enter the exhaust closer to the X line, at active separatrices, evolve into a thermalized exhaust. Here, the cold populations are heated through a combination of thermalization at the separatrices and pitch angle scattering in the curved magnetic field around the neutral plane. Depending on where the ions enter the exhaust, and how long time they have spent there, they are accelerated to different energies. The superposition of separately thermalized ion populations that have been accelerated to different energies form the hot exhaust population