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We analyze the local dynamics of magnetotail reconnection onset using Magnetospheric Multiscale (MMS) data. In conjunction with MMS, the macroscopic dynamics of this event were captured by a number of other ground and space‐based observatories, as is reported in a companion paper. We find that the local dynamics of the onset were characterized by the rapid thinning of the cross‐tail current sheet below the ion inertial scale, accompanied by the growth of flapping waves and the subsequent onset of electron tearing. Multiple kinetic‐scale magnetic islands were detected coincident with the growth of an initially sub‐Alfvénic, demagnetized tailward ion exhaust. The onset and rapid enhancement of parallel electron inflow at the exhaust boundary was a remote signature of the intensification of reconnection Earthward of the spacecraft. Two secondary reconnection sites are found embedded within the exhaust from a primary X‐line. The primary X‐line was designated as such on the basis that (a) while multiple jet reversals were observed in the current sheet, only one reversal of the electron inflow was observed at the high‐latitude exhaust boundary, (b) the reconnection electric field was roughly five times larger at the primary X‐line than the secondary X‐lines, and (c) energetic electron fluxes increased and transitioned from anti‐field‐aligned to isotropic during the primary X‐line crossing, indicating a change in magnetic topology. The results are consistent with the idea that a primary X‐line mediates the reconnection of lobe magnetic field lines and accelerates electrons more efficiently than its secondary X‐line counterparts. Key Points: Magnetotail reconnection onset was triggered by electron tearing during a solar wind pressure pulseOnset was characterized by the rapid collapse of the current sheet thickness and kinetic‐scale flux rope formationA primary X‐line was established within minutes of the onset [ABSTRACT FROM AUTHOR]

Characterization of Earth's magnetic field is key to understanding dynamics of the core. We assess whether Iridium Communications magnetometer data can be used for this purpose since. The 66 Iridium satellites are in 86° inclination, 780 km altitude, circular orbits, with 11 satellites in each of six orbit planes. In one day the constellation returns 300,000 measurements spanning the globe with <2° spacing. We used data from January 2010 through November 2015, and compared against International Geomagnetic Reference Field (IGRF-11) to inter-calibrate all data to the same model. Geomagnetically quiet 24-h intervals were selected using the total Birkeland current, auroral electrojet, and ring current indices. The z-scores for these quantities were combined and the quietest 16 intervals from each quarter selected for analysis. Residuals between the data and IGRF-11 yield consistent patterns that evolve gradually from 2010 to 2015. Residuals for each day were binned in 9° latitude by 9° longitude and the distributions about the mean in each bin are Gaussian with 1-sigma standard errors of ~3 nT. Spherical harmonic coefficients for each quiet day were computed and time series of the coefficients used to identify artifacts at the orbit precession (8 months) and seasonal (12 months) periods and their harmonics which were then removed by notch filtering. This analysis yields time series at 800 virtual geomagnetic observatories each providing a global field map using a single day of data. The results and CHAOS 7.4 generally agree, but systematic differences larger than the statistical uncertainties are present that warrant further exploration. [ABSTRACT FROM AUTHOR]

We use the magnetohydrodynamic (MHD) with embedded particle‐in‐cell model (MHD‐EPIC) to study the Geospace Environment Modeling (GEM) dayside kinetic processes challenge event at 01:50–03:00 UT on 18 November 2015, when the magnetosphere was driven by a steady southward interplanetary magnetic field (IMF). In the MHD‐EPIC simulation, the dayside magnetopause is covered by a PIC code so that the dayside reconnection is properly handled. We compare the magnetic fields and the plasma profiles of the magnetopause crossing with the MMS3 spacecraft observations. Most variables match the observations well in the magnetosphere, in the magnetosheath, and also during the current sheet crossing. The MHD‐EPIC simulation produces flux ropes, and we demonstrate that some magnetic field and plasma features observed by the MMS3 spacecraft can be reproduced by a flux rope crossing event. We use an algorithm to automatically identify the reconnection sites from the simulation results. It turns out that there are usually multiple X‐lines at the magnetopause. By tracing the locations of the X‐lines, we find that the typical moving speed of the X‐line endpoints is about 70 km/s, which is higher than but still comparable with the ground‐based observations. Key Points: The MHD‐EPIC simulation magnetic fields and plasma data match MMS3 observations well during the magnetopause crossingThere are usually multiple X‐lines at the magnetopause in the MHD‐EPIC simulationThe MHD‐EPIC simulation shows complex movement and spreading of the X‐lines [ABSTRACT FROM AUTHOR]

Using a three-dimensional (3-D) global-scale hybrid code, the Magnetospheric Multiscale (MMS) reconnection event around 02:13 UT on 18 November 2015, highlighted in the Geospace Environment Modeling (GEM) Dayside Kinetic Challenge, is simulated, in which the interplanetary magnetic field (IMF) points southward and the geomagnetic field has a -27° dipole tilt angle. Strong southward plasma jets are found near the magnetopause as a result of the dayside reconnection. Our results indicate that the subsolar magnetopause reconnection X line shifts from the subsolar point toward the Northern Hemisphere due to the effect of the tilted geomagnetic dipole angle, consistent with the MMS observation. Subsequently, the reconnection X lines or sites and reconnection flux ropes above the equator propagate northward along the magnetopause. The formation and global distribution of the X lines and the structure of the magnetopause reconnection are investigated in detail with the simulation. Mirror mode waves are also found in the middle of the magnetosheath downstream of the quasi-perpendicular shock where the plasma properties are consistent with the mirror instability condition. As a special outcome of the GEM challenge event, the spatial and temporal variations in reconnection, the electromagnetic power spectra, and the associated D-shaped ion velocity distributions in the simulated reconnection event are compared with the MMS observation. [ABSTRACT FROM AUTHOR]

The development of large‐scale Birkeland currents is examined using the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), which measures global Birkeland currents continuously on 10‐min time scales. The integrated current was used to identify onsets of at least 1 MA preceded by periods of quiescence lasting at least 3 hr. The Region 1 currents do not fully form without Region 2. Rather, they develop together, first on the dayside, then on the nightside, and lastly, they fill in and intensify at all local times to form the nominal statistical pattern. The onsets are closely correlated with enhancements of magnetospheric forcing as indicated by the solar wind electric field and the orientation of the interplanetary magnetic field. Nightside onsets correspond to intensifications of the auroral electrojet as reflected in the AE index; they are delayed by ~40 min relative to the increase of the dayside current and are 2.8 times more rapid than the dayside current increase. After nightside onset, Birkeland currents expand toward dawn and dusk and merge with the dayside currents while also intensifying at all local times. The dayside current pattern depends on the sign of the interplanetary magnetic field BY. The nightside current distributions are the same for positive and negative BY and display a Harang discontinuity independent of the sign of BY. The predominant development and intensification of Birkeland currents occur after nightside onset at all local times with roughly 75% of the total current, both Regions 1 and 2, appearing after nightside onset. Plain Language Summary: This study uses data from the 66 Iridium Communications satellites to track the development of electric currents that drive aurora. These currents can turn on in less than about an hour. However, the currents can only be measured from satellites orbiting the Earth above the atmosphere, and in those orbits, satellites take about 100 min to orbit the Earth. With the Iridium constellation, we remeasure these currents every 10 min and are able to track where they start and how they grow. This happens in two steps: Currents appear first in the day where they remain. About 30 min later, new currents start near midnight and then spread toward dawn and dusk and increase until they completely encircle the poles in an oval pattern. The total current finally reaches millions of Amperes. The two‐step process implies that explosive dynamics in the magnetic tail of the Earth play a key role in creating and generating the auroral currents. This study shows that the effects they have on the temporal sequence of activity onset need to be included to predict the effects of the solar wind and solar storms on the Earth's ionosphere and upper atmosphere. Key Points: Analysis of Birkeland current growth following extended intervals of low current reveals the development sequence of the global systemCurrents begin on the dayside with both Regions 1 and 2 and do not appear on the nightside until a sudden onset about 30 min laterAt onset, nightside Regions 1 and 2 appear together, intensify, and expand to the dayside forming the complete system after about an hour [ABSTRACT FROM AUTHOR]