Dawn Magnetopause 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.

Investigating the Occurrence of Kelvin‐Helmholtz Instabilities at Jupiter's Dawn Magnetopause

AbstractWe use the Kelvin‐Helmholtz instability (KHI) condition with particle and magnetic field observations from Jovian Auroral Distributions Experiment and MAG on Juno along the dawn flank of Jupiter's magnetosphere. We identify the occurrence of magnetopause crossings that show evidence of being KH (Kelvin‐Helmholtz) unstable. When estimating the k vector to be parallel to the velocity shear, we find that 25 of 62 (40%) magnetopause crossings satisfy the KHI condition. When considering the k vector of the maximum growth rate through a solid angle approach, we find that 60 of 62 (97%) events are KH unstable. This study shows evidence of KH waves at Jupiter's dawn flank, including primary drivers such as high velocity shears and changes in plasma pressure. Signatures of magnetic reconnection were also observed in ∼25% of the KH unstable crossings. We discuss these results and their implication for the prevalence of KHI at Juno's dawn magnetopause as measured by Juno.

Investigating the Occurrence of Magnetic Reconnection at Jupiter's Dawn Magnetopause During the Juno Era

AbstractIn this study, observations from the Jovian auroral distributions experiment, Jupiter energetic particle detector instrument, and Magnetic field investigation instruments on Juno are used to identify signatures of magnetic reconnection at Jupiter's dawn magnetopause and relate these signatures to the local plasma environment. Magnetopause crossings occurred between 73–114 Jovian radii and 4.3–6.2 magnetic local time at low latitudes. Reconnection signatures include plasma energization and ion velocity enhancements resembling reconnection jets. We test for diamagnetic suppression which considers the magnetic shear and plasma beta (β) before and after a magnetopause crossing. Findings show that a large majority of these dawn magnetopause crossings at Jupiter have a low likelihood for local magnetic reconnection (are diamagnetically suppressed) because of high delta β values and/or low magnetic shear angles across the magnetopause boundary. These crossings exhibit no evidence of local reconnection while crossings that are not diamagnetically suppressed show multiple signatures of reconnection, adhering to the Swisdak relation.

Characteristics of the Flank Magnetopause: MMS Results

AbstractWe have used a large number of magnetopause crossings by the Magnetospheric Multiscale (MMS) mission to investigate macroscopic properties of this current sheet, with emphasis on the flanks of the magnetopause. Macroscopic features such as thickness, location, and motion of the magnetopause were calculated as a function of local time sector. The results show that the flanks of the magnetopause are significantly thicker than the dayside magnetopause. Thicknesses vary from about 650 km near noon to over 1,000 km near the terminator. Current densities vary in a similar manner, with average current densities around noon almost twice as high as near the terminator. We also find a dawn‐dusk asymmetry in many of the macroscopic parameters; the dawn magnetopause is thicker than at dusk, while the dusk flank is more dynamic, with a higher average normal velocity.

Survey of Jupiter's Dawn Magnetosheath Using Juno

AbstractThe Juno spacecraft crossed Jupiter's bow shock and into the magnetosheath on 24 June 2016. It then went into 53‐day polar orbits with its apojoves at 113 Jovian radii on Jupiter's dawn side. These orbits have given us a unique opportunity to survey Jupiter's dawn magnetosheath. Using data from the Jovian Auroral Distributions Experiment, the Magnetic Field Investigation, and the Waves Investigation, we have identified 91 magnetosheath crossings for a total of 48.2 days of data. We present the statistical properties of the magnetosheath plasma, including density, velocity, magnetic field strength, temperature, plasma beta, pressures, and Mach numbers. We then show correlations between these various parameters. We confirm the rotation of the magnetic field to align with Jupiter's spin axis and find that this rotation correlates with several of the plasma properties, including magnetic field strength and plasma flow velocity. Finally, we discuss how these results can affect magnetic reconnection and the Kelvin‐Helmholtz instability at Jupiter's dawn magnetopause boundary.

The Difference Between Isolated Flux Transfer Events and Flux Transfer Event Cascades

AbstractThis flux transfer event (FTE) study is based on 984 FTEs originally identified by Wang et al. (2005, https://doi:10.1029/2005JA011150) in Cluster data. Due to Cluster's orbit, the FTE list exclusively contains events detected at the high‐latitude dayside magnetopause and low‐latitude flanks. The focus of this study is on FTE separation time. The results show that FTEs appearing in cascades are mainly located at the northern dusk and southern dawn magnetopause, while isolated FTEs are equally spread over the region covered by Cluster. This difference may be explained by the different interplanetary magnetic field (IMF) conditions during which the subsets occur. For isolated FTEs, average IMF By is close to zero. During such conditions, FTEs are expected to form at arbitrary longitudes along an equatorial merging line. After formation, they propagate northward and southward, causing an equal distribution at higher latitudes. In contrast, FTE cascades typically occur during weakly southward IMF with a negative By component. Their asymmetric distribution at higher latitudes is consistent with both the component and the antiparallel merging model for nonzero By. In both scenarios, newly formed FTEs are expected to move to the northern dusk and southern dawn regions, as observed. Many FTE cascades appearing during northward IMF are located close to the low‐latitude flanks, confirming previous reports. We discovered that such FTEs appear during large IMF values. Another new result is that 16% of all isolated FTEs appear during small IMF cone angles, suggesting that these may form as a result of magnetosheath jets impacting on the magnetopause.

Accelerated flows at Jupiter's magnetopause: Evidence for magnetic reconnection along the dawn flank

AbstractWe report on plasma and magnetic field observations from Juno's Jovian Auroral Distributions Experiment and Magnetic Field Investigation at 18 magnetopause crossings when the spacecraft was located at ~6 h magnetic local time and 73–114 Jovian radii from Jupiter. Several crossings showed evidence of plasma energization, accelerated ion flows, and large magnetic shear angles, each representing a signature of magnetic reconnection. These signatures were observed for times when the magnetosphere was in both compressed and expanded states. We compared the flow change magnitudes to a simplified Walén relation and found ~60% of the events to be 110% or less of the predicted values. Close examination of two magnetopause encounters revealed characteristics of a rotational discontinuity and an open magnetopause. These observations provide compelling evidence that magnetic reconnection can occur at Jupiter's dawn magnetopause and should be incorporated into theories of solar wind coupling and outer magnetosphere dynamics at Jupiter.

Plasma flows in Saturn's nightside magnetosphere

AbstractThe plasma properties, especially the flow parameters, obtained from numerical integration of Cassini Plasma Spectrometer (CAPS) Ion Mass Spectrometer measurements during intervals when the CAPS field of view encompassed both inward and outward flow directions relative to corotation are examined for nightside data (18–06 local time) during 2006, 2009, and 2010. The results show good agreement with previously reported values derived using different selection criteria and a different analysis technique. Nightside flows are predominantly in or near the corotation direction, indicating continuing influence of connection to the ionosphere. There is no evidence for a quasi‐steady reconnection x line within the surveyed region of the tail, although dynamic events attributable to transient reconnection have been observed. There is a net radial mass outflow, leading to an estimated net mass loss between 18 and 03 local time of ~34 kg/s. Part of this mass loss occurs as a “planetary wind” along the dusk flank. The remainder probably occurs ultimately much deeper in the tail and along the distant dawn magnetopause, when the mass disconnects from the weakened planetary magnetic field.

MESSENGER observations of large flux transfer events at Mercury

Six flux transfer events (FTEs) were encountered during MESSENGER's first two flybys of Mercury (M1 and M2). For M1 the interplanetary magnetic field (IMF) was predominantly northward and four FTEs with durations of 1 to 6 s were observed in the magnetosheath following southward IMF turnings. The IMF was steadily southward during M2, and an FTE 4 s in duration was observed just inside the dawn magnetopause followed ∼32 s later by a 7‐s FTE in the magnetosheath. Flux rope models were fit to the magnetic field data to determine FTE dimensions and flux content. The largest FTE observed by MESSENGER had a diameter of ∼1 RM (where RM is Mercury's radius), and its open magnetic field increased the fraction of the surface exposed to the solar wind by 10–20 percent and contributed up to ∼30 kV to the cross‐magnetospheric electric potential.

Configuration and dynamics of the Jovian magnetosphere

A massive rotating equatorial plasma sheet dominates Jupiter's magnetosphere and the solar wind and the interplanetary magnetic field (IMF) are not thought to be as important as at Earth. However, in a recent simulation study we found that for a purely northward IMF the Jovian magnetosphere reached an unstable state in which a nearly periodic series of magnetic X and O lines were launched tailward. In this study we have carried out three‐dimensional global magnetohydrodynamic simulations to investigate the causes of this dynamic behavior. First, we examined the effects of dynamic pressure on the magnetospheric configuration in the absence of an IMF. We examined a series of northward IMF simulations in which we varied both the IMF magnitude and dynamic pressure. If the outer edge of the rotating plasma sheet is far from the X‐line (for large pressure and small IMF), the reconnected flow reaches the dawn magnetopause and exits down the tail. If the neutral line forms closer to the rotation boundary (small pressure and IMF), then the reconnected flux tubes can convect all of the way around Jupiter. When they reach the nightside, they become stretched tailward and can reconnect again. This leads to the periodic behavior. If the neutral line forms very close to the rotation boundary (large pressure and IMF), the Jupiterward flow compresses the rotating plasma sheet. In this case the flow goes around Jupiter but the flux tubes return along the dusk magnetopause and do not participate in reconnection a second time.

Characteristics of the near-tail dawn magnetopause and boundary layer

Abstract. The paper discusses properties of the near-tail dawnside and boundary layer, as obtained from Cluster plasma and magnetic field measurements during a single skimming orbit on 4 and 5 July 2001 that included 24 well-defined crossings by all four spacecraft. As a result of variations of the interplanetary magnetic field, the magnetic shear across the local varied between ~0° and ~180°. Using an improved method, which takes into account acceleration and thickness variation, we have determined the orientation, speed, thickness and current for the 96 individual crossings. The orientations show clear evidence of surface waves. Magnetopause thicknesses range from ~100 to ~2500km, with an average of 753km. The speeds range from less than 10up to more than 300, with an average of 48. Both results are consistent with earlier ISEE and AMPTE results obtained for the dayside magnetopause. Importantly, scaling the thicknesses to the ion gyro radius or the ion inertial length did not reduce the large dynamic range. There is also no significant dependence of thickness on magnetic shear. Current densities range from ~0.01 up to ~0.3uA, with an average value of 0.05 . By including some extra crossings that did not involve all four spacecraft, we were able to apply the Walén test to a total of 60 by Cluster 1 and 3, and have classified 19 cases as rotational discontinuities (RDs), of which 12 and 7 were sunward and tailward of an X-line, respectively. Of these 60 crossings, 26 show no trace of a boundary layer. The only with substantial boundary layers are into the plasma mantle. Of the 26 without a boundary layer, 8 were identified as RDs. Since reconnection produces wedge-shaped boundary layers emanating from the X-line, RDs without may be considered close to the X-line, in which case the observed magnetic shear and Alfvén Mach number should be representative of the conditions at the X-line itself. It is therefore important that four of the eight cases had shear angles ≤100, i.e. the reconnecting fields were far from being anti-parallel, and that all eight cases had Alfvén Mach numbers MA>1 in the adjoining magnetosheath. Another important conclusion can be drawn from the without a that were tangential discontinuities (TDs). To observe TDs with no at such large distances from the subsolar point appears to rule out diffusion over large portions of the as an effective means for plasma transport across the magnetopause.

The flank magnetopause: INTERBALL observations

The magnetopause represents a key boundary in the solar wind — magnetosphere coupling. Our analysis of corresponding INTERBALL-1 and MAGION-4 spacecraft observations of the high-latitude and flank magnetopause reveals a variety of transient processes superimposed upon a complicated boundary layer structure. The turbulent motion of the plasma compressed behind the bow shock can cause a local deformation of the magnetopause and its inward motion with respect to the mean position determined from magnetopause models. The presented statistics shows that this process prevails on the dawn magnetopause. The effects of the magnetospheath turbulence are further amplified by reconnection if the magnetic shear is high enough.

Surface waves on the dawn magnetopause: Connection with ground PC 5 pulsations

Observations of the Geotail spacecraft show that ion flow oscillations in the Pc 5 frequency range are fairly common at the equatorial region in the dawn magnetosphere (−5 ≤ X GSM ≤ 5). The amplitudes of these oscillations sometimes exceed 250 km/s. We examined in detail several events associated with temporal magnetopause crossings during the course of oscillations. It is found that plasma flow oscillations near the magnetopause correspond closely with magnetic field pulsations at latitudes of 70 – 75 degrees on the ground, although the form of the flow oscillation is spiky rather than sinusoidal. The boundary normal inclines sunward (tailward), when the spacecraft enters the magnetosheath (magnetosphere). Most of magnetopause crossings analyzed are cases with high magnetic shears (Bz in the magnetopause was negative in most cases). Outward and sunward flows, whose magnitudes are greater than those in the magnetosheath, were observed in the magnetosphere after inbound crossings, while inward and tailward flows were detected before outbound crossings. These characteristics are not necessarily consistent with the excitation of surface waves by the Kelvin-Helmholtz instability. A high-speed flow with the sunward and outward component was also observed in the magnetosphere after the inbound crossing with high magnetic shear in an isolated and temporal magnetopause crossing event drivent by a pressure pulse.

A global magnetohydrodynamic simulation of the Jovian magnetosphere

We have developed a three‐dimensional global magnetohydrodynamic simulation of the interaction between the solar wind and a rapidly rotating magnetosphere and applied it to Jupiter. For fixed solar wind dynamic pressure the rotating model Jovian magnetosphere extends farther toward the Sun and has greater extent in the east–west direction than a model without rotation but is little different in the north south direction. There is a pronounced dawn‐dusk asymmetry with the dawn magnetopause displaced farther from Jupiter. The middle Jovian magnetosphere contains a thin plasma sheet dominated by rotating plasmas. On the day side this plasma sheet thickens near the magnetopause. Near the dawnside magnetopause where rotating Jovian convection is opposed to the solar wind induced flow, a pressure ridge forms where the magnetospheric flow slows and forms a stagnation region. In the magnetotail the rotating flow is diverted tailward. For x < −100RJ x‐type and o‐type neutral lines form. When the solar wind pressure was decreased, the boundaries moved away from Jupiter and the dayside field lines became stretched into a more tail‐like configuration. A flow vortex formed in the evening middle magnetosphere.

Energetic particles in the vicinity of the dawn magnetopause

This paper presents first results of two-point measurements carried out onboard two satellites of the INTERBALL project which was launched into a highly elliptical orbit in August, 1995. Both satellites are provided with instrumentation for charged particle measurements in a wide energy range (from 0.1 keV to 1 MeV). The distance between the satellites was about 1000 km for the presented events. During the crossings of the magnetopause region, the flux and angular distribution of electrons just inside the low-latitude magnetosphere are compared with those just outside the boundary layer and the electron leakage from the magnetosphere is examined. The peculiarities of the distributions observed by different satellites are discussed in terms of boundary position shifting. The results of the study of the low-latitude magnetopause are compared with the measurements carried out in the plasma mantle region.

Correction to: “A filament of energetic particles near the high‐latitude dawn magnetopause”

In the paper A filament of energetic particles near the high-latitude dawn magnetopause by A. T. Y. Lui, D. $. Williams, R. W. McEntire, S. P. Christon, C. $acquey, V. Angelopoulos, T. Yamamoto, S. Kokubun, L. A. Frank, K. L. Ackerson, and K. Tsuruda (Oeophys. Res. Lett., 21 (25), 3011-3014, 1994), on page 3014, the second sentence of the last paragraph should read as follows: Given that these two ion species have different field-aligned velocities but share a common convection velocity perpendicular to the magnetic field, the lack of a spatial separation between them allows us to put an upper limit on how far upstream the detachment of the filament could have taken place.

A filament of energetic particles near the high‐latitude dawn magnetopause

The Geotail satellite detected a filament of tailward‐streaming energetic particles spatially separated from the boundary layer of energetic particles at the high‐latitude dawn magnetopause at a downstream distance of ∼80 RE on October 27, 1992. During this event, the composition and charge states of energetic ions at energies above ∼10 keV show significant intermix of ions from solar wind and ionospheric sources. Detailed analysis leads to the deduction that the filament was moving southward towards the neutral sheet at an average speed of ∼80 km/s, implying an average duskward electric field of ∼1 mV/m. Its north‐south dimension was ∼1 RE and it was associated with an earthward‐directed field‐aligned current of ∼5 mA/m. The filament was separated from the energetic particle boundary layer straddling the magnetopause by ∼0.8 RE and was inferred to be detached from the boundary layer at a downstream distance beyond ∼70 RE in the distant tail.

Anomalous aspects of magnetosheath flow and of the shape and oscillations of the magnetopause during an interval of strongly northward interplanetary magnetic field

On February 15, 1978, the orientation of the interplanetary magnetic field (IMF) remained steadily northward for more than 12 hours. The ISEE 1 and 2 spacecraft were located near apogee on the dawnside flank of the magnetotail. IMP 8 was almost symmetrically located in the magnetosheath on the dusk flank and IMP 7 was upstream in the solar wind. Using plasma and magnetic field data, we show that (1) the magnetosheath flow speed on the flanks of the magnetotail steadily exceeded the solar wind speed by 20%, (2) surface waves of ∼5‐min period and very nonsinusoidal waveform were persistently present on the dawn magnetopause and waves of similar period were present in the dusk magnetosheath, and (3) the magnetotail ceased to flare at an antisunward distance of 15 RE. We propose that the acceleration of the magnetosheath flow is achieved by magnetic tension in the draped field configuration for northward IMF and that the reduction of tail flaring is consistent with a decreased amount of open magnetic flux and a larger standoff distance of the subsolar magnetopause. Results of a three‐dimensional magnetohydrodynamic simulation support this phenomenological model.

Intensity variations in plasma flow at the dawn magnetopause

Voyager 1, exiting the earth's magnetosphere along the dawn meridian at a velocity of ~11 km/sec, measured strong tailward flows of ions ( E≳30 keV) immediately outside the magnetopause. These flows are found to originate sunward of the dawn meridian and to exhibit significant variabilities on the time scale of 400 msec. The variations are not related to changes in the magnetosheath magnetic fields and are likely produced up-stream by the leakage of magnetosphere protons or by a magnetopause particle energization process. The intensities of the dawn meridian ion flows are greater in the magnetosheath than in the magnetosphere. The flows appear to penetrate inside the dawn magnetosphere to a depth ≲0.1 R> E , less than an ion gyroradius.

Voyager 2: Energetic Ions and Electrons in the Jovian Magnetosphere

The Voyager 2 encounter has enhanced our understanding of earlier results and provided measurements beyond 160 Jupiter radii (R(J)) in the magnetotail. Significant fluxes of energetic sulfur and oxygen nuclei (4 to 15 million electron volts per nucleon) of Jovian origin were observed inside 25 R(J), and the gradient in phase space density at 12 R(J) indicates that the ions are diffusing inward. A substantially longer time delay versus distance was found for proton flux maxima in the active hemisphere in the magnetotail at Jovicentric longitudes lambda(III), = 260 degrees to 320 degrees than in the inactive hemisphere at lambda(III), = 85 degrees to l10 degrees . These delays can be related to the radial motion of plasma expanding into the magnetotail, and differences in the expansion speeds between the active and inactive hemispheres can produce rarefaction regions in trapped particles. It is suggested that the 10-hour modulation of interplanetary Jovian electrons may be associated with the arrival at the dawn magnetopause of a rarefaction region each planetary rotation.