Magnetopause Boundary Layer Research Articles

Investigating Boundary Layer Properties at Jupiter's Dawn Magnetopause

AbstractWe survey crossings of Jupiter's dawn magnetopause during the Juno prime mission to identify and characterize Jupiter's magnetopause boundary layer. Using plasma and magnetic field observations from Jovian Auroral Distributions Experiment and Juno Magnetic Field investigation, we identify 53 boundary layer events from the 62 magnetopause crossings studied here. We find that the boundary layer generally exhibits mixed properties of magnetosheath and magnetosphere electron distributions, including lower characteristic electron energies and denser ion populations than in the magnetosphere, but higher characteristic electron energies and less dense ion populations than in the magnetosheath. Boundary layer proton speeds are on average slower than both the magnetosheath and magnetosphere. Other proton parameters in the boundary layer have intermediate values between the magnetosheath and magnetosphere. Through ion composition analysis in regions adjacent to the magnetopause, we find evidence of solar wind and magnetospheric plasma in the boundary layer that suggests plasma is transported across the magnetopause in both directions. This mass and energy transport may be the result of solar wind interactions such as magnetic reconnection and Kelvin‐Helmholtz instabilities. However, many boundary layer events do not exhibit local signatures of these solar wind interactions and plasma may be transported by a non‐local process or diffusively transported.

Multiple Flux Rope Dynamics: MMS Observations and Reconstruction Results

A series of six ion-scale magnetic flux ropes (FR1–6) and a thin current sheet, encountered by Magnetospheric Multiscale spacecraft in the magnetosheath side of the dayside magnetopause boundary layer, are studied for multiple flux rope dynamics in terms of energy conversion and two-dimensional geometry structure. The thin current sheet is identified in between FR1 and FR2. The energy exchange between electromagnetic fields and plasmas is dynamic in FR1–5 and also surrounding the flux ropes, while a low-energy conversion rate is seen in FR6. Different energy conversions exist in the flux ropes: energy dynamo is predominant in FR1 and FR5, while energy dissipation is dominated in FR2–4. Both the energy dynamo and dissipation primarily result from j ∥ E ∥. Strong dissipation, surrounded by an energy dynamo, happens at the thin current sheet and is accompanied by ion agyrotropic behavior. From the reconstructed magnetic field maps, the estimated aspect ratios of the six flux ropes are 0.78, 0.16, 0.66, 0.11, 0.40, and 0.46 in order, and the thickness of the thin current sheet is ∼63 km (∼1.8 ion inertial length). Evidently, the magnetic field map shows that FR4 is a coalescing flux rope where a pronounced dissipation is present. The overall finding agrees with the previous simulation studies of multiple island coalescence.

The Electric Properties of the Magnetopause Boundary Layer

The magnetopause plays a pivotal role in the coupling among solar wind, the magnetosheath, and the magnetosphere. By analyzing magnetopause crossing events using MMS, we reveal a local non-neutrality of electric charges in the magnetopause boundary layer and the associated electric field. There are two types of electric structures. In one group, which typically occurs on the dusk side, the electric field directs towards the Earth. In the other, which generally occurs on the day side, the field directs away from the Earth. The spatial extent of this electric non-neutrality spans approximately 600 km, which is at the scale of ion gyrational motion. These findings provide valuable insights into the fine structures of the magnetopause and the coupling between the magnetosheath and the magnetosphere.

TRICE‐2/SuperDARN Observations and Comparison With the Associated MMS Magnetopause Crossing

AbstractTwo sounding rockets, designated TRICE‐2, were launched on 8 December 2018 into the northern cusp region. The two rockets were designated the high‐ and low‐flyers, respectively, and launched 2 min apart to investigate cusp structures, specifically their spatial or temporal nature. 2 hr prior to the cusp encounter by the TRICE‐2 rockets, the MMS satellites, located in the magnetopause boundary layer, observed switching ion beams under very similar IMF conditions as later observed by TRICE‐2. The observed ion beam switch in the boundary layer defined the location of the primary dayside X‐line. Both, TRICE‐2 and MMS, also observed the signatures of multiple X‐lines at the magnetopause, overlapping ion‐energy dispersions in the cusp and counterstreaming ion beams in the magnetopause boundary layer, respectively. In addition to the TRICE‐2 cusp observations, ionospheric convection patterns from the SuperDARN radar are used to explain the vastly different cusp ion signatures observed by the TRICE‐2 rockets. While the high‐flyer rocket progressed north through the center of the cusp, the low‐flyer rocket drifted off to the east and crossed into the dusk convection cell, traveling perpendicular to the ionospheric convection direction before reaching the poleward oriented section of the convection cell also observed by the high‐flyer counterpart.

Small-scale Field-aligned Currents in the Magnetopause Boundary Layer

Based on high-resolution measurements from the Magnetospheric Multiscale mission from 2015 May to 2018 June, we statistically investigate the properties of small-scale field-aligned currents (SFACs) in the magnetopause boundary layer. A total of 2235 SFACs are successfully identified. The durations of SFACs mainly fall between 0.2 and 0.3 s. Over 90% of SFACs have a width of less than 1 ion inertia length and are primarily distributed from 5 to 25 electron inertia lengths, implying that the SFACs belong to the kinetic-scale current layer. The main carriers of SFACs are electrons, and over 70% of SFACs exhibit net energy dissipation (i.e., J · E ′ > 0) with the majority of energy dissipation taking place in the parallel direction. SFACs are widely distributed spatially, and the occurrence rate of SFACs is higher in the boundary layer closer to the magnetosphere. Additionally, less than half of the total SFACs are identified in well-known structures, including the magnetic reconnection region, flux transfer event, Kelvin–Helmholtz vortex, and exhaust region, and 54% of the SFACs are in the “others” unknown structures. These results improve our comprehension of the current system at the magnetopause and the roles of SFACs in the coupling between the solar wind and magnetosphere.

Dynamics of the Storm Time Magnetopause and Magnetosheath Boundary Layers: An MMS‐THEMIS Conjunction

AbstractThis letter uses simultaneous observations from Magnetosphere Multiscale (MMS) and Time History of Events and Macroscale Interactions during Substorms (THEMIS) to address the dynamics of the magnetopause and magnetosheath boundary layers during the main phase of a storm during which the interplanetary magnetic field (IMF) reverses from south to north. Near the dawn terminator, MMS observes two boundary layers comprising open and closed field lines and containing energetic electrons and ring current oxygen. Some closed field line regions exhibit sunward convection, presenting an avenue to replenish dayside magnetic flux lost during the storm. Meanwhile, THEMIS observes two boundary layers in the pre‐noon sector which strongly resemble those observed at the flank by MMS. Observations from the three THEMIS spacecraft indicate the boundary layers are still evolving several hours after the IMF has turned northward. These observations advance our knowledge of the dynamic magnetopause and magnetosheath boundary layers under the combined effects of an ongoing storm and changing IMF.

Energy Conversion by a Twisted Magnetic Structure at Magnetopause Boundary Layer: MMS Observations

AbstractThe Earth's magnetopause is an ion‐scale boundary that separates the magnetosphere from the shocked solar wind. At such boundary, energy‐conversion processes frequently occur. Previous studies suggested that such energy conversions are related to the magnetic reconnection process. Here, we report a new mechanism that the coaction between the twisted magnetic structure and lower‐hybrid waves can also drive energy conversion (J⋅ E, J is current density and E is electric field) at the magnetopause boundary layer, by using high‐resolution measurements of the four Magnetospheric Multiscale spacecraft. We find that such energy conversion is efficient (with magnitude up to 20 nW/m3) and is attributed to an intense current filament (j ≈ 3,800 nA/m2) and a fluctuating electric field driven by lower‐hybrid waves. With the help of the First‐Order Taylor Expansion method, we find that the intense current filament is driven by a twisted magnetic structure at electron scale. Our study improves the understanding of energy‐conversion processes at the Earth's magnetopause.

Multi‐Band Periodic Poleward‐Moving Auroral Arcs at the Postdawn Sector: A Case Study

AbstractPoleward‐moving auroral arcs (PMAAs) with recurrence periods on the order of minutes have been established as a distinct form of auroral activities. Their origin and characteristics, especially those on the dayside, are less understood due to the paucity of observations. Here, we investigate a periodic PMAA event at the postdawn sector based on optical measurements from the high‐latitude Yellow River Station and magnetic measurements from multiple ground‐based stations. The optical observations demonstrate a long duration (∼2.5 hr) of the periodic auroral arcs at not only the well‐established red‐line but also the green‐ and the blue‐lines. This multi‐band feature, together with images at far‐ultraviolet wavelengths, demonstrates a wide energy range of precipitating electrons from hundreds of eV to several keV, with a characteristic energy of ∼2 keV. The auroral luminosity variations at different wavebands show a consistent periodicity within 1.67–3 mHz, which is also shown in the ground‐based magnetic pulsations. The auroral arc and magnetic field observations display signatures of field line resonance, which are likely driven by perturbations at the magnetopause boundary layer. These results complement our knowledge of the dayside periodic PMAAs and improve our understanding of their relationship with ultra‐low frequency waves.

Auroral, Ionospheric and Ground Magnetic Signatures of Magnetopause Surface Modes

AbstractSurface waves on Earth's magnetopause have a controlling effect upon global magnetospheric dynamics. Since spacecraft provide sparse in situ observation points, remote sensing these modes using ground‐based instruments in the polar regions is desirable. However, many open conceptual questions on the expected signatures remain. Therefore, we provide predictions of key qualitative features expected in auroral, ionospheric, and ground magnetic observations through both magnetohydrodynamic theory and a global coupled magnetosphere‐ionosphere simulation of a magnetopause surface eigenmode. These show monochromatic oscillatory field‐aligned currents (FACs), due to both the surface mode and its non‐resonant Alfvén coupling, are present throughout the magnetosphere. The currents peak in amplitude at the equatorward edge of the magnetopause boundary layer, not the open‐closed boundary as previously thought. They also exhibit slow poleward phase motion rather than being purely evanescent. We suggest the upward FAC perturbations may result in periodic auroral brightenings. In the ionosphere, convection vortices circulate the poleward moving FAC structures. Finally, surface mode signals are predicted in the ground magnetic field, with ionospheric Hall currents rotating perturbations by approximately (but not exactly) 90° compared to the magnetosphere. Thus typical dayside magnetopause surface modes should be strongest in the East‐West ground magnetic field component. Overall, all ground‐based signatures of the magnetopause surface mode are predicted to have the same frequency across L‐shells, amplitudes that maximize near the magnetopause's equatorward edge, and larger latitudinal scales than for field line resonance. Implications in terms of ionospheric Joule heating and geomagnetically induced currents are discussed.

MMS Observations of Storm‐Time Magnetopause Boundary Layers in the Vicinity of the Southern Cusp

AbstractDuring a storm‐time interval around winter solstice, observations by the Magnetospheric Multi‐Scale (MMS) Mission show multiple distinct magnetopause boundary layers (BLs) in the vicinity of the southern cusp. The microphysics of the solar wind‐magnetosphere interaction during storm times are not well understood, because the observations are relatively lacking. This event enables the opportunity to probe the storm‐time magnetopause, and observations support that MMS was near a reconnection site equatorward of the southern cusp, suggesting active reconnection in close proximity to closed magnetic flux regions in the BL. The Grid Agnostic magnetohydrodynamics (MHD) for Extended Research Applications global MHD simulation shows evidence for transient secondary reconnection sites near the southern cusp, demonstrating mechanisms to form closed field line regions of the BL.

Plasma Observations During the 7 June 2021 Ganymede Flyby From the Jovian Auroral Distributions Experiment (JADE) on Juno

AbstractWe report on plasma observations from Juno/Jovian Auroral Distributions Experiment during the Ganymede flyby on 7 June 2021. Juno approached Ganymede from southern latitudes, passed through the wake region, then through its magnetosphere to closest approach (1,046 km from the surface) on the night side, and then back into Jupiter's plasma disk. We describe general plasma properties in the regions explored along the trajectory. We infer that Juno traversed a region of open field lines where one end intercepts Ganymede and the other Jupiter. The observations do not support Juno crossing into the closed field line region. The ion composition near Ganymede is very different than that of the nearby plasma environment. H2+ and H3+ ions were detected near Ganymede and in the wake region. Low energy (∼0.1–1 keV) electrons are enhanced just outside the magnetopause, in the wake (inbound trajectory) and in the magnetopause boundary layer (outbound trajectory).

Jupiter's Sheared Flow Unstable Magnetopause Boundary Observed by Juno

AbstractThe interaction between the solar wind and giant magnetospheres (i.e., Jupiter's and Saturn's magnetospheres) is fundamentally important for magnetospheric physics, in which the viscous interaction (presumably driven by the Kelvin‐Helmholtz (KH) instability) is often expected to play an important role. Previous simulation and theory studies suggested that Jupiter's low‐latitude dawn side flank region is KH unstable due to the large sheared flow between the fast tailward solar wind and magnetospheric sunward corotational flow. The onset of the KH instability can strongly modify the magnetopause boundary layer, which is consistent with the identification of 194 boundary crossings during Juno's first 14 orbits. We applied four different boundary normal analysis methods to illustrate that there is always a wide range of local normal directions, suggesting Jupiter's dawn‐side flank region is often highly perturbed. The distribution of local normal directions is insensitive to the upstream solar wind dynamic pressure, Juno's inward/outward boundary crossing direction, and the location along the z direction of the boundary crossing. We also used a 2‐D magnetohydrodynamic simulation to demonstrate that such a wide distribution can be formed by the KH instability even at the beginning of the nonlinear stage.

Intense Energy Conversion Events at the Magnetopause Boundary Layer

AbstractThis paper investigates intense energy conversion events (IECEs) at the dayside magnetopause boundary layer (MBL) using high‐resolution data from the Magnetospheric Multiscale (MMS) spacecraft. Most events were detected by MMS in a duration less than 0.5 s, corresponding to a thickness below one ion inertia length. About three‐quarters of the events occurred in local reconnecting current sheets. Interestingly, there are more electron‐only reconnections than standard reconnections. Most of the reconnection IECEs are within the exhaust of the primary reconnection and the Kelvin‐Helmholtz vortices, suggesting that secondary reconnections are important building blocks for the overall energy conversion at the magnetopause. The nonreconnection events are also primarily within the reconnection exhaust and the Kelvin‐Helmholtz vortices, highlighting the importance of reconnection and Kelvin‐Helmholtz instability in regulating the energy conversion at the MBL.

Statistics of the Intense Current Structure in the Dayside Magnetopause Boundary Layer

AbstractThis paper presents a comprehensive study of the intense current structures (ICSs) at the dayside magnetopause, by using the high‐resolution data from the Magnetospheric Multiscale (MMS) mission. About 3,600 ICSs with a current density exceeding 1.2 μA/m2 have been detected within the magnetopause boundary layer (MBL). We find that most ICSs crossed the spacecraft in less than 1 s, corresponding to an average thickness of less than one ion inertial length (di). The number of ICSs decreases with the thickness increasing from the electron‐scale to the ion‐scale. Boundary layers closer to Earth have more ICS than boundary layers further away from Earth, probably caused by the large solar wind dynamic pressure. The occurrence rate of the ICS is higher in the dusk sector near the meridian. For most ICSs, the current is carried by electrons. The perpendicular current (90° to the magnetic field) is larger than the parallel current (0° or 180° to the magnetic field) for more ICSs. The energy conversion between the magnetic field and plasma as measured by is primarily through the perpendicular current and electric field, while the energy dissipation is mainly dominated by the parallel component. ICSs provide much stronger energy conversion and dissipation compared to the ambient plasma in the MBL. This study improves our understanding of the characteristics of the ICS and its role in solar wind‐magnetosphere coupling.

Measurements of the Net Charge Density of Space Plasmas

AbstractSpace plasmas are composed of charged particles that play a key role in electromagnetic dynamics. However, to date, there has been no direct measurement of the distribution of such charges in space. In this study, three schemes for measuring charge densities in space are presented. The first scheme is based on electric field measurements by multiple spacecraft. This method is applied to deduce the charge density distribution within Earth's magnetopause boundary layer using Magnetospheric MultiScale constellation (MMS) four‐point measurements, and indicates the existence of a charge separation there. The second and third schemes proposed are both based on electric potential measurements from multiple electric probes. The second scheme, which requires 10 or more electric potential probes, can yield the net charge density to first‐order accuracy, while the third scheme, which makes use of seven to eight specifically distributed probes, can give the net charge density with second‐order accuracy. The feasibility, reliability, and accuracy of these three schemes are successfully verified for a charged‐ball model. These charge density measurement schemes could potentially be applied in both space exploration and ground‐based laboratory experiments.

Magnetic Field Gradient Across the Flank Magnetopause

Magnetic pressure inside the magnetopause is usually balanced with a sum of thermal plasma and magnetic pressures on the magnetosheath side. However, observations reveal that the magnetosheath magnetic field can be frequently larger than that in the magnetosphere (inverse magnetic field gradient across the magnetopause), and thus, the enhanced pressure from the magnetosheath side seems to be uncompensated. Such events are rare in the subsolar region, but their occurrence rate increases toward flanks. The analysis, based on statistical processing of about 35,000 THEMIS magnetopause crossings collected in the course of the years 2007–2017, shows that these events are more frequently observed under enhanced geomagnetic activity that is connected with a strong southward IMF. Case studies reveal that such a state of the magnetopause boundary layers can persist for several hours. This study discusses conditions and mechanisms keeping the pressure balance across the magnetopause under these conditions.

Long and Active Magnetopause Reconnection X‐Lines During Changing IMF Conditions

AbstractThe magnetic reconnection process at the Earth's magnetopause is observed at various times and places as either anti‐parallel or component reconnection. The two scenarios are combined in a model known as the Maximum Magnetic Shear model, which predicts the location of long and continuous reconnection X‐lines across the dayside magnetopause along the ridge of maximum magnetic shear. This study investigates a Magnetospheric Multiscale Mission (MMS) magnetopause encounter where the spacecraft had repeated contact with the boundary layers for about 40 min. While in contact with the magnetopause boundary layers, the Interplanetary Magnetic Field (IMF) slowly and continuously rotated, magnetically connecting the MMS spacecraft to different sections along an extended dayside reconnection X‐line. These magnetic connection points slide along component reconnection tilted X‐lines and also connect several times to the anti‐parallel section of the magnetopause X‐line. Despite the continuously changing IMF conditions, each magnetopause boundary layer encounter shows accelerated ion beams, indicating an active reconnection X‐line at the magnetopause that covers a length of about 20 RE.

Field‐Aligned Currents in Auroral Vortices

AbstractAuroral bright spots have been observed at Earth, Jupiter, and Saturn in regions that map to the magnetopause boundary layer. It has been suggested that the bright spots are associated with the Kelvin‐Helmholtz (KH) instability. We utilize a quasistatic magnetosphere‐ionosphere coupling model driven by a vortex in the boundary layer to determine how the field‐aligned current structure depends on ionospheric and boundary layer parameters. We compare vortex induced currents with shear‐flow induced currents. We find that the strength of the maximum currents are comparable, but the structure is significantly different. For a vortex, the current and electron precipitation maximize when the vortex size mapped to the ionosphere is approximately 1.5L, where is the auroral scale length, ΣP is the Pedersen conductivity, and κ is the Knight parameter. For a vortex, the current width provides a direct measure of the size, Δ, of the boundary layer structure, while the current width of shear‐flow aurora is generally determined by the larger of Δ or L. For comparison with observations, an event is considered where auroral bright spots in the ionosphere are detected by DMSP SSUSI FUV when KH structures are observed on the dusk flank by THEMIS.

Flux Transfer Event With an Electron‐Scale Substructure Observed by the Magnetospheric Multiscale Mission

AbstractOn 12 November 2015 the Magnetospheric Multiscale (MMS) spacecraft traversed the magnetopause from the magnetosphere to the magnetosheath encountering evidence of magnetic reconnection and a tiny flux transfer event (FTE) on the magnetosheath side of the magnetopause boundary layer. The FTE exhibited a large negative‐positive bipolar variation in the normal magnetic field component (BN), an enhanced (negative) north‐south component direction (BL), and a variation in the third component BM resembling a “W” shape with negative values close to the edges and positive values near the center. Using the tetrahedron formation, we estimate that the FTE moved southward and duskward with a speed of VFTE = 324 km/s. The FTE size in the transverse direction is 518 km, which corresponds to 4.42 ion gyroradii. We identify an internal layer where the electron bulk flow velocity exhibits different behaviors at each of the four spacecraft and the current density was more intense at two spacecraft. Within the FTE's core region, the ion bulk flow behavior was similar for all four spacecraft. Using the velocity obtained from timing analysis, we estimate a dimension of this core region to be 181.4 km, which corresponds to 1.5 ion gyroradii. It is evident that the internal region is not large enough to affect the ion behavior but can do so for electrons. We conclude that the FTE's core is an electron‐scale substructure.

The 18 November 2015 Magnetopause Crossing: The GEM Dayside Kinetic Challenge Event Observed by MMS/HPCA

AbstractAt Earth's dayside magnetopause, the fundamental process known as magnetic reconnection is responsible for connecting geomagnetic field lines to interplanetary magnetic field lines and converting magnetic energy into particle energy and heat. As part of a coordinated analysis effort by the Geospace Environment Modeling (GEM) community to determine the reconnection location and other aspects of reconnection, a magnetopause crossing by the Magnetospheric Multiscale (MMS) mission on 18 November 2015 was selected. The crossing occurred during stable solar wind and dominantly southward interplanetary magnetic field conditions. The MMS satellites were located close to the subsolar region and observed southward accelerated ion jets during encounters with the magnetopause boundary layers, indicating the presence of an active X‐line north of the satellites. Using proton distributions from the Hot Plasma Composition Analyzer (HPCA) instruments, the distance from the spacecraft to the reconnection site was initially determined to be between 5 to 7 RE. A slight rotation of the interplanetary magnetic field caused a change of the magnetic topology at the magnetopause. This topology change produced conditions which are known to change the location of the X‐line. After the change, the location of the component reconnection line was estimated to be less than 4 RE, but most likely was much closer to the observing satellites.