Whistler waves that can produce anomalous resistivity by affecting electrons' motion have been suggested as one of the mechanisms responsible for magnetic reconnection in the electron diffusion ...region (EDR). Such type of waves, however, has rarely been observed inside the EDR so far. In this study, we report such an observation by Magnetospheric Multiscale (MMS) mission. We find large‐amplitude whistler waves propagating away from the X line with a very small wave‐normal angle. These waves are probably generated by the perpendicular temperature anisotropy of the ~300 eV electrons inside the EDR, according to our analysis of dispersion relation and cyclotron resonance condition; they significantly affect the electron‐scale dynamics of magnetic reconnection and thus support previous simulations.
Key Points
Whistler waves are observed inside the EDR by MMS
The whistlers are propagating away from the X line
The pancake distribution of electrons in the EDR generates the whistlers
New Magnetospheric Multiscale (MMS) observations of small-scale (approx. 7 ion inertial length radius) flux transfer events (FTEs) at the dayside magnetopause are reported. The 1O km MMS tetrahedron ...size enables their structure and properties to be calculated using a variety of multispacecraft techniques, allowing them to be identified as flux ropes, whose flux content is small (approx. 22 kWb).The current density, calculated using plasma and magnetic field measurements independently, is found to be filamentary. lntercomparison of the plasma moments with electric and magnetic field measurements reveals structured non-frozen-in ion behavior. The data are further compared with a particle-in-cell simulation. It is concluded that these small-scale flux ropes, which are not seen to be growing, represent a distinct class of FTE which is generated on the magnetopause by secondary reconnection.
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.
Using observations of Earth's bow shock by the Magnetospheric Multiscale mission, we show for the first time that active magnetic reconnection is occurring at current sheets embedded within the ...quasi‐parallel shock's transition layer. We observe an electron jet and heating but no ion response, suggesting we have observed an electron‐only mode. The lack of ion response is consistent with simulations showing reconnection onset on sub‐ion time scales. We also discuss the impact of electron heating in shocks via reconnection.
Plain Language Summary
For the first time, we document an observation of magnetic reconnection occurring at Earth's bow shock. The observations have been made by NASA's Magnetospheric Multiscale mission while the bow shock is under a “quasi‐parallel” geometry, which typically results in a highly disordered structure. Models of shock waves in space plasmas do not currently account for reconnection. This therefore introduces a new avenue of research into how shocks can repartition energy when slowing the solar wind from supersonic to subsonic flow. The observations also introduce a new regime for magnetic reconnection, for which we observe only an electron response at an ion scale reconnecting structure. This work will also attract interest from the broader astrophysics community, as reconnection at shocks may influence cosmic ray generation.
Key Points
Reconnecting current sheets have been observed at a quasi‐parallel bow shock
The ion‐scale current sheet exhibits only an electron jet and heating, with no ion response
Consistent with kinetic simulations, reconnection relaxes complexity in the shock transition region
We report evidence for reconnection between colliding reconnection jets in a compressed current sheet at the center of a magnetic flux rope at Earth's magnetopause. The reconnection involved nearly ...symmetric Inflow boundary conditions with a strong guide field of two. The thin (2.5 ion-skin depth (d(sub i) width) current sheet (at approximately 12 d(sub i) downstream of the X line) was well resolved by MMS, which revealed large asymmetries in plasma and field structures in the exhaust. Ion perpendicular heating, electron parallel heating, and density compression occurred on one side of the exhaust, while ion parallel heating and density depression were shifted to the other side. The normal electric field and double out-of-plane (bifurcated) currents spanned almost the entire exhaust. These observations are in good agreement with a kinetic simulation for similar boundary conditions, demonstrating in new detail that the structure of large guide field symmetric reconnection is distinctly different from antiparallel reconnection.
Spatial and high-time-resolution properties of the velocities, magnetic field, and 3-D electric field within plasma turbulence are examined observationally using data from the Magnetospheric ...Multiscale mission. Observations from a Kelvin-Helmholtz instability (KHI) on the Earth's magnetopause are examined, which both provides a series of repeatable intervals to analyze, giving better statistics, and provides a first look at the properties of turbulence in the KHI. For the first time direct observations of both the high-frequency ion and electron velocity spectra are examined, showing differing ion and electron behavior at kinetic scales. Temporal spectra exhibit power law behavior with changes in slope near the ion gyrofrequency and lower hybrid frequency. The work provides the first observational evidence for turbulent intermittency and anisotropy consistent with quasi two-dimensional turbulence in association with the KHI. The behavior of kinetic-scale intermittency is found to have differences from previous studies of solar wind turbulence, leading to novel insights on the turbulent dynamics in the KHI.
We compare case studies of Magnetospheric Multiscale (MMS)‐observed magnetopause electron diffusion regions (EDRs) to determine how the rate of work done by the electric field,
J→·(E→+v→e×B→)≡J→·E→′ ...varies with shear angle. We analyze MMS‐observed EDR event with a guide field approximately the same size as the magnetosheath reconnecting field, which occurred on 8 December 2015. We find that
J→·E→′ was largest and positive near the magnetic field reversal point, though patchy lower amplitude
J→·E→′ also occurred on the magnetosphere side EDR near the electron crescent point. The current associated with the large
J→·E→′ near the X point was carried by electrons with a velocity distribution function (VDF) resembling the magnetosheath inflow, shifted in the −v∥ direction. At the magnetosphere side EDR, the current was carried by electrons with a crescent‐like VDF. We compare this 8 December event to 10 other EDRs with different guide field strengths. The dual‐region
J→·E→′ was observed in three other moderate‐shear EDR events, whereas three high‐shear events had a strong positive
J→·E→′ near the electron crescent point and one low‐shear event had a strong positive
J→·E→′ only near the BL=0 point. The dual‐region
J→·E→′>0 was seen for one of three “intermediate”‐shear EDRs with guide fields of ∼0.2–0.3. We propose a physical relationship between the shear angle and mode of energy conversion where (a) a guide field provides an efficient mechanism for carrying a current at the field reversal point (streaming) and (b) a guide field may limit the formation of crescent electron VDFs, limiting the current carried near the stagnation point.
Plain Language Summary
At the boundary between the two, the magnetic fields of the Earth and Sun often interconnect, explosively releasing energy. This reconnection of magnetic fields takes place in a very small region of our magnetosphere's outermost boundary, but the process of reconnection effects nearly the entire magnetosphere. NASA's Magnetospheric Multiscale (MMS) mission was designed to investigate the small scale reconnection region. Within the small reconnection region, the process of exchange energy between electric fields and the surrounding plasma may depend on how the geometry of the connecting magnetic fields. Here we analyze multiple observations of the reconnection region by MMS and show that the strength of the nonreconnecting, out‐of‐the‐reconnection plane portion of the magnetic field may be a crucial factor in governing where this energy release occurs.
Key Points
We determined the location where
J→·E→′>0 for 11 asymmetric EDRs with different guide fields
Increasing the guide field strength appears to move
J→·E→′>0 from the electron crescent to the X point
Guide field allows electron streaming at X point, which takes work by the electric field
Studies of solar wind turbulence traditionally employ high-resolution magnetic field data, but high-resolution measurements of ion and electron moments have been possible only recently. We report the ...first turbulence studies of ion and electron velocity moments accumulated in pristine solar wind by the Fast Plasma Investigation (FPI) instrument on board the Magnetospheric Multiscale Mission. Use of these data is made possible by a novel implementation of a frequency domain Hampel filter, described herein. After presenting procedures for processing of the data, we discuss statistical properties of solar wind turbulence extending into the kinetic range. Magnetic field fluctuations dominate electron and ion-velocity fluctuation spectra throughout the energy-containing and inertial ranges. However, a multispacecraft analysis indicates that at scales shorter than the ion inertial length, electron velocity fluctuations become larger than ion-velocity and magnetic field fluctuations. The kurtosis of ion-velocity peaks around a few ion inertial lengths and returns to a near Gaussian value at sub-ion scales.
A method is described to model the magnetic field in the vicinity of three‐dimensional constellations of satellites (at least four) using field and plasma current measurements. This quadratic model ...matches the measured values of the magnetic field and its curl (current) at each spacecraft, with ∇ • B zero everywhere, and thus extends the linear curlometer method to second order. Near the spacecraft, it predicts the topology of magnetic structures, such as reconnecting regions or flux ropes, and allows a tracking of the motion of these structures relative to the spacecraft constellation. Comparisons to particle‐in‐cell simulations estimate the model accuracy. Reconstruction of two electron diffusion regions definitively confirms the expected field line structure. The model can be applied to other small‐scale phenomena (e.g., bow shocks) and can also be modified to reconstruct the electric field, allowing tracing of particle trajectories.
Key Points
Three‐dimensional model of magnetic field is constructed using magnetic field and current data
The constructions are able to visualize the local magnetic topology around spacecraft
Motion of magnetic structures can be derived
Using data from the MMS mission and the First‐Order Taylor Expansion (FOTE) method, here we reveal electron distribution functions around a reconnection X‐line at the Earth's magnetopause. We find ...cigar distribution of electrons in both the magnetosphere‐side and magnetosheath‐side inflow regions, isotropic distribution of electrons at the separatrix, and loss of high‐energy electrons in the antiparallel direction in the magnetosheath‐side inflow region. We interpret the formation of cigar distribution in the inflow regions using the Fermi mechanism—as suggested in previous simulations, the loss of high‐energy electrons in the magnetosheath side using the parallel electric fields—which evacuate electrons to escape the diffusion region along the antiparallel direction, and the isotropic distribution at the separatrix using the pitch angle scattering by whistler waves—which exist frequently at the separatrix. We also find that the electron distribution functions can change rapidly (within 60 ms) from isotropic to cigar as the spacecraft moves slightly away from the separatrix.
Plain Language Summary
Magnetic reconnection is a key process responsible for many explosive phenomena in nature such as solar flares and magnetospheric substorms. Up to now, the electron behaviors (particularly electron distribution functions) around the reconnection X‐line have not been well revealed, because the X‐line topology is unavailable in previous study. Using data from the MMS mission and the newly developed FOTE method, here we investigate electron distribution functions around a reconnection X‐line and explain the formation mechanisms of these distributions successfully.
Key Points
For the first time, we reveal electron distributions around a reconnection X‐line using the FOTE method, rather than cartoons or simulations
We find different types of electron distributions around the X‐line, and explain the formation mechanisms of these distributions successfully
We find a rapid change of electron distribution within 60 ms. Such rapid change can only be revealed by FOTE but not other methods
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