An $LMN$ coordinate system for magnetic reconnection events is sometimes determined by defining $N$ as the direction of the gradient across the current sheet and $L$ as the direction of maximum variance of the magnetic field. The third direction, $M$, is often assumed to be the direction of zero gradient, and thus the orientation of the X line. But when there is a guide field, the X line direction may have a significant component in the L direction defined in this way. For a 2D description, a coordinate system describing such an event would preferably be defined using a different coordinate direction $M’$ oriented along the X line. Here we use a 3D particle-in-cell simulation to show that the X line is oriented approximately along the direction bisecting the asymptotic magnetic field directions on the two sides of the current sheet. We describe two possible ways to determine the orientation of the X line from spacecraft data, one using the minimum gradient direction from Minimum Directional Derivative analysis at distances of the order of the current sheet thickness from the X line, and another using the bisection direction based on the asymptotic magnetic fields outside the current sheet. We discuss conditions for validity of these estimates, and we illustrate these conditions using several Magnetospheric Multiscale (MMS) events. We also show that intersection of a flux rope due to secondary reconnection with the primary X line can destroy invariance along the X line and negate the validity of a two-dimensional description.

A document by Jason M H Beedle. Click on the document to view its contents.

Whistler waves are often observed in magnetopause reconnection associated with electron beams. We analyze seven MMS crossings surrounding the electron diffusion region (EDR) to study the role of electron beams in whistler excitation. Waves have two major types: (1) Narrow-band waves with high ellipticities and (2) broad-band waves that are more electrostatic with significant variations in ellipticities and wave normal angles. While both types of waves are associated with electron beams, the key difference is the anisotropy of the background population, with perpendicular and parallel anisotropies, respectively. The linear instability analysis suggests that the first type of wave is mainly due to the background anisotropy, with the beam contributing additional cyclotron resonance to enhance the wave growth. The second type of distribution excites broadband waves via Landau resonance, and as seen in one event, the beam anisotropy induces an additional cyclotron mode. The results are supported by particle-in-cell simulations. We infer that the first type occurs downstream of the central EDR, where background electrons experience Betatron acceleration to form the perpendicular anisotropy; the second type occurs in the central EDR of guide field reconnection. A parametric study is conducted with linear instability analysis. A beam anisotropy alone of above ~3 likely excites the cyclotron mode waves. Large beam drifts cause Doppler shifts and may lead to left-hand polarizations in the ion frame. Future studies are needed to determine whether the observation covers a broader parameter regime and to understand the competition between whistler and other instabilities.

We present observations in Earth’s magnetotail by the Magnetospheric Multiscale spacecraft that are consistent with magnetic field annihilation, rather than magnetic topology change, causing fast magnetic-to-electron energy conversion in an electron-scale current sheet. Multi-spacecraft analysis for the magnetic field reconstruction shows that an electron-scale magnetic island was embedded in the observed electron diffusion region (EDR), suggesting an elongated shape of the EDR. Evidence for the annihilation was revealed in the form of the island growing at a rate much lower than expected for the standard collisionless reconnection, which indicates that magnetic flux injected into the EDR was not ejected from the X-point or accumulated in the island, but was dissipated in the EDR. This energy conversion process is in contrast to that in the standard EDR of a reconnecting current sheet where the energy of antiparallel magnetic fields is mostly converted to electron bulk-flow energy. Fully kinetic simulation also demonstrates that an elongated EDR is subject to the formation of electron-scale magnetic islands in which fast but transient annihilation can occur. Consistent with the observations and simulation, theoretical analysis shows that fast magnetic diffusion can occur in an elongated EDR in the presence of nongyrotropic electron effects. We suggest that the annihilation in elongated EDRs may contribute to the dissipation of magnetic energy in a turbulent collisionless plasma.

Reconnection in the magnetotail occurs along so-called X-lines, where magnetic field lines tear and detach from plasma on microscopic spatial scales (comparable to particle gyroradii). In 2017–2020 the Magnetospheric MultiScale (MMS) mission detected X-lines in the magnetotail enabling their investigation on local scales. However, the global structure and evolution of these X-lines, critical for understanding their formation and total energy conversion mechanisms, remained virtually unknown because of the intrinsically local nature of observations and the extreme sparsity of concurrent data. Here we show that mining a multi-mission archive of space magnetometer data collected over the last 25+ years and then fitting a magnetic field representation modeled using flexible basis-functions, faithfully reconstructs the global pattern of X-lines; 24 of the 26 modeled X-lines match ($B_z=0$ isocontours are within $\sim2$ Earth radii or $R_E$) or nearly match ($B_z=2$ nT isocontours are within $\sim2 R_E$) the locations of the MMS encountered reconnection sites.

We calculate the aspect ratio of the electron diffusion region (EDR) during symmetric magnetic reconnection using magnetic field gradients measured by the Magnetospheric Multiscale mission (MMS). The technique introduced in this paper is validated using a particle-in-cell (PIC) simulation, compared with the reconnection rate for three MMS-observed magnetotail EDRs, then compared with the EDR aspect ratio predicted by scaling dependencies on asymptotic upstream electron $\beta$. For the MMS events, we find that the EDR aspect ratio determined using the magnetic field gradients are within uncertainty of the normalized reconnection rate found by previous studies for three MMS-observed EDRs. Because the magnetic field gradients are velocity-frame independent and typically very well measured by MMS, the technique can be used to obtain the normalized reconnection rate with higher fidelity than established methods.

In situ spacecraft missions are powerful assets to study processes that occur in space plasmas. One of their main limitations, however, is extrapolating such local measurements to the global scales of the system. To overcome this problem at least partially, multi-point measurements can be used. There are several multi-spacecraft missions currently operating in the Earth’s magnetosphere, and the simultaneous use of the data collected by them provides new insights into the large-scale properties and evolution of magnetospheric plasma processes. In this work, we focus on studying the Earth’s magnetopause using a conjunction between the MMS and Cluster fleets, when both missions skimmed the magnetopause for several hours at distant locations during radial IMF conditions. The observed magnetopause positions as a function of the evolving solar wind conditions and compared to model predictions of the magnetopause. We observe an inflation of the magnetosphere (˜0.7 RE), consistent with magnetosheath pressure decrease during radial IMF conditions, which is less pronounced on the flank (< 0.2 RE). There is observational evidence of magnetic reconnection in the subsolar region for the whole encounter, and in the dusk flank for the last portion of the encounter, suggesting that reconnection was extending more than 15 RE. However, reconnection jets were not always observed, suggesting that reconnection was patchy, intermittent or both. Shear flows reduce the reconnection rate up to ˜30% in the dusk flank according to predictions, and the plasma ß enhancement in the magnetosheath during radial IMF favors reconnection suppression by the diamagnetic drift.

In order to determine particle velocities and electric field in the frame of the magnetic structure, one first needs to determine the velocity of the magnetic structure in the frame of the spacecraft observations. Here, we show how to determine a two dimensional magnetic structure velocity for the magnetic reconnection event observed in the magnetotail by the Magnetospheric Multiscale (MMS) spacecraft on 11 July 2017. We use two different multi-spacecraft methods, Spatio-Temporal Difference (STD) and the recently developed polynomial reconstruction method. Both of these methods use the magnetic field measurements, and the reconstruction technique also uses the current density measured by the particle instrument. We find rough agreement between the results of our methods and with other velocity determinations previously published. We also explain a number of features of STD and show that the polynomial reconstruction technique is only likely to be valid within a distance of two spacecraft spacings from the centroid of the MMS spacecraft. Both of these methods are susceptible to contamination by magnetometer calibration errors.

We examine the July 11th, 2017 electron diffusion region (EDR) observed by the Magnetospheric Multiscale (MMS) mission using Poynting’s theorem. The terms in Poynting’s theorem are determined using a linear gradient approximation to obtain barycentric averages within the MMS tetrahedron. We find that Poynting’s theorem is approximately balanced in the EDR, and the balance is improved if the calculation of ∇∙S is restricted to the LN plane. The work rate per unit volume J∙E is mostly balanced by the divergence of the electromagnetic energy flux ∇∙S, indicating that the electromagnetic energy density remains relatively constant within the EDR during the encounter. We also use particle-in-cell (PIC) simulations to examine Poynting’s theorem near an x-line evolving in time. The central EDR in the simulation is characterized by approximate time independent balance in Poynting’s theorem during reconnection growth, while the outer EDR exhibits time-dependent fluctuations indicative of more chaotic behavior.