We describe methods for polynomial reconstruction of the magnetic field close to a cluster of spacecraft and apply that to reconstruction of the magnetic field observed during a magnetic reconnection ...event on 10 August 2017 by the Magnetospheric Multiscale spacecraft. Four different models are described, which vary in complexity between a 12‐parameter linear model, which has only linear variation with respect to the spatial coordinates, and a 27‐parameter quadratic model, which has the full quadratic expansion except that the second derivative with respect to the Minimum Directional Derivative minimum gradient coordinate
m has been neglected. In contrast to previous reconstruction techniques, these reconstructions can be found using only the magnetic field and current density measured at a single time by the cluster of spacecraft. The equations satisfying
∇·B=0 are satisfied exactly, while the equations specifying the model fields at the spacecraft locations are satisfied for most models in a best least squares sense. For this magnetotail event, the models have very small errors in magnetic field components (
<0.1 nT) at a distance from the nearest spacecraft on the order of the spacecraft separation,
Lsc, here equal to 20.5 km. The magnetic structures found using the quadratic models are very time dependent, with a stretched field leading to plasmoid formation at one point in time.
Key Points
Polynomial reconstruction methods using both magnetic field and particle current density for input are described
For a magnetotail event, the magnetic field error is small (
<0.1 nT) up to about one spacecraft separation from the nearest spacecraft
The quadratic reconstructions are very dynamic, with stretched magnetic fields leading to formation of plasmoids
We report Magnetospheric Multiscale observations of electron pressure gradient electric fields near a magnetic reconnection diffusion region using a new technique for extracting 7.5 ms electron ...moments from the Fast Plasma Investigation. We find that the deviation of the perpendicular electron bulk velocity from E × B drift in the interval where the out-of-plane current density is increasing can be explained by the diamagnetic drift. In the interval where the out-of-plane current is transitioning to in-plane current, the electron momentum equation is not satisfied at 7.5 ms resolution.
Magnetic reconnection is of fundamental importance to plasmas because of its role in releasing and repartitioning stored magnetic energy. Previous results suggest that this energy is predominantly ...released as ion enthalpy flux along the reconnection outflow. Using Magnetospheric Multiscale data we find the existence of very significant electron energy flux densities in the vicinity of the magnetopause electron dissipation region, orthogonal to the ion energy outflow. These may significantly impact models of electron transport, wave generation, and particle acceleration.
Magnetotail reconnection plays a crucial role in explosive energy conversion in geospace. Because of the lack of in-situ spacecraft observations, the onset mechanism of magnetotail reconnection, ...however, has been controversial for decades. The key question is whether magnetotail reconnection is externally driven to occur first on electron scales or spontaneously arising from an unstable configuration on ion scales. Here, we show, using spacecraft observations and particle-in-cell (PIC) simulations, that magnetotail reconnection starts from electron reconnection in the presence of a strong external driver. Our PIC simulations show that this electron reconnection then develops into ion reconnection. These results provide direct evidence for magnetotail reconnection onset caused by electron kinetics with a strong external driver.
We report observations of turbulent dissipation and particle acceleration from large‐amplitude electric fields (E) associated with strong magnetic field (B) fluctuations in the Earth's plasma sheet. ...The turbulence occurs in a region of depleted density with anti‐earthward flows followed by earthward flows suggesting ongoing magnetic reconnection. In the turbulent region, ions and electrons have a significant increase in energy, occasionally >100 keV, and strong variation. There are numerous occurrences of |E| >100 mV/m including occurrences of large potentials (>1 kV) parallel to B and occurrences with extraordinarily large J · E (J is current density). In this event, we find that the perpendicular contribution of J · E with frequencies near or below the ion cyclotron frequency (fci) provide the majority net positive J · E. Large‐amplitude parallel E events with frequencies above fci to several times the lower hybrid frequency provide significant dissipation and can result in energetic electron acceleration.
Plain Language Summary
The Magnetospheric Multiscale mission is able to examine dissipation associated with magnetic reconnection with unprecedented accuracy and frequency response. The observations show that roughly 80% of the dissipation is from the perpendicular currents and electric fields. However, large‐amplitude parallel electric fields appear to play a strong role in turbulent dissipation into electrons and in electron acceleration.
Key Points
MMS observations reveal characteristics of turbulent dissipation and particle acceleration associated with magnetic reconnection
Perpendicular electric fields and large‐amplitude parallel electric fields structures have dominant roles in turbulent dissipation
Turbulent electric fields in a magnetic structure is shown to play a key role in accelerating electrons to greater than 100 keV energies
One of the major unresolved questions regarding the magnetic reconnection phenomenon is how plasma waves impact the process. In 2015, the National Aeronautics and Space Administration launched the ...four‐satellite Magnetospheric Multiscale Mission to study magnetic reconnection, especially on the electron scale. Since launch, it has identified several wave modes below the electron plasma frequency that occur near the dayside reconnection X‐line. These include large‐amplitude parallel electrostatic waves, whistler mode waves, lower hybrid waves, and turbulence associated with a corrugated current structure. We survey 23 electron diffusion region events observed by Magnetospheric Multiscale Mission at the dayside magnetopause to help understand how these wave modes impact the reconnection process. Common wave modes are identified, as well as their typical location within the reconnection layer (e.g., electron diffusion region, ion diffusion region, separatrix, and inflow and outflow jets). We find that, with a few exceptions, electromagnetic whistlers are most commonly confined to the separatrices of the exhaust boundary. Lower hybrid waves are found on the magnetosphere side of the current layer and do not make it to the X‐line. The wave modes that typically occur closest to the dissipation region are the corrugated current structures and large‐amplitude parallel electrostatic waves.
Key Points
We survey waves below the electron plasma frequency near 23 dayside magnetopause electron diffusion region events
The main waves investigated are whistlers, parallel electrostatic waves, current corrugation, and the lower hybrid drift instability
We find waves most typically occur on the magnetospheric side, with current corrugation most likely to reach the diffusion region
The role and properties of lower hybrid waves in the ion diffusion region and magnetospheric inflow region of asymmetric reconnection are investigated using the Magnetospheric Multiscale (MMS) ...mission. Two distinct groups of lower hybrid waves are observed in the ion diffusion region and magnetospheric inflow region, which have distinct properties and propagate in opposite directions along the magnetopause. One group develops near the ion edge in the magnetospheric inflow, where magnetosheath ions enter the magnetosphere through the finite gyroradius effect and are driven by the ion‐ion cross‐field instability due to the interaction between the magnetosheath ions and cold magnetospheric ions. This leads to heating of the cold magnetospheric ions. The second group develops at the sharpest density gradient, where the Hall electric field is observed and is driven by the lower hybrid drift instability. These drift waves produce cross‐field particle diffusion, enabling magnetosheath electrons to enter the magnetospheric inflow region thereby broadening the density gradient in the ion diffusion region.
Key Points
Two groups of lower hybrid waves are observed in the ion diffusion and magnetospheric inflow regions
In the magnetospheric inflow region lower hybrid waves develop when cold magnetospheric ions are present and can heat cold ions
In the diffusion region lower hybrid waves develop at the density gradient and can cause cross‐field particle diffusion
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