Dusk Flank Research Articles

The impact of a hot sodium ion population on the growth of the Kelvin‐Helmholtz instability in Mercury's magnetotail

AbstractObservations of Mercury's local plasma environment by MErcury Surface, Space ENvironment, GEochemistry, and Ranging have revealed that the planet hosts a strongly asymmetric magnetosphere as a result of an off‐axis dipolar or quadrupolar internal field and significant finite Larmor radius effects at the boundary layer between magnetospheric and solar wind plasma environments. One important asymmetry appears in the growth and evolution of Kelvin‐Helmholtz (K‐H) waves at the dawn and dusk flanks of the magnetopause. Linear analysis and global hybrid simulations support a dusk‐dawn asymmetry in the growth rate caused by finite Larmor radius effects, and indeed, K‐H waves have been almost exclusively observed at the dusk magnetopause during northward interplanetary magnetic field. Observations of these K‐H waves at sodium gyroscales invite investigation into the impact of the hot planetary sodium ion population, itself distributed preferentially on the dusk flank, on the growth of the K‐H instability and associated plasma transport. We present local two‐dimensional hybrid simulations of the dusk and dawn boundary layers, with varying magnetospheric sodium ion number density, and examine the associated changes in the growth rates of the K‐H instability, K‐H wave spectra, and cross‐boundary particle transport. We show that gyroresonance between growing K‐H vortices and sodium ion gyration introduces a strong spectral peak at sodium gyroscales at the dusk magnetopause, that an increase in sodium ion number density increases dawn‐dusk asymmetry of K‐H growth rates, and that cross‐boundary particle transport decreases with sodium number density at the dawn flank.

A statistical study of the dawn‐dusk asymmetry of ion temperature anisotropy and mirror mode occurrence in the terrestrial dayside magnetosheath using THEMIS data

AbstractWe present a statistical study of ion temperature anisotropy and mirror mode activity in the Earth's dayside magnetosheath using 6 years of Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations focusing on the quantification of dawn‐dusk asymmetry as a function of upstream conditions and distance from the magnetopause. Our statistical data show a pronounced dusk favored asymmetry of T⊥/T∥ which drives a similar asymmetry of mirror mode activity. T⊥/T∥ decreases with increasing solar wind Alfvén Mach number, whereas mirror mode occurrence increases. In both cases, the relative asymmetry between the dawn and dusk flanks decrease with increasing Alfvén Mach number. In addition, during the transition from low/moderate MA, there was a shift in our data set from dips to peaks, suggesting that the magnetosheath strongly favors peaks during intervals of higher Mach number. We also observed more mirror modes and larger asymmetry during atypical inward ortho‐Parker spiral interplanetary magnetic field compared to the more statistically relevant Parker spiral configuration. Our results are consistent with previous experimental studies of mirror modes and to some extent, numerical models.

Jupiter's deep magnetotail boundary layer

In 2007 the New Horizons (NH) spacecraft flew by Jupiter for a gravity assist en route to Pluto. After closest approach on day of year (DOY) 58, 2007, NH followed a tailward trajectory that provided a unique opportunity to explore the deep jovian magnetotail and the surrounding magnetosheath. After DOY 132, 16 magnetopause crossings were observed between 1654 and 2429 Jupiter radii (Rj) along the dusk flank tailward of the planet. In some cases the crossings were identified as rapid transitions from the magnetotail to the magnetosheath and vice versa. In other cases a boundary layer was observed just inside the magnetopause. Solar Wind Around Pluto (SWAP) is an instrument on board NH that obtained spectra of low energy ions during the flyby period. We use a forward model including the SWAP instrument response to derive plasma parameters (density, temperature and velocity) which best reproduce the observations. We also vary the plasma parameters in our model in order to fit the observations more accurately on occasions where the measurements exhibit significant variability. We compare the properties of the plasma in the boundary layer with those of the magnetosheath plasma derived in our earlier work. We attempt to estimate the magnetic field in the boundary layer assuming pressure balance between it and the magnetosheath. Finally, we investigate several possible scenarios to assess if magnetopause movement and structure could cause the variations seen in the data.

A statistical study into the spatial distribution and dawn‐dusk asymmetry of dayside magnetosheath ion temperatures as a function of upstream solar wind conditions

AbstractThe magnetosheath contains the shocked solar wind and behaves as a natural filter to the solar wind plasma before it reaches the magnetosphere. The redistribution of kinetic energy at the bow shock results in significant thermalization of the solar wind plasma, resulting in a magnetosheath temperature profile which is highly nonhomogeneous and nonisotropic and differs between the dawn and dusk flanks. The present study attempts to study the spatial distribution of magnetosheath ion temperature as a function of upstream solar wind conditions. We pay particular attention to the dawn/dusk asymmetry in which we attempt to quantify using experimental data collected over a 7 year period. We also compare these data to simulated data from both the Block‐Adaptive‐Tree‐Solarwind‐Roe‐Upwind‐Scheme (BATS‐R‐US) MHD code and a kinetic hybrid model. We present evidence that the dawn flank is consistently hotter than the dusk flank for a variety of upstream conditions. Our statistical data also suggest a dependency on solar wind speed such that the level of asymmetry increases with faster speeds. We conclude that the dawn‐favored asymmetry of the magnetosheath seed population is insufficient to explain the dawn asymmetry (30–40%) of cold component ions in the cold, dense plasma sheet, and therefore, other mechanisms are likely required.

Review of Solar Wind Entry into and Transport Within the Plasma Sheet

The plasma sheet is populated in part by the solar wind plasma. Four solar entry mechanisms are examined: (1) double cusp or double lobe reconnection, (2) Kelvin-Helmholtz Instability (KHI), (3) Kinetic Alfven waves (KAW), and (4) Impulsive Penetration. These mechanisms can efficiently fill the plasma sheet with cold dense ions during northward interplanetary magnetic field (IMF). The solar wind ions appear to have been heated upon entry along the plasma sheet dawn flank. The cold-component (solar wind origin) ion density is higher on the dawn flank than the dusk flank. The asymmetric evolution of the KAW and magnetic reconnection in association with the KHI at the dawn and dusk flank magnetopause may partly produce the dawn-dusk temperature and density asymmetries. Solar wind that crosses the magnetopause lowers the specific entropy (s=p/ρ γ ) of the plasma sheet along the flanks. Subsequent transport of the cold ions from the flanks to the midnight meridian increases s by a factor of 5. T i , T e , s i , and s e increase when the solar wind particles are transported across the magnetopause, but T i /T e is roughly conserved. Within the magnetotail, E×B and curvature and gradient drifts play important roles in the plasma transport and can explain the large features seen in the plasma sheet. Turbulence can also play a significant role, particularly in the cold plasma transport from the flanks to the midnight meridian. Total entropy (S=pV γ ) conservation provides important constraints on the plasma sheet transport, e.g., fast flows.

Characteristics of the flank magnetopause: Cluster observations

AbstractThe magnetopause is a current sheet forming the boundary between the geomagnetic field on one side and the shocked solar wind on the other side. This paper discusses properties of the low‐latitude dawn and dusk flanks of the magnetopause. The reported results are based on a large number of measurements obtained by the Cluster satellites during magnetopause traversals. Using a combination of single‐spacecraft and multispacecraft techniques, we calculated macroscopic features such as thickness, location, and motion of the magnetopause. The results show that the typical flank magnetopause is significantly thicker than the dayside magnetopause and also possesses a pronounced and persistent dawn‐dusk asymmetry. Thicknesses vary from 150 to 5000 km, with an median thickness of around 1400 km at dawn and around 1150 km at dusk. Current densities are on average higher on dusk, suggesting that the total current at dawn and dusk are similar. Solar wind conditions and the interplanetary magnetic field cannot fully explain the observed dawn‐dusk asymmetry. For a number of crossings we were also able to derive detailed current density profiles. The profiles show that the magnetopause often consists of two or more adjacent current sheets, each current sheet typically several ion gyroradii thick and often with different current direction. This demonstrates that the flank magnetopause has a structure that is more complex than the thin, one‐dimensional current sheet described by a Chapman‐Ferraro layer.

Radial distribution of magnetic field in earth magnetotail current sheet

Knowing the magnetic field distribution in the magnetotail current sheet (CS) is essential for exploring magnetotail dynamics. In this study, using a joint dataset of Cluster/TC-1, the radial profile of the magnetic field in the magnetotail CS with radial distances covering 8<r<20RE under different geomagnetic activity states (i.e., AE≤100nT for quiet intervals while AE>100nT for active times) and solar wind parameters are statistically surveyed. Our new findings demonstrate that, independent of the activity state, the field strength and Bz component (GSM coordinates) start the monotonic increase prominently as r decreases down to ~11.5RE, which means the dipole field starts to make a significant contribution from there. At least in the surveyed radial range, the Bz component is found to be weaker in the midnight and dusk sectors than that in the dawn sector, displaying a dawn–dusk asymmetry. The occurrence rate of negative Bz in active times also exhibits a similar asymmetric distribution, which implies active dynamics may occur more frequently at midnight and dusk flank. In comparison with that in quiet intervals, several features can be seen in active times: (1) a local Bz minimum between 10.5<r<12.5RE is found in the dusk region, (2) the Bz component around the midnight region is generally stronger and experiences larger fluctuations, and (3) a sharp positive/negative-excursion of the By component occurs at the dawn/dusk flank regions inside r<10RE. The response to solar wind parameters revealed that the Bz component is generally stronger under higher dynamic pressure (Pdy>5nPa), which may support the dawn–dusk squeezing effect as presented by Miyashita et al. (2010). The CS By is generally correlated with the interplanetary magnetic field (IMF) By component, and the correlation quality is found to be better with higher penetration coefficient (the ratio of CS By to IMF By) when IMF Bz is positive. The implications of the present results are discussed.

Saturn's dynamic magnetotail: A comprehensive magnetic field and plasma survey of plasmoids and traveling compression regions and their role in global magnetospheric dynamics

AbstractWe present a comprehensive study of the magnetic field and plasma signatures of reconnection events observed with the Cassini spacecraft during the tail orbits of 2006. We examine their “local” properties in terms of magnetic field reconfiguration and changing plasma flows. We also describe the “global” impact of reconnection in terms of the contribution to mass loss, flux closure, and large‐scale tail structure. The signatures of 69 plasmoids, 17 traveling compression regions (TCRs), and 13 planetward moving structures have been found. The direction of motion is inferred from the sign of the change in the Bθ component of the magnetic field in the first instance and confirmed through plasma flow data where available. The plasmoids are interpreted as detached structures, observed by the spacecraft tailward of the reconnection site, and the TCRs are interpreted as the effects of the draping and compression of lobe magnetic field lines around passing plasmoids. We focus on the analysis and interpretation of the tailward moving (south‐to‐north field change) plasmoids and TCRs in this work, considering the planetward moving signatures only from the point of view of understanding the reconnection x‐line position and recurrence rates. We discuss the location spread of the observations, showing that where spacecraft coverage is symmetric about midnight, reconnection signatures are observed more frequently on the dawn flank than on the dusk flank. We show an example of a chain of two plasmoids and two TCRs over 3 hours and suggest that such a scenario is associated with a single‐reconnection event, ejecting multiple successive plasmoids. Plasma data reveal that one of these plasmoids contains H+ at lower energy and W+ at higher energy, consistent with an inner magnetospheric source, and the total flow speed inside the plasmoid is estimated with an upper limit of 170 km/s. We probe the interior structure of plasmoids and find that the vast majority of examples at Saturn show a localized decrease in field magnitude as the spacecraft passes through the structure. We take the trajectory of Cassini into account, as, during 2006, the spacecraft's largely equatorial position beneath the hinged current sheet meant that it rarely traversed the center of plasmoids. We present an innovative method of optimizing the window size for minimum variance analysis (MVA) and apply this MVA across several plasmoids to explore their interior morphology in more detail, finding that Saturn's tail contains both loop‐like and flux rope‐like plasmoids. We estimate the mass lost downtail through reconnection and suggest that the apparent imbalance between mass input and observed plasmoid ejection may mean that alternative mass loss methods contribute to balancing Saturn's mass budget. We also estimate the rate of magnetic flux closure in the tail and find that when open field line closure is active, it plays a very significant role in flux cycling at Saturn.

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.

Plasma convection in the nightside magnetosphere of Saturn determined from energetic ion anisotropies

The Cassini Ion and Neutral Camera measures intensities of hydrogen and oxygen ions and neutral atoms in the Saturnian magnetosphere and beyond. We use the measured intensity spectrum and anisotropy of energetic hydrogen and oxygen ions to detect, qualify, and quantify plasma convection. We find that the plasma azimuthal convection speed relative to the local rigid corotation speed decreases with radial distance, lagging the planetary rotation rate, and has no significant local time dependences. Plasma in the dusk-midnight quadrant sub-corotates at a large fraction of the rigid corotation speed, with the primary velocity being azimuthal but with a distinct radially outward component. The duskside velocities are similar to those obtained from earlier orbits in the midnight-dawn sector, in contrast to the depressed velocities measured at Jupiter using Energetic Particles Detector measurements on the Galileo spacecraft in the dusk-midnight quadrant. We find significant radial outflow in most of the nightside region. The radial component of the flow decreases with increasing local time in the midnight-dawn sector and reverses as dawn is approached. This and previous results are consistent with a plasma disk undergoing a centrifugally induced expansion as it emerges into the nightside, while maintaining partial rotation with the planet. The magnetodisk expansion continues as plasma rotates across the tail to the dawnside. We do not see evidence in the convection pattern for steady state reconnection in Saturn's magnetotail. The outermost region of the magnetodisk, having undergone expansion upon emerging from the dayside magnetopause confinement, is unlikely to recirculate back into the dayside. We conclude that plasma in the outer magnetodisk [at either planet] rotates from the dayside, expands at the dusk flank, but remains magnetically connected to the respective planet while moving across the tail until it interacts with and is entrained into the dawnside magnetosheath flow. This interaction causes plasma in the outer magnetospheric regions of Jupiter and Saturn to decouple from the planet and exhaust tailward down a dawnside low latitude boundary layer. Magnetospheric plasma will also interact with the dayside magnetosheath plasma, moving across the boundary [enhanced by shear instability] and into the magnetosheath, where it is lost to the magnetosphere with the magnetosheath flow.

Impact of MHD shock physics on magnetosheath asymmetry and Kelvin‐Helmholtz instability

We have performed 13 three‐dimensional global magnetohydrodynamic (MHD) simulations of the magnetosheath plasma and magnetic field properties for Parker spiral (PS) and ortho‐Parker spiral interplanetary magnetic field (IMF) orientations corresponding to a wide range of solar wind plasma conditions. To study the growth of the Kelvin‐Helmholtz instability on the dawn and dusk flank magnetopause, we have performed 26 local two‐dimensional MHD simulations, with the initial conditions taken from global simulations on both sides of the velocity shear layer at the dawn‐dusk terminator. These simulations indicate that while the MHD physics of the fast shocks does not directly lead to strong asymmetry of the magnetosheath temperature for typical solar wind conditions, the magnetosheath on the quasi‐parallel shock side has a smaller tangential magnetic field along the magnetosheath flow which enables faster growth of the Kelvin‐Helmholtz instability (KHI). Because the IMF is statistically mostly in the PS orientation, the KHI formation may statistically favor the dawnside flank. For all the 26 simulations, the growth rates of the KHI correlated well with the ratio of the velocity shear and Alfvén speed along the wave vector, k. Dynamics of the KHI may subsequently lead to formation of kinetic Alfvén waves and reconnection in the Kelvin‐Helmholtz vortices which can lead to particle energization. This may partly help to explain the observed plasma sheet asymmetry of cold‐component ions, which are heated more on the dawnside plasma sheet.

Asymmetry of magnetosheath flows and magnetopause shape during low Alfvén Mach number solar wind

AbstractPrevious works have emphasized the significant influence of the solar wind Alfvén Mach number (MA) on magnetospheric dynamics. Here we report statistical, observational results that pertain to changes in the magnetosheath flow distribution and magnetopause shape as a function of solar wind MA and interplanetary magnetic field (IMF) clock angle orientation. We use all Cluster 1 data in the magnetosheath during the period 2001–2010, using an appropriate spatial superposition procedure, to produce magnetosheath flow distributions as a function of location in the magnetosheath relative to the IMF and other parameters. The results demonstrate that enhanced flows in the magnetosheath are expected at locations quasi‐perpendicular to the IMF direction in the plane perpendicular to the Sun‐Earth line; in other words, for the special case of a northward IMF, enhanced flows are observed on the dawn and dusk flanks of the magnetosphere, while much lower flows are observed above the poles. The largest flows are adjacent to the magnetopause. Using appropriate magnetopause crossing lists (for both high and low MA), we also investigate the changes in magnetopause shape as a function of solar wind MA and IMF orientation. Comparing observed magnetopause crossings with predicted positions from an axisymmetric semi‐empirical model, we statistically show that the magnetopause is generally circular during high MA, while is it elongated (albeit with moderate statistical significance) along the direction of the IMF during low MA. These findings are consistent with enhanced magnetic forces that prevail in the magnetosheath during low MA. The component of the magnetic forces parallel to the magnetopause produces the enhanced flows along and adjacent to the magnetopause, while the component normal to the magnetopause exerts an asymmetric pressure on the magnetopause that deforms it into an elongated shape.

Magnetic signatures of Kelvin‐Helmholtz vortices on Saturn's magnetopause: Global survey

Saturn's rapid rotation combined with relatively weak magnetic fields in the outer magnetosphere and sheath lead to conditions that are favorable for the Kelvin‐Helmholtz (KH) instability. A Kelvin‐Helmholtz unstable magnetopause boundary has important consequences for Saturn's interaction with the solar wind due to mass, momentum, and energy transport that can occur at the magnetopause boundary. Previous attempts to identify vortices have been hampered by limited plasma data to unambiguously reveal vortical flow. The magnetic field data, on the other hand, may be able to identify the KH instability due to intense magnetic fluctuations that are associated with KH vortices. We have conducted two‐dimensional hybrid code simulations of Saturn's magnetopause boundary to illustrate the expected magnetic field signatures of KH. Specifically, our simulations show strong field‐aligned current sheet filaments or strong bipolar fluctuations of the in‐plane magnetic field components, bounding the KH vortices. A global search for these characteristic magnetic field signatures near the magnetopause boundary was made of the Cassini mission data from 2004 to 2009. We find that most of the potential KH activity is found on the dusk flank, contrary to expectations. We suggest that KH growth is supported in the prenoon and subsolar regions and that these vortices are transported through coupling to the rotating planet, past noon and tailward on the dusk flank. In addition, we find many instances in the subsolar magnetosphere of possible plasmoid formation (Bz northward) in conjunction with these intense magnetic field fluctuations.

Cluster observations of the dusk flank magnetopause near the sash: Ion dynamics and flow‐through reconnection

Compared to the dayside, dynamics on the flanks of the magnetopause are poorly understood. To help bridge this knowledge gap we analyzed Cluster plasma and field measurements acquired during a 90‐min period on 20 November 2003 when Cluster crossed the magnetopause four times in the vicinity of the sash. MHD simulations provide a context for Cluster observations. Crossings were between the magnetosheath and an S‐shaped plasma sheet, rather than to the open‐field lobes of the magnetotail. Cluster encountered two regions of MHD‐breaking differences between perpendicular ion velocities andE × B convection. Ion adiabatic expansion parameter (δi) calculations show that ion gyrotropy was not broken during an episode of strong Alfvén wave activity in the magnetosheath. However, gyrotropy was broken (δi&gt; 1) during the fourth magnetopause crossing. In the magnetosheath, ion guiding‐center motion was maintained but inertial effects associated with temporally varying electric fields are probable sources of velocity differences. Regarding the magnetopause crossing, the generalized Ohm's law limits possible sources for breaking ion gyrotropy to inertial forces and/or electron pressure gradients associated with a nearby reconnection event. We suggest that Cluster witnessed effects of a temporally varying and spatially limited, flow‐through reconnection event between open mantle field lines from the two polar caps adding new closed flux to the LLBL at the sash. Future modeling of flank dynamics must consider inertial forces as significant drivers at the magnetopause and in the adjacent magnetosheath.

Cluster observations of band‐limited Pc 1 waves associated with streaming H+ and O+ions in the high‐altitude plasma mantle

Bursts of band‐limited Pc 1 waves (0.2 to ∼1.0 Hz) with normalized frequency f/fH+∼ 0.5 have been observed by the Cluster spacecraft during many passes through the high‐latitude plasma mantle. These transverse, left‐hand polarized waves are associated with regions of H+ and O+ ions streaming away from Earth along magnetic field lines at the same velocity (∼140 km/s). Waves were observed only when H+fluxes increased by factors of 10–1000 and energies of both ion species increased by factors of up to 10. We present two satellite‐ground conjunctions to demonstrate the high latitude localization of these waves and their ability to reach the polar ionosphere and two extended examples of waves and associated ion distribution functions near the southern dusk flank magnetopause. We also present the results of a search for all such events during Cluster's 2002 and 2003 passages through the magnetotail, with orbital precession covering dawn to dusk on Earth's night side (June through December). A total of 46 events (band‐limited Pc 1–2 waves accompanied by a sustained population of streaming H+ and O+ ions, separated by at least 12 min) were observed on 29 days. The waves were generally associated with intervals of southward IMF Bz and/or large IMF By (times of active cusp reconnection), and often but not always occurred during the main phase or early recovery phase of magnetic storms. Analysis of selected events shows that the waves are associated with large H+ temperature anisotropy, and that the waves propagate opposite to the direction of the streaming ions. A wave instability analysis using the WHAMP code confirms that the generation of these waves, via the ion cyclotron instability, is basically consistent with known physics. Their extended region of wave growth is likely, however, to reach tailward significantly beyond the Cluster orbit.

A statistical study of magnetopause structures: Tangential versus rotational discontinuities

A statistical study of the structure of Earth's magnetopause is carried out by analyzing two‐year AMPTE/IRM plasma and magnetic field data. The analyses are based on the minimum variance analysis (MVA), the deHoffmann‐Teller (HT) frame analysis and the Walén relation. A total of 328 magnetopause crossings are identified and error estimates associated with MVA and HT frame analyses are performed for each case. In 142 out of 328 events both MVA and HT frame analyses yield high quality results which are classified as either tangential‐discontinuity (TD) or rotational‐discontinuity (RD) structures based only on the Walén relation: Events withSWA ≤ 0.4 (SWA ≥ 0.5) are classified as TD (RD), and rest (with 0.4 &lt; SWA &lt; 0.5) is classified as “uncertain,” where SWA refers to the Walén slope. With this criterion, 84% of 142 events are TDs, 12% are RDs, and 4% are uncertain events. There are a large portion of TD events which exhibit a finite normal magnetic field component Bnbut have insignificant flow as compared to the Alfvén velocity in the HT frame. Two‐dimensional Grad‐Shafranov reconstruction of forty selected TD and RD events show that single or multiple X‐line accompanied with magnetic islands are common feature of magnetopause current. A survey plot of the HT velocity associated with TD structures projected onto the magnetopause shows that the flow is diverted at the subsolar point and accelerated toward the dawn and dusk flanks.

Survival of flux transfer event (FTE) flux ropes far along the tail magnetopause

During intervals of southward IMF, magnetic reconnection can result in the formation of flux transfer events (FTEs) on the dayside magnetopause which travel along the magnetopause in the anti‐sunward direction. Of particular interest is their fate and the role they play transporting solar wind plasma into the magnetosphere. We present the discovery of FTEs far along the distant tail magnetopause (x = −67 Earth radii) using data from ARTEMIS on the dusk flank magnetopause under southward/duskward IMF conditions. The identification of several events is further supported by excellent fits to a force‐free flux rope model. The axis of each structure is principally north‐south, i.e., perpendicular to the Sun‐Earth line. Simultaneous observations by THEMIS on the dayside magnetopause indicate that FTEs are being produced there, although perhaps 2–4 times smaller in size. The convection time from the dayside magnetopause to ARTEMIS is 30 min, and the FTEs have a flux content comparable to those typically observed on the dayside magnetopause, indicating that these features are in quasi‐equilibrium as they are convected downtail. By considering the relative orientations of the FTEs observed by THEMIS and ARTEMIS, the magnetic field geometry is consistent with the FTEs being produced on the dayside magnetopause along an extended X‐line in the presence of IMF By and bending as they are convected to the flanks.

Conditions at the expanded Jovian magnetopause and implications for the solar wind interaction

Using idealized models of the magnetosheath and magnetosphere magnetic fields, plasma densities, and plasma flow, we test for the steady state viability of processes mediating the interaction between the solar wind and the Jovian magnetosphere. The magnetopause is modeled as an asymmetric paraboloid with variable asymmetry. The subsolar standoff of the magnetopause has been shown to exhibit a bimodal probability distribution (Joy et al., 2002). Only the expanded magnetopause is considered, with a standoff of ∼90 RJ. We test where on the magnetopause surface large‐scale reconnection may be affected by either a shear flow or diamagnetic drift due to a pressure gradient across the magnetopause boundary. We also test for the onset of the Kelvin‐Helmholtz instability. We find that reconnection is inhibited on the dawn flank due to the large shear flows in this region, regardless of magnetopause shape or interplanetary magnetic field orientation. The presence of a high energy plasma population in the magnetosphere may inhibit reconnection over much of the magnetopause area, except when the fields are antiparallel. Additionally, most of the dawn flank of the magnetopause is Kelvin‐Helmholtz unstable, regardless of magnetopause asymmetry; and the dusk flank tailward of the planet is Kelvin‐Helmholtz unstable when the magnetopause is highly oblate.

Spatial distribution of rolled up Kelvin-Helmholtz vortices at Earth's dayside and flank magnetopause

Abstract. The Kelvin-Helmholtz Instability (KHI) can drive waves at the magnetopause. These waves can grow to form rolled-up vortices and facilitate transfer of plasma into the magnetosphere. To investigate the persistence and frequency of such waves at the magnetopause we have carried out a survey of all Double Star 1 magnetopause crossings, using a combination of ion and magnetic field measurements. Using criteria originally used in a Geotail study made by Hasegawa et al. (2006) (forthwith referred to as H2006), 17 candidate events were identified from the entire TC-1 mission (covering ~623 orbits where the magnetopause was sampled), a majority of which were on the dayside of the terminator. The relationship between density and shear velocity was then investigated, to identify the predicted signature of a rolled up vortex from H2006 and all 17 events exhibited some level of rolled up behavior. The location of the events had a clear dawn-dusk asymmetry, with 12 (71%) on the post noon, dusk flank suggesting preferential growth in this region.

Diversion of plasma due to high pressure in the inner magnetosphere during steady magnetospheric convection

Steady magnetospheric convection (SMC) events in the Earth's magnetosphere are thought to result from balancing the rate of opening flux through solar wind‐magnetosphere reconnection at the dayside magnetopause to the rate of closing flux through reconnection in the magnetotail. For this to occur, reconnected flux in the tail must return to the dayside to balance the dayside reconnection rate. Using Geotail and THEMIS data over a span of 14 years, we examine the average plasma conditions and fast Earthward flows during SMC intervals and compare them to other types of geomagnetic activity, such as quiet intervals, isolated substorm phases, and the two hours before an SMC (Pre‐SMC intervals). We show that the average total pressure in the inner magnetosphere is higher during SMC events than for other types of activity. This higher pressure region extends to larger radial distances, and causes fast Earthward flows to divert toward the dawn or dusk flanks and continue to the dayside. This pattern is contrasted to substorms, during which flows are directed toward the inner magnetosphere and flux remains there in the “pile‐up region.” We suggest that the SMC pattern of flow deflection carries enough flux from the tail to the dayside to allow for balanced reconnection. Finally, the Pre‐SMC intervals have plasma conditions that are similar to, but slightly weaker than, SMC events. Since most SMCs begin with a substorm, this indicates that preconditioning of the magnetosphere by prior geomagnetic activity is important in setting up the magnetotail for an SMC state.