We report unambiguous in situ observation of the coalescence of macroscopic flux ropes by the magnetospheric multiscale (MMS) mission. Two coalescing flux ropes with sizes of ∼1 R_{E} were ...identified at the subsolar magnetopause by the occurrence of an asymmetric quadrupolar signature in the normal component of the magnetic field measured by the MMS spacecraft. An electron diffusion region (EDR) with a width of four local electron inertial lengths was embedded within the merging current sheet. The EDR was characterized by an intense parallel electric field, significant energy dissipation, and suprathermal electrons. Although the electrons were organized by a large guide field, the small observed electron pressure nongyrotropy may be sufficient to support a significant fraction of the parallel electric field within the EDR. Since the flux ropes are observed in the exhaust region, we suggest that secondary EDRs are formed further downstream of the primary reconnection line between the magnetosheath and magnetospheric fields.
Collisionless shock nonstationarity arising from microscale physics influences shock structure and particle acceleration mechanisms. Nonstationarity has been difficult to quantify due to the small ...spatial and temporal scales. We use the closely spaced (subgyroscale), high-time-resolution measurements from one rapid crossing of Earth's quasiperpendicular bow shock by the Magnetospheric Multiscale (MMS) spacecraft to compare competing nonstationarity processes. Using MMS's high-cadence kinetic plasma measurements, we show that the shock exhibits nonstationarity in the form of ripples.
We report observations from the Magnetospheric Multiscale (MMS) satellites of a large guide field magnetic reconnection event. The observations suggest that two of the four MMS spacecraft sampled the ...electron diffusion region, whereas the other two spacecraft detected the exhaust jet from the event. The guide magnetic field amplitude is approximately 4 times that of the reconnecting field. The event is accompanied by a significant parallel electric field (E(sub parallel lines) that is larger than predicted by simulations. The high-speed (approximately 300 km/s) crossing of the electron diffusion region limited the data set to one complete electron distribution inside of the electron diffusion region, which shows significant parallel heating. The data suggest that E(sub parallel lines) is balanced by a combination of electron inertia and a parallel gradient of the gyrotropic electron pressure.
Electron inflow and outflow velocities during magnetic reconnection at and near the dayside magnetopause are measured using satellites from NASA's Magnetospheric Multiscale (MMS) mission. A case ...study is examined in detail, and three other events with similar behavior are shown, with one of them being a recently published electron‐only reconnection event in the magnetosheath. The measured inflow speeds of 200–400 km/s imply dimensionless reconnection rates of 0.05–0.25 when normalized to the relevant electron Alfvén speed, which are within the range of expectations. The outflow speeds are about 1.5–3 times the inflow speeds, which is consistent with theoretical predictions of the aspect ratio of the inner electron diffusion region. A reconnection rate of 0.04 ± 25% was obtained for the case study event using the reconnection electric field as compared to the 0.12 ± 20% rate determined from the inflow velocity.
Key Points
Electron inflow velocities are determined for reconnection at the magnetopause and in the magnetosheath
For four events inflow velocities of 200–400 km/s imply normalized reconnection rates of 0.05–0.25
Reconnection rates using electron inflow velocities (0.12) and the reconnection electric field (0.04) are compared for one event
Plain Language Summary
When the solar wind impacts the Earth's magnetosphere, an explosive energy conversion process called magnetic reconnection opens the door for solar wind energy to enter the magnetosphere by interconnection of the magnetic fields of the solar wind and of Earth. In this process, magnetic energy is converted to charged‐particle energy. Magnetic reconnection is fairly well understood at large scales and even down to the ion scale. However, the breaking and linking of field lines and the acceleration of electrons occur at much smaller scales, which are only recently being accessed by the NASA Magnetospheric Multiscale mission. This paper analyzes the speed at which electrons flow into and out of reconnection sites. The inflow speeds are crucial because they provide a measurement of the rate at which reconnection proceeds.
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
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.
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.
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.
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
PDF
