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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. publishedVersion

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. publishedVersion

Heavier plasma species such as oxygen ions can have a large impact on the magnetic reconnection process. It has been hypothesized that the acceleration of demagnetized oxygen ions by the Hall electric field will lead to the formation of an oxygen wave that expands into the exhaust. By comparing data from NASA's Magnetospheric Multiscale mission to a fully kinetic particle-in-cell simulation, we can for the first time provide observational evidence of such an expanding oxygen wave. The wave is characterized by an oxygen jet consisting of cold ions directed toward the neutral sheet associated with a density cavity. This density cavity forms as the O+ are subject to collective acceleration by the Hall electric field leaving behind a region of low-density oxygen ions. Our results are important for the understanding of the role and effect of oxygen ions in magnetic reconnection. publishedVersion

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. publishedVersion

We investigate the role of a background oxygen population in magnetic reconnection, using particle‐in‐cell simulations. We run several simulations, with different initial oxygen temperatures and densities, to understand how the reconnection rate is influenced, as oxygen is captured by the reconnection process. The oxygen remains approximately demagnetized on the relevant time and spatial scales and therefore has little direct (i.e., immediate mass loading) effect on the reconnection process itself. The reconnection rate is independent of the initial oxygen temperature but clearly dependent on the density. The reduced reconnection rate is twice as fast as predicted by mass loading. We describe a mechanism where the oxygen population (and the associated electrons) acts as an energy sink on the system, altering the energy partitioning. Based on a scaling analysis, we derive an estimate of the reconnection electric field that scales as (1+no/np)−1, where no and np is the oxygen and proton densities, respectively. publishedVersion

A new look at the structure of the electron diffusion region in collision less magnetic reconnection is presented. The research is based on a particle-in-cell simulation of asymmetric magnetic reconnection, which includes a temperature gradient across the current layer in addition to density and magnetic field gradient. We find that none of X-point, flow stagnation point, and local current density peak coincide. Current and energy balance analyses around the flow stagnation point and current density peak show consistently that current dissipation is associated with the divergence of nongyrotropic electron pressure. Furthermore, the same pressure terms, when combined with shear-type gradients of the electron flow velocity, also serve to maintain local thermal energy against convective losses. These effects are similar to those found also in symmetric magnetic reconnection. In addition, we find here significant effects related to the convection of current, which we can relate to a generalized diamagnetic drift by the nongyrotropic pressure divergence. Therefore, only part of the pressure force serves to dissipate the current density. However, the prior conclusion that the role of the reconnection electric field is to maintain the current density, which was obtained for a symmetric system, applies here as well. Finally, we discuss related features of electron distribution function in the electron diffusion region (EDR). Specifically, we analyze both new crescent substructures as well as outer, higher energy crescents generated by accelerated magnetospheric particles. publishedVersion

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. publishedVersion

Using 2.5-D Particle-In-Cell (PIC) simulations, we investigate how magnetotail reconnection is affected by a cold, streaming, oxygen plasma population, attributed to an ionospheric source, in the inflow region. As the tailward streaming oxygen reaches the current layer, a tailward motion of the reconnection site is induced. Due to the much longer cyclotron period of the oxygen ions, oxygen cannot couple as directly into the reconnection dynamics as protons. We find that the oxygen ions couple indirectly by means of impacting the electron dynamics. Therefore, a demagnetized species can, in fact, alter the dynamics of the reconnection site. We see further that the reconnection rate remains unchanged relative to a nonstreaming run. Our results may prove useful for understanding the development and dynamics of magnetospheric substorms and storms. publishedVersion

Combined with the magnetic field, the distribution of charged particles in the inflow region is expected to control the rate of magnetic reconnection. This paper investigates how the reconnection process is altered by a cold, asymmetrically distributed, oxygen population, which is initially located away from the current layer in the inflow regions. A particle-in-cell simulation is used to gain further insight into the dynamics of the system. The time evolution of the reconnection process proceeds rapidly compared to the cyclotron period of O urn:x-wiley:grl:media:grl59941:grl59941-math-0001. Therefore, the oxygen remains, to a good approximation, demagnetized. Thus, Alfvén scaling is not an adequate description of the reconnection rate. A scaling relation for the reconnection rate for an asymmetrically distributed, demagnetized species has been developed. Additionally, we find that an asymmetric density configuration leads to a distinct motion of the reconnection site and generates an asymmetry of the diffusion region and the Hall electric field. publishedVersion

In this study we investigate the performance of the University of Michigan’s Space Weather Modeling Framework (SWMF) in prediction of ground magnetic perturbations (ΔB) and their rate of change with time (dB/dt), which is directly connected to geomagnetically induced currents (GICs). We use the SWMF set-up where the global magnetosphere provided by the Block Adaptive Tree Solar-wind Roe-type Upwind Scheme (BATS-R-US) MHD code, is coupled to the inner magnetosphere and the ionospheric electrodynamics. The validation is done for ΔB and dB/dt separately. The performance is evaluated via data-model comparison through a metrics-based approach. For ΔB, the normalized root mean square error (nRMS) and the correlation coefficient are used. For dB/dt, the probability of detection, the probability of false detection, the Heidke skill score, and the frequency bias are used for different dB/dt thresholds. The performance is evaluated for eleven ground magnetometer stations located between 59° and 85° magnetic latitude and spanning about five magnetic local times. Eight geomagnetic storms are studied. Our results show that the SWMF predicts the northward component of the perturbations better at lower latitudes (59°–67°) than at higher latitudes (>67°), whereas for the eastward component, the model performs better at high latitudes. Generally, the SWMF performs well in the prediction of dB/dt for a 0.3 nT/s threshold, with a high probability of detection ≈0.8, low probability of false detection (B/dt prediction performance generally decreases as the threshold is raised, except for the probability of false detection, which improves.

We employ a 2.5D particle-in-cell simulation to study a scenario where the reconnection process captures cold streaming protons. As soon as the tailward streaming protons become involved, they contribute to the overall momentum balance, altering the initially symmetric dynamics. Adding tailward-directed momentum to the reconnection process results in a tailward propagation of the reconnection site. We investigate how the reconnection process reorganizes itself due to the changing momentum conditions on the kinetic scale and how the reconnection rate is affected. We find that adding tailward momentum does not result in a significantly different reconnection rate compared to the case without cold streaming protons, when scaled to the total Alfvén velocity. This implies that the effect of changing inflow conditions due to the motion of the reconnection site appears to be minimal. The dynamics of the particles are, however, significantly different depending on whether they enter on the tailward or Earthward side of the reconnection site. On the Earthward side they are reflected and thermalized, while on the tailward side they are picked up and accelerated. The cold proton density and Ez on the Earthward side are turbulent, while the tailward side has laminar cold proton density striations and an embedded Ez layer. Also, since the initial plasma sheet population is swept up on one side and flushed out on the other, asymmetries in the densities and strength of Hall fields emerge. Our results are important for understanding the development and dynamics of magnetospheric substorms and storms. publishedVersion