Nominal Magnetopause Research Articles

Magnetopause Indentation Induced by the Magnetosheath Fast Flow

Recent studies indicated that the magnetopause indentation plays an important role in magnetosphere-ionosphere coupling. Confirmation of magnetopause indentation requires joint observations with multiple satellites. So far, there have been few magnetopause indentation events reported. In this paper, a case of magnetopause indentation induced by fast magnetosheath flow is reported with multiple spacecraft analysis based on the observations of five THEMIS probes. During the interval from 10:00 UT to 10:45 UT on 21 July 2007, when the five THEMIS probes are located near the subsolar magnetopause, a fast anti-sunward flow (with a velocity of 400 km·s^-1) was observed in the magnetosheath just before THEMIS crossed the magnetopause to the magnetosphere. A magnetopause local indentation event was identified by comparing the nominal magnetopause and the tangential magnetopause plane calculated using the MVA method. In order to explore the origin of this magnetosheath fast flow, solar wind data observed by WIND satellite at L1 point were analyzed. It is found that the solar wind is very stable during this period. The Interplanetary Magnetic Field (IMF) is mainly radial and the component of the north-south direction is very small. It is speculated that the generation of this magnetosheath fast anti-sunward flow may be related to the radial IMF.

Chandra Observations of Jupiter's X‐ray Auroral Emission During Juno Apojove 2017

AbstractJupiter's auroral X‐rays have been observed for 40 years with an unknown driver producing quasiperiodic emission, concentrated into auroral hot spots. In this study we analyze an 10‐hr Chandra observation from 18:56 on 18 June 2017. We use a new Python pipeline to analyze the auroral morphology, perform timing analysis by incorporating Rayleigh testing, and use in situ Juno observations to infer the magnetosphere that was compressed during the Chandra interval. During this time Juno was near its apojove position of 112 , on the dawn flank of the magnetosphere near the nominal magnetopause position. We present new dynamical polar plots showing an extended X‐ray hot spot in the northern auroral region traversing across the Jovian disk. From this morphology, we propose setting a numerical threshold of 7 photons per 5° System III longitude 5° latitude to define a photon concentration of the northern hot spot region. Our timing analysis finds two significant quasiperiodic oscillations (QPOs) of 37 and 26 min within the extended northern hot spot. No statistically significant QPOs were found in the southern X‐ray auroral emission. The Rayleigh test is combined with Monte Carlo simulation to find the statistical significance of any QPOs found. We use a flux equivalence mapping model to trace the possible origin of the QPOs, and thus the driver, to the dayside magnetopause boundary.

Small‐Scale Flux Transfer Events Formed in the Reconnection Exhaust Region Between Two X Lines

AbstractWe report MMS observations of the ion‐scale flux transfer events (FTEs) that may involve two main X lines and tearing instability between the two X lines. The four spacecraft detected multiple isolated regions with enhanced magnetic field strength and bipolar Bn signatures normal to the nominal magnetopause, indicating FTEs. The currents within the FTEs flow mostly parallel to B, and the magnetic tension force is balanced by the total pressure gradient force. During these events, the plasma bulk flow velocity was directed southward. Detailed analysis of the magnetic and electric field and plasma moments variations suggests that the FTEs were initially embedded within the exhaust region north of an X line but were later located southward/downstream of a subsequent X line. The cross sections of the individual FTEs are in the range of ~2.5–6.8 ion inertial lengths. The observations suggest the formation of multiple secondary FTEs. The presence of an X line in the exhaust region southward of a second X line results from the southward drift of an old X line and the reformation of a new X line. The current layer between the two X lines is unstable to the tearing instability, generating multiple ion‐scale flux‐rope‐type secondary islands.

Transient events at the magnetopause and bipolar magnetic signatures

Bipolar signatures in the magnetic field component perpendicular to a nominal magnetopause surface accompanied with an increase of the magnetic field strength are regularly attributed to flux transfer events (FTEs) crossing the spacecraft. The detailed analysis of one such event shows that the magnetic signatures are consistent with the FTE but the timing of multipoint observations and the interpretation of changes of plasma parameters in terms of FTEs requires additional assumptions. We argue that although the event exhibits clear FTE signatures, an explanation of the observations as a local magnetopause surface deformation associated with a change of the magnetosheath density better fits to the data. The deformation caused by this density depression at the magnetopause is associated with a rotation of the magnetosheath magnetic field.

Three‐dimensional MHD simulation of FTEs produced by merging at an isolated point in a sheared magnetic field configuration

We present predictions for the evolution of FTEs generated by localized bursts of reconnection on a planar magnetopause that separates a magnetosheath region of high densities and weak magnetic field from a magnetospheric region of low densities and strong magnetic field. The magnetic fields present a shear angle of 105 degrees. Reconnection forms a pair of FTEs each crossing the magnetopause in the field reversal region and bulging into the magnetosphere and magnetosheath. At their initial stage they can be characterized as flux tubes since the newly reconnected magnetic field lines are not twisted. Reconnection launches Alfvenic perturbations that propagate along the FTEs generating high‐speed jets, which move the pair of FTEs in opposite directions. As the FTE moves, it displaces the ambient magnetic field and plasma producing bipolar magnetic field and plasma velocity signatures normal to the nominal magnetopause in the regions surrounding the FTE. The combination of the ambient plasma with the FTE flows generates a vortical velocity pattern around the reconnected field lines. During its evolution the FTE evolves to a flux rope configuration due to the twist of the magnetic field lines. The alfvenic perturbations propagate faster along the part of the FTE bulging into the magnetosphere than in the magnetosheath, and due to the differences between the plasma and magnetic field properties the perturbations have slightly different signatures in the two regions. As a consequence, the FTEs have different signatures depending on whether the satellite encounters the part bulging into the magnetosphere or into the magnetosheath.

Concerning the motion and orientation of flux transfer events produced by component and antiparallel reconnection

We employ the Cooling et al. (2001) model to predict the location, orientation, motion, and signatures of flux transfer events (FTEs) generated at the solstices and equinoxes along extended subsolar component and high ]latitude antiparallel reconnection curves for typical solar wind plasma conditions and various interplanetary magnetic field (IMF) strengths and directions. In general, events generated by the two mechanisms maintain the strikingly different orientations they begin with as they move toward the terminator in opposite pairs of magnetopause quadrants. The curves along which events generated by component reconnection form bow toward the winter cusp. Events generated by antiparallel reconnection form on the equatorial magnetopause during intervals of strongly southward IMF orientation during the equinoxes, form in the winter hemisphere and only reach the dayside equatorial magnetopause during the solstices when the IMF strength is very large and the IMF points strongly southward, never reach the equatorial dayside magnetopause when the IMF has a substantial dawnward or duskward component, and never reach the equatorial flank magnetopause during intervals of northward and dawnward or duskward IMF orientation. Magnetosheath magnetic fields typically have strong components transverse to events generated by component reconnection but only weak components transverse to the axes of events generated by antiparallel reconnection. As a result, much stronger bipolar magnetic field signatures normal to the nominal magnetopause should accompany events generated by component reconnection. The results presented in this paper suggest that events generated by component reconnection predominate on the dayside equatorial and flank magnetopause for most solar wind conditions.

High kinetic energy density jets in the Earth’s magnetosheath: A case study

We study several high kinetic energy density jets observed during a traversal of the dayside magnetosheath by the Cluster spacecraft on March 17, 2001, at various distances from the magnetopause, generally characterised by anomalously high values of the local magnetosonic Mach number. We concentrate on two jets observed just outside the magnetopause, the first almost parallel to the GSM x axis and the second directed northward–tailward along the nominal magnetopause surface. We present evidence that none of them can be ascribed to magnetic reconnection at the magnetopause and show that the magnetopause is severely deformed by the jets, so that its local normal forms an angle of 97° with the quiet time magnetopause normal. On these grounds, we suggest that the indentation of the magnetopause is caused by an anti-sunward jet ramming into the magnetopause slightly equatorward of the northern cusp and that the northward–tailward jet is the result of its reflection at the deformed magnetopause. Finally, we briefly discuss our results by comparing them with past studies of events which in some way recall the one analysed herein.

Geotail observations of FTE velocities

Abstract. We discuss the plasma velocity signatures expected in association with flux transfer events (FTEs). Events moving faster than or opposite the ambient media should generate bipolar inward/outward (outward/inward) flow perturbations normal to the nominal magnetopause in the magnetosphere (magnetosheath). Flow perturbations directly upstream and downstream from the events should be in the direction of event motion. Flows on the flanks should be in the direction opposite the motion of events moving at subsonic and subAlfvénic speeds relative to the ambient plasma. Events moving with the ambient flow should generate no flow perturbations in the ambient plasma. Alfvén waves propagating parallel (antiparallel) to the axial magnetic field of FTEs may generate anticorrelated (correlated) magnetic field and flow perturbations within the core region of FTEs. We present case studies illustrating many of these signatures. In the examples considered, Alfvén waves propagate along event axes away from the inferred reconnection site. A statistical study of FTEs observed by Geotail over a 3.5-year period reveals that FTEs within the magnetosphere invariably move faster than the ambient flow, while those in the magnetosheath move both faster and slower than the ambient flow.

THEMIS observations of the dayside traveling compression region and flows surrounding flux transfer events

We present multipoint observations of Flux Transfer Events (FTEs) by the THEMIS spacecraft in a string‐of‐pearls configuration, at the near‐equatorial magnetopause. Common characteristics from a number of cases, examined at various local times, are exemplified here by a case study of FTE trains observed in the pre‐noon sector, on Aug. 18, 2007. We show that the magneto‐pause‐normal flow velocity is consistently towards and away from the magnetopause on the inbound and outbound transit of the FTE, on all spacecraft sufficiently away from the magnetopause. The velocity just outside the FTE at the time of closest proximity to the FTE core is opposite to the FTE direction of motion even when the FTE moves in the same direction as the adjacent sheath. The total pressure inside and just outside the FTE is consistently larger than expected from pressure balance at a nominal (unperturbed) magnetopause and therefore the result of local compression by the passing FTE. The multi‐spacecraft observation enables the reconstruction of the Dayside Traveling Compression Region (DTCR), whose result is consistent with previous theoretical results. Semi‐periodic compressional velocity oscillations were observed and are likely driven by nearby occurrences of FTE trains, implying recurrent reconnection with a similar periodicity.

The changing topology of the duskside magnetopause boundary layer in relation to IMF orientation

On 7 December 2000, Cluster made an extended outbound radial traversal of the duskside magnetopause boundary layer. The long duration of the crossing, during which Cluster spent several hours within 2 R E of the nominal magnetopause, allows us to deconvolve the structure of this boundary layer in its dependence on interplanetary parameters. We present evidence that as the interplanetary magnetic field (IMF) changed in discontinuous jumps from southward to northward, the magnetic topology of the boundary layer evolved from open to closed. Reconnection signatures, including enhanced flows observed locally at Cluster and ion energy dispersion signatures observed by FAST near Cluster’s magnetic footpoint, appeared while the IMF was southward. When the IMF turned nearly due northward, Cluster observed 75-s (Pc 4) oscillations which we attribute to the Kelvin–Helmholtz instability.

CLUSTER encounters with the high altitude cusp: boundary structure and magnetic field depletions

Abstract. Data from the four spacecraft Cluster mission during a high altitude cusp crossing on 13 February 2001 are presented. The spacecraft configuration has one leading spacecraft, with the three trailing spacecraft lying in a plane that corresponds roughly to the nominal magnetopause surface. The typical spacecraft separation is approximately 600km. The encounter occurs under conditions of strong and steady southward Interplanetary Magnetic Field (IMF). The cusp is identified as a seven-minute long depression in the magnetic field, associated with ion heating and a high abundance of He+. Cusp entry involves passage through a magnetopause boundary that has undergone very significant distortion from its nominal shape, is moving rapidly, and exhibits structure on scales of the order of the spacecraft separation or less. This boundary is associated with a rotation of the magnetic field, a normal field component, and a plasma flow into the cusp of approximately 35 km/s. However, it cannot be identified positively as a rotational discontinuity. Exit from the cusp into the lobe is through a boundary that is initially sharp, but then retreats tailward at a few km/s. As the leading spacecraft passes through this boundary, there is a plasma flow out of the cusp of approximately 30km/s, suggesting that this is not a tangential discontinuity. A few minutes after exit from the cusp, the three trailing the spacecraft see a single cusp-like signature in the magnetic field. There is an associated temperature increase at two of the three trailing spacecraft. Timing measurements indicate that this is due to cusp-like regions detaching from the rear of the cusp boundary, and moving tailward. The magnetic field in the cusp is highly disordered, with no obvious relation between the four spacecraft, indicative of structure on scales

Properties of the magnetic field in the Jovian magnetotail

Magnetometer data acquired as the Galileo Orbiter apoapsis rotated from dawn to dusk across the magnetotail of the Jovian magnetosphere between late 1995 and the end of May 2000 are used to characterize the magnetic field and the distribution of magnetic pressure in the inner part of the Jovian magnetotail. The distances probed extend to ∼150RJor roughly 3 times the distance to the nose of the magnetopause, analogous to distances within 30REin the magnetotail of Earth. The magnetic pressure in the center of the plasma sheet is typically almost an order of magnitude smaller than the lobe pressure, which therefore is roughly equal to the peak thermal plasma pressure in the plasma sheet. The lobe magnetic pressure decrease with radial distance can be described roughly as a power law with an exponent of −2.74, and the lobe field magnitude decreases with distance to the −1.37. In comparing radial variations of the lobe magnetic fields of Jupiter and Earth we argue that rescaled lengths based on nominal magnetopause standoff distances and rescaled field magnitudes based on planetary dipole field strengths are required. Comparison of rescaled fits suggest that the power law dependence on radial distance should not be extrapolated beyond 150RJand that beyond this distance the trend may become asymptotically constant. Systematic asymmetries of the field structure and magnetic pressure across the midnight meridian in the region beyond 25RJdowntail are notable, with the flux tubes being less stretched (with larger equatorialBz) near dusk than near dawn. The lobe pressure attains its minimum value in the dusk sector where the plasma sheet magnetic pressure maximizes. We argue that only a small fraction of the magnetic flux remaining in the lobes in the magnetotail beyond 100RJcloses across the plasma sheet, but the structure of open and closed flux tubes may be nonuniform across the tail. We discuss briefly mechanisms that may lead to the observed asymmetry.

Multi-spacecraft observations of broadband waves near the lower hybrid frequency at the Earthward edge of the magnetopause

Abstract. Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have amplitudes at least up to 5 mV/m. These waves are similar on all four Cluster satellites, i.e. they are likely to be distributed over large areas of the boundary. The strongest electric fields occur during a few seconds, i.e. over distances of a few hundred km in the frame of the moving magnetopause, a scale length comparable to the ion gyroradius. The strongest magnetic oscillations in the same frequency range are typically found in the boundary layer, and across the magnetopause. During an event studied in detail, the magnetopause velocity is consistent with a large-scale depression wave, i.e. an inward bulge of magnetosheath plasma, moving tailward along the nominal magnetopause boundary. Preliminary investigations indicate that a rather flat front side of the large-scale wave is associated with a rather static small-scale electric field, while a more turbulent backside of the large-scale wave is associated with small-scale time varying electric field wave packets.Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers) – Space plasma physics (waves and in-stabilities)

Simultaneous satellite and ground observations of transient events near the morningside magnetopause

We present observations of transient events on April 13, 1996, using simultaneous measurements in the solar wind, near the magnetopause, at geosynchronous orbit, and on the ground at high latitudes. On this day, Geotail passed outbound through the morningside magnetopause. When Geotail was in the magnetosphere, a clear bipolar perturbation in the magnetic field component normal to the nominal magnetopause and transient outbound (from the magnetosphere to the magnetosheath)/inbound (from the magnetosheath to the magnetosphere) magnetopause crossings were observed. When Geotail was in the magnetosheath, inbound/outbound crossings were observed. During the bipolar magnetic field signature, Geotail observed asymmetric bipolar (less inward/more outward) flow normal to the nominal magnetopause accompanied by sunward flow. Inward flow was observed when the total magnetic field of the event was enhanced, whereas outward flow was observed when the total magnetic field was depressed. We observed that some of the transient outbound/inbound crossings show significantly different signatures in the plasma flow and magnetic field, that is, weak inward flow and enhanced magnetic field before the outbound crossing and strong outward flow and decreased magnetic field after the inbound crossing. The field and flow variations during the transient outbound/inbound crossings are qualitatively similar to those during the bipolar magnetic field signature. These observations suggest that the bipolar field signature and the transient magnetopause crossings would be similar phenomena on the magnetopause. We discuss whether the observed field and flow variations during the transient events are caused by inward/outward magnetopause motion due to compression/expansion of the magnetosphere by comparing the data sets obtained from different regions of the solar wind‐magnetosphere system.

Pulsed boundary layers observed during a Geotail magnetopause skimming orbit

On January 8, 1996, the Geotail satellite skimmed along the dayside magnetopause from the dawnside to the duskside and observed a series of quasi‐periodically recurring pulsed boundary layers in the prenoon (∼0900–1000 LT) and the postnoon (∼1400–1500 LT) sectors. For both intervals the interplanetary magnetic field observed by the Wind satellite upstream of the bow shock was inferred to be southward from an estimated convection time between Wind and Geotail. During the pulsed events, Geotail observed bipolar signatures in the magnetic field component normal to the nominal magnetopause. In the prenoon sector the bipolar variation had the inward‐then‐outward polarity, while in the postnoon sector the polarity was opposite (i.e., outward‐then‐inward). The plasma flows normal to the nominal magnetopause showed bipolar signatures with the inward‐then‐outward polarity in the prenoon and postnoon sectors. They were accompanied by sunward flows for the lower‐density pulsed events and by tailward flows for the higher‐density pulsed events. We have interpreted the sunward flow as the local plasma response to a tailward moving flux tube. This may imply that the sunward plasma flow near the magnetopause does not always reflect the direction of the motion of the flux tube. On the basis of these observations, we suggest that the observed pulsed boundary layers are attributed to the passage of a train of flux transfer events propagating tailward.

Substorm-associated shrinkage of the mid-tail magnetosphere: IACG Campaign #2

In the framework of IACG Campaign #2, we study a case in which the GEOTAIL spacecraft was close to the mid-tail magnetopause when three substorms took place. GEOTAIL was mainly located within the dawnside plasmasheet, and its average speed normal to the nominal magnetopause was very small (∼0.2 R E /hr) because it was near its apogee. It exited to the LLBL∼magnetosheath during the expansion to recovery phases of each substorm, stayed there for a while, and then returned to the plasmasheet near the end of the recovery phase. This pattern suggests a shrinkage of the mid-tail magnetosphere after the expansion onset so that the tail radius becomes smaller than that in the quiet time. We interpret it as a result of strong tailward flows from the near-Earth neutral line which transports plasma in the mid-tail region to the distant tail and causes a rarefaction in the mid-tail region.

A model for plasma flows and shocks in the high‐altitude cusp

A new model for the interaction of a supersonic magnetosheath flow with the high‐altitude cusp is proposed. Archival and recent observations have suggested that the cusp is a region of indentation, where solar wind plasma is found earthward of the nominal magnetopause position. It is proposed that a supersonic solar wind flow interacts with such an indentation by being first accelerated through an expansion fan and then decelerated by a shock. For most realistic parameters the shock cannot originate from the deepest part of the cusp but must be detached. The rarefaction of the magnetosheath plasma leads to a rather shallow cusp (< 1Re) and also implies that the cusp becomes less indented the more tailward it is located.

A statistical study of the magnetic signatures of FTEs near the dayside magnetopause

During magnetopause crossings, the AMPTE CCE satellite frequently observed flux transfer events (FTEs) characterized by fluctuations in the magnetic field strength (B) and bipolar signatures in the field component (BN) normal to the nominal magnetopause. In this study, we survey 110 events observed from October to December 1984 and during January 1986. Nearly all events exhibited increases in B, and although the majority of events exhibited a symmetric bipolar signature in BN, a significant number (31 of 110) had asymmetric bipolar signatures in which the trailing pulse was dominant. Most of the asymmetric events were observed near the magnetic equator. This is consistent with an explanation in which FTEs form via merging along a single subsolar X line with strongly asymmetric signatures but that these signatures evolve into the familiar symmetric bipolar form with distance from the merging line.

Internal structure of flux transfer events produced by the onset of merging at a single X line

We present a new, higher‐order, two‐dimensional, magnetohydrodynamic model for the detailed internal structure of bubbles produced by the onset of merging along a single subsolar X line. The key feature of the model is to implement a nonlinear filter to postprocess results provided by the pseudospectral element method. The devised nonlinear filter (1) eliminates only the high‐frequency oscillations, (2) retains conservation laws, and (3) enforces a total variation diminished criterion to guarantee the elimination of spurious oscillations near discontinuities. For realistic parameters, the model predicts that satellites which remain in the magnetosheath will observe asymmetric bipolar magnetic field signatures normal to the magnetopause, magnetic field strength increases, density and temperature decreases, and velocities opposite the motion of the events. Satellites in the magnetosheath which enter the events observe asymmetric bipolar signatures normal to the magnetopause, magnetic field strength decreases, densities intermediate between those in the magnetosheath and magneto‐sphere, temperature increases, and velocities in the direction of event motion. Satellites initially located just outside the the magnetopause observe greatly depressed densities and enhanced temperatures (but no significant bipolar signatures normal to the magnetopause) followed by velocity jets of magnetosheath‐like plasma on northward magnetospheric magnetic field lines. Satellites in the magnetosphere which enter the events observe depressed magnetic field strengths, enhanced densities and temperatures, and accelerated flows surrounding a core region of accelerated magnetosheath‐like plasma. Neither they nor satellites which remain in the magnetosphere observe any significant bipolar signatures normal to the nominal magnetopause.

Interplanetary magnetic field orientation for transient events in the outer magnetosphere

It is generally believed that flux transfer events (FTEs) in the outer dayside magnetosphere, usually identified by transient (∼1 min) bipolar magnetic field perturbations in the direction normal to the nominal magnetopause, occur when the magnetosheath magnetic field has a southward component. We compare the results of three methods for determining the magnetosheath magnetic field orientation at the times of previously identified UKS/IRM events: (1) the average magnetosheath magnetic field orientation in the 30‐min period adjacent to the nearest magnetopause crossing, (2) the magnetosheath magnetic field orientation observed just outside the magnetopause, and (3) the lagged interplanetary magnetic field (IMF) orientation at the time of the transient events. Whereas the results of method 2 indicate that the events tend to occur for a southward magnetosheath magnetic field, the results of methods 1 and 3 show no such tendency. The fact that the three methods yield significantly different results emphasizes the need for caution in future studies.