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.
Magnetic reconnection is a fundamental physical process in plasmas whereby stored magnetic energy is converted into heat and kinetic energy of charged particles. Reconnection occurs in many ...astrophysical plasma environments and in laboratory plasmas. Using measurements with very high time resolution, NASA's Magnetospheric Multiscale (MMS) mission has found direct evidence for electron demagnetization and acceleration at sites along the sunward boundary of Earth's magnetosphere where the interplanetary magnetic field reconnects with the terrestrial magnetic field. We have (i) observed the conversion of magnetic energy to particle energy; (ii) measured the electric field and current, which together cause the dissipation of magnetic energy; and (iii) identified the electron population that carries the current as a result of demagnetization and acceleration within the reconnection diffusion/dissipation region.
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
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
We report Magnetospheric Multiscale observations of macroscopic and electron-scale current layers in asymmetric reconnection. By intercomparing plasma, magnetic, and electric field data at multiple ...crossings of a reconnecting magnetopause on 22 October 2015, when the average interspacecraft separation was approximately 10 km, we demonstrate that the ion and electron moments are sufficiently accurate to provide reliable current density measurements at 30ms cadence. These measurements, which resolve current layers narrower than the interspacecraft separation, reveal electron-scale filamentary Hall currents and electron vorticity within the reconnection exhaust far downstream of the X line and even in the magnetosheath. Slightly downstream of the X line, intense (up to 3 μA/m2) electron currents, a super-Alfvenic outflowing electron jet, and nongyrotropic crescent shape electron distributions were observed deep inside the ion-scale magnetopause current sheet and embedded in the ion diffusion region. These characteristics are similar to those attributed to the electron dissipation/diffusion region around the X line.
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
In the Earth's magnetotail, magnetic reconnection releases stored magnetic energy and drives magnetospheric convection. The rate at which magnetic flux is transferred from the reconnection inflow to ...outflow regions is determined by the reconnection electric field Er, which is often referred to as the unnormalized reconnection rate. To better quantify the efficiency of reconnection, this electric field Er is often normalized by the characteristic Alfvén speed and the reconnecting magnetic field. This parameter is generally called the normalized or dimensionless reconnection rate R. In this paper, we employ a two‐dimensional fully kinetic simulation to model a magnetotail reconnection event with weak geomagnetic activity (<200 nT of the AE index) observed by the Magnetospheric Multiscale (MMS) mission on 11 July 2017. We obtain R and Er from direct measurements in the diffusion region and indirect measurements of the rate at the separatrix using a recently proposed remote sensing technique. The measured normalized rate for this MMS event is R ∼0.15–0.2, consistent with theoretical and simulation models of fast collisionless reconnection. This corresponds to an unnormalized rate of Er ∼2–3 mV/m. Based on quantitative consistencies between the simulation and the MMS observations, we conclude that our estimates of the reconnection rates are reasonably accurate. Given that past studies have found Er of the order ∼10 mV/m during strong geomagnetic substorms, these results indicate that the local Er in magnetotail reconnection may be an important parameter controlling the amplitude of geomagnetic disturbances.
Key Points
Reliable reconnection rates are obtained based on virtual observations in a fully kinetic simulation of an MMS tail reconnection event
The normalized rates obtained from the simulation and MMS data are 0.15–0.2, indicating the occurrence of fast reconnection
The observed unnormalized rate is 2–3 mV/m, while higher rates were observed in other events with stronger geomagnetic activities
We discuss methods to determine L‐M‐N coordinate systems for current sheet crossings observed by the Magnetospheric Multiscale (MMS) spacecraft mission during ongoing reconnection, where eL is the ...direction of the reconnecting component of the magnetic field, B, and eN is normal to the magnetopause. We present and test a new hybrid method, with eL estimated as the maximum variance direction of B (MVAB) and eN as the direction of maximum directional derivative of B, and then adjust these directions to be perpendicular. In the best case, only small adjustment is needed. Results from this method, applied to an MMS crossing of the dayside magnetopause at 1305:45 UT on 16 October 2015, are discussed and compared with those from other methods for which eN is obtained by other means. Each of the other evaluations can be combined with eL from MVAB in a generalized hybrid approach to provide an L‐M‐N system. The quality of the results is judged by eigenvalue ratios, constancy of directions using different data segments and methods, and expected sign and magnitude of the normal component of B. For this event, the hybrid method appears to produce eN accurate to within less than 10°. We discuss variance analysis using the electric current density, J, or the J × B force, which yield promising results, and minimum Faraday residue analysis and MVAB alone, which can be useful for other events. We also briefly discuss results from our hybrid method and MVAB alone for a few other MMS reconnection events.
Plain Language Summary
We discuss methods for determining coordinate systems in order to study magnetic reconnection events at the magnetopause, the boundary between the ionized gas in the region of space dominated by the Earth's magnetic field and the ionized gas coming from the solar wind. We introduce a new method that combines results from multiple methods in order to determine the three coordinate directions in space. We demonstrate this method by applying it to an event observed by the Magnetospheric Multiscale spacecraft on 16 October 2015 and at other times.
Key Points
Methods to determine L‐M‐N current sheet coordinates are described and tested
Quality of results is judged by eigenvalue ratios and consistency using different data intervals and methods and with the geophysical context
For the interval examined here, the uncertainty of the normal direction was at least several degrees but probably less than 10°
We present a series of electron holes observed simultaneously on four Magnetospheric Multiscale spacecraft in the plasma sheet boundary layer. The multispacecraft probing shows that the electron ...holes propagated quasi‐parallel to the local magnetic field with velocities of a few thousand kilometers per second with parallel spatial scales of a few kilometers (a few Debye lengths). The simultaneous multispacecraft probing allows analyzing the 3‐D configuration of the electron holes. We estimate the electric field gradients and charge densities associated with the electrons holes. The electric fields are fit to simple 3‐D electron hole models to estimate their perpendicular scales and demonstrate that the electron holes were generally not axially symmetric with respect to the local magnetic field. We emphasize that most of the electron holes had a complicated structure not reproduced by the simple models widely used in single‐spacecraft studies.
Plain Language Summary
We present the first measurement of electron holes (electrostatic solitary waves) by four Magnetospheric Multiscale spacecraft simultaneously. Such observation has allowed us to directly measure the charge density and to address the 3‐D structure of these electron holes both for the first time. The analysis of 3‐D configuration of the electron holes can be valuable for analysis of electron holes observed in space plasmas.
Key Points
Simultaneous observations of electron phase space holes at four MMS spacecraft are presented
The charge density within the electron holes is computed using the electric field measurements at four spacecraft
The three‐dimensional configuration of the electron holes is analyzed, and the perpendicular scales are estimated
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