Midnight Meridian Research Articles

Northward interplanetary magnetic field plasma sheet entropies

The plasma sheet pressure, temperature, and density profiles inferred from DMSP observations are used to investigate northward interplanetary magnetic field plasma sheet specific entropy s = p/ργ or p/nγ (for a single ion species). Profiles of s for hot and cold populations are considered. The hot ion population s profile suggests a duskward heat flux that is consistent with the curvature and gradient drift. In contrast, s of the cold population is approximately conserved in the X direction but has a strong gradient toward the midnight meridian. The total entropy (S = ∫p1/γ dl/B) and s under various entry mechanisms are estimated for comparisons with observed values. The cold population s is higher than that of the magnetosheath, suggesting the entry process may heat the ions. Cusp reconnection by itself may not increase the magnetosheath ion s, but cusp reconnection with kinetic Alfvén waves (KAW) may provide the necessary heating. Localized reconnection in Kelvin‐Helmholtz vortices may increase s if the plasma expands nonadiabatically into the large magnetospheric flux tube volume. KAWs may lead to diffusion and heating consistent with the observations. The cold population s increases by a factor of 5 from the flanks to the midnight meridian, which provides a constraint for transport mechanism(s) within the plasma sheet. A simple calculation is performed to demonstrate that the spatial gradients of s in the plasma sheet could result from the temporal dependence of transport processes in the magnetopause boundary layer.

Radial propagation velocity of energetic particle injections according to measurements onboard the Cluster satellites

Injections of energetic electrons with a dispersion over energies were observed during the February 23, 2004 (at about 03:20 UT) substorm onboard the Cluster satellites in the vicinity of perigee near the midnight meridian. The delays in the particle observation caused by the energy dependence of the magnetic drift velocities made it possible to determine the position and time of the beginning of the drift, tracing the trajectories of the leading center of particles back in time in the magnetospheric model. The comparisons of the measurements of four satellites allowed us to determine the radial propagation of the injection front with a velocity of 100–150 km/s at a distance of 7–9 RE. The comparison with a few previous measurements shows a substantial slowing down of injections as they approached the Earth, and this confirms the prospects of this method for more detailed study of propagation of plasma injection into the inner magnetosphere.

Morphology and dynamics of Venus oxygen airglow from Venus Express/Visible and Infrared Thermal Imaging Spectrometer observations

Images obtained by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)‐M channel instrument onboard Venus Express have been used to retrieve maps and apparent motions of the O2 (1Δ) infrared nightglow on Venus at 1.27 μm. The nightglow distribution is highly inhomogeneous with the regions of brightest emission generally located at low latitudes near the midnight meridian. Unexpectedly some orbits show also intense airglow activity over the south polar region. The spatially resolved airglow is spectacularly variable not only in its morphology and intensity but also in the apparent motions of the airglow small‐ and large‐scale structures. Visual tracking of the bright features allowed to obtain mean zonal and meridional motions related to the subsolar to antisolar circulation. The zonal velocity is dominated by an intense prograde jet (contrary to the retrograde planetary rotation) from dawn to midnight extending up to 22 hours in local time with lower velocities and reversed sign from dusk. Typical zonal velocities range between +60 (prograde) to −50 (retrograde) m/s, whereas most meridional velocities range from −20 (poleward) to +100 m/s (equatorward) with an average meridional circulation of +20 m/s toward low latitudes. The brightest small‐scale (∼100 km) features appear correlated with locations of apparent convergence which may be a signature of compression and downwelling, whereas this is not evident for the large‐scale structures suggesting slow subsidence over large areas mixed with horizontal motions. We argue that part of the tracked motions are representative of real motions at the mesosphere over an altitude range of 95–107 km.

Distribution of the O2 infrared nightglow observed with VIRTIS on board Venus Express

We present characteristics of the statistical horizontal distribution of the O2 infrared nightglow over most of the southern hemisphere observed with the VIRTIS instrument over a period spanning nearly 11 months of low solar activity. We show that the distribution is inhomogeneous with the regions of brightest emission reaching ∼3 MegaRayleighs (MR) located at low latitude near and dawnward of the midnight meridian. The hemispherically averaged nadir brightness is 1.3 MR, in very good agreement with earlier ground based observations. We show that the dayside supply of O atoms is sufficient to produce the observed global O2 nightglow if approximately 50% of the dayside O production is carried to the nightside by the subsolar to antisolar global circulation. Limb profiles observed at northern mid‐latitudes exhibit large intensity variations over short time periods. Calculations with a one‐dimensional chemical diffusive model produce an airglow peak at 96 km, in agreement with the limb observations. The atomic oxygen density derived from the best fits to O2 airglow limb profiles reaches a maximum of 1.8–3.5 × 1011 cm−3 at 104 km.

Formula omitted] transport across the polar cap

The plasma sheet, inner magnetosphere, and high-latitude magnetosphere all contain significant amounts of O + ions during active times. Singly charged oxygen ions unambiguously come from the ionosphere, making them an excellent tracer species. We test the cleft ion fountain theory, which asserts that O + ions escape from the cleft, cross the polar cap, and then enter the plasma sheet. Statistical studies of O + density in the cleft, high-altitude polar cap, and plasma sheet all indicate that the O + density increases with increasing solar wind dynamic pressure. In order to examine O + transport more directly, we use polar cap ion outflow measurements and the 2001 Tsyganenko magnetic field model driven with advanced composition explorer (ACE) solar wind parameters. We calculate the distance between the cleft and the ionospheric footpoints of magnetic field lines mapped from the polar spacecraft along the noon–midnight meridian. Using the observed outflow speed and the magnetic field line length we calculate the travel time for the ions. When we examine the distance from the cleft versus the O + travel time for individual passes, the slope of the line is consistent with the measured ionospheric convection speed across the polar cap. We conclude that O + ions emanating principally from the cleft are transported across the polar cap, and these O + ions have access to the ring current and plasma sheet.

A superposed epoch study of SuperDARN convection observations during substorms

Case studies of flow patterns during substorms have revealed characteristic patterns in the period leading to expansion onset. Velocities in the premidnight region have been observed to become strongly sheared, and the premidnight convection reversal has been observed to extend across the midnight meridian. At expansion onset, the shear decreases and flow at midnight becomes meridional, out of the polar cap. The study presented here superposes flow patterns from 10 substorm intervals to determine the average flow patterns. The study confirms the findings of the case studies and extends them to additional local time sectors. In addition, changes to the flow pattern after expansion onset are examined.

Multi-spacecraft observation of plasma dipolarization/injection in the inner magnetosphere

Abstract. Addressing the origin of the energetic particle injections into the inner magnetosphere, we investigate the 23 February 2004 substorm using a favorable constellation of four Cluster (near perigee), LANL and Geotail spacecraft. Both an energy-dispersed and a dispersionless injection were observed by Cluster crossing the plasma sheet horn, which mapped to 9–12 RE in the equatorial plane close to the midnight meridian. Two associated narrow equatorward auroral tongues/streamers propagating from the oval poleward boundary could be discerned in the global images obtained by IMAGE/WIC. As compared to the energy-dispersed event, the dispersionless injection front has important distinctions consequently repeated at 4 spacecraft: a simultaneous increase in electron fluxes at energies ~1..300 keV, ~25 nT increase in BZ and a local increase by a factor 1.5–1.7 in plasma pressure. The injected plasma was primarily of solar wind origin. We evaluated the change in the injected flux tube configuration during the dipolarization by fitting flux increases observed by the PEACE and RAPID instruments, assuming adiabatic heating and the Liouville theorem. Mapping the locations of the injection front detected by the four spacecraft to the equatorial plane, we estimated the injection front thickness to be ~1 RE and the earthward propagation speed to be ~200–400 km/s (at 9–12 RE). Based on observed injection properties, we suggest that it is the underpopulated flux tubes (bubbles with enhanced magnetic field and sharp inner front propagating earthward), which accelerate and transport particles into the strong-field dipolar region.

Timescale for the formation of the cold‐dense plasma sheet: A case study

The timescale for the formation of cold‐dense plasma sheet ions was investigated with an event in which the interplanetary magnetic field (IMF) was northward for almost one day. The plasma sheet dawn and dusk flanks appear to reach cold dense states (n > 1 cm−3; T < 2 keV) within a few hours after IMF northward turning. Closer to the center (midnight meridian), the ion temperatures reach < 2 keV within a few hours of IMF northward turning, but the ion densities do not reach above 1 cm−3 for at least ∼8 hours after IMF northward turning. The connection between solar wind ions and plasma sheet cold‐component ions is demonstrated. The plasma sheet dawn flank ions appear to lag the solar wind ions by about 3 hours. This study confirms the previous statistical results: (a) the densification of the plasma sheet can be attributed to the influx of the cold‐component (magnetosheath/solar wind origin) ions; and (b) the cooling of the plasma sheet can be attributed not only to the influx of the solar wind ions, but also to the cooling of the hot components. Order of magnitude calculations of the plasma sheet filling rate from reconnection and diffusion suggest that both entry mechanisms could result in roughly comparable filling rates. Hence, the dawn‐dusk asymmetries would be key in distinguishing the roles of the various proposed entry mechanisms.

Dawn‐dusk asymmetries, ion spectra, and sources in the northward interplanetary magnetic field plasma sheet

During periods of northward IMF, plasma sheet ions often have two components: hot (magnetospheric origin) and cold (magnetosheath origin). The temperatures of the cold‐component ions are ∼30–40% higher in the dawn sector compared to the dusk sector, implying the dawnside magnetosheath ion heating of ∼30–40%. As a result, the magnetosheath ions are less distinguishable from the hot‐component ions, which have lower temperatures on the dawnside, leading to a higher occurrence of the ions having (apparent) one‐component distribution. As the duration of the hourly averaged IMF being northward (Δt) increases from 1 to 10 hours, the occurrence of two‐component ions increases from 65% to 83% in the dusk flank, but in the dawn flank it remains relatively stable at around 45%. In contrast, the occurrence of ions best characterized by kappa (κ) distribution increases from 25% to 35% in the dawn flank whereas in the dusk flank it remains relatively insensitive to Δt (10%). The occurrence of a one‐component Maxwellian distribution appears to be most pronounced in the region of the plasma sheet close to the midnight meridian, and these ions appear to be characteristic of the nominal plasma sheet (hot component) ions. The densification of the plasma sheet, as Δt increases, mainly results from the influx of the magnetosheath ions. However, the cooling of the plasma sheet ions can be attributed not only to the influx of the cold magnetosheath ions but also to the cooling of the nominal plasma sheet ions. The dawn‐dusk asymmetries observed in the cold magnetosheath ion profiles should provide constraints that can help determine the roles of various proposed magnetosheath ion entry mechanisms.

Convection during strong driving in MHD simulations: Evolution of flux tube volume

In this work, we examine convection during very strong driving during magnetic storms using global MHD simulations of the interaction of the solar wind with the terrestrial magnetosphere. Specifically, we examine the variation of the flux tube volume along the midnight meridian using the simulation of the January 10, 1997, magnetic storm. We find that during substorm-like periods in the simulation, the flux tube volume varies significantly, rising during the growth phase and shrinking during the expansion phase. During main phase of the storm, the flux tube volume grows and remains at elevated values. We suggest that this is an important feature of convection during storms.

Cluster observations of magnetospheric substorm behavior in the near- and mid-tail region

The Cluster constellation of spacecraft has returned substantial new data on particle and field variations in the near- and mid-magnetotail regions of Earth’s magnetosphere. Using the Research with Adaptive Particle Imaging Detectors (RAPID) system onboard the four Cluster vehicles, we have identified substorm-related energetic ( E > 20 keV) electron enhancement events during the period March 2001 through October 2001 in the geocentric radial range of 4–19 Earth radii. We have used concurrent data from other Cluster instruments as well as from the POLAR, IMAGE, FAST, GPS, and geostationary orbit spacecraft in order to understand particle injection and transport phenomena throughout this key region of the magnetotail. Electron enhancements in the plasma sheet at intermediate radial distances have been studied in a global substorm context. A particularly well-observed substorm case occurred on August 27, 2001 when Cluster was almost exactly in the midnight meridian. We find evidence that Cluster was very near the near-Earth substorm neutral line and that magnetic reconnection began some seven minutes prior to the substorm auroral brightening of the expansive phase onset. We also study in some detail the recovery phase of the substorm and the associated expansion of the plasma sheet over the 4-satellite Cluster constellation.

Modeling the transition of the inner plasma sheet from weak to enhanced convection

We seek to determine whether the adiabatic plasma transport and energization resulting from electric and magnetic drift can quantitatively account for the plasma sheet under weak and enhanced convection observed by Geotail presented in the companion paper [Wang et al., 2004]. We use a modified Magnetospheric Specification Model to simulate the dynamics and distributions of protons originating from the deep tail and low‐latitude boundary layer (LLBL) under an assigned, slowly increasing convection electric field. The magnetic field is Tsyganenko 96 model, modified so that force balance is maintained along the midnight meridian. Our simulation results reproduce well the observed radial profiles and magnitudes of pressure and magnetic field. The changes of these parameters with convection strength are also well reproduced, indicating that the electric and magnetic drift control the large‐scale structure of the plasma sheet. The plasma flows near midnight are diverted toward dusk by diamagnetic drift. We obtain a steady state plasma sheet under strong and steady convection, showing that magnetic drift and field line stretching bring the plasma sheet away from possible convection disruption. The protons from the LLBL strongly affect the plasma sheet density and temperature during quiet times but not during enhanced convection. For the same cross–polar cap potential, stronger shielding of the convection electric field results in smaller energization. The penetration electric field is important in moving the plasma sheet to smaller geocentric radial distance. Our results suggest that the frozen‐in condition E = −v × B is not valid in the inner plasma sheet because of strong diamagnetic drift.

Quiet time magnetotail plasma flow: Coordinated Polar ultraviolet images and Geotail observations

A search for fast flows in the plasma sheet during quiet times was conducted in order to further test the previously found close association between fast flows and auroral brightenings. We first identified “quiet intervals” of at least 30‐min duration in the Polar ultraviolet global images of auroras. During 16 months of Polar observations from January 1997 through April 1998, 41 quiet intervals were identified when Geotail was between 8 and 31 RE down the tail and within 10 RE of the midnight meridian. Fast flows were more rare during the quiet intervals than at other times. For 4 of the 41 intervals identified, Geotail detected flows faster than 300 km s−1 in the plasma sheet. All the four flows were earthward parallel flows accompanied by northward magnetic field beyond 20 RE, and there were no perpendicular flows faster than 300 km s−1. To understand the nature of the fast (but not perpendicular) flows during the quiet intervals, an earthward flow at 21 RE down the tail was studied in detail. The plasma beta was higher than unity during the earthward flow, but the fastest ion velocity moments consisted of the two counterstreaming components flowing along the magnetic field, a characteristic known to define the plasma sheet boundary layer. Our results further support the close association between auroral brightenings and clear fast flows as seen in previous studies, because no perpendicular fast flows were found during quiet intervals. Since some perpendicular flows slower than 300 km s−1 were found beyond ∼20 RE down the tail, a velocity of ∼300 km s−1 appears to statistically make a difference in the geoeffectiveness of perpendicular flows.

Substorm expansion phase: Observations from Geotail, Polar and IMAGE network

We describe the signature of a substorm detected in the midtail while Geotail was located close to the midnight meridian. At the same time, the UVI imager on board Polar identifies a bulge which develops at low latitude and rapidly expands towards the north, east, and west, corresponding to the expansion phase. The magnetograms of the IMAGE network are consistent with these observations; during the expansion phase they give evidence for a northward expansion of the magnetic perturbation. Mapping indicates that the Geotail footprint is located north of the initial bulge and south of the high‐latitude oval. During the expansion phase, Geotail is located in the center of the neutral sheet and detects an ion flow velocity, perpendicular to Bo and directed tailward while Bz changes from positive to negative. During the recovery phase, Geotail, which is not at the center of the sheet anymore, detects an ion velocity directed earthward but essentially field aligned, while Bz is positive and the high‐latitude auroral structure is located north of Geotail footprint. The radial component of the velocity is always dominant. We interpret these observations as evidence for a tailward moving dipolarization front that first destroys the inner part of the thin current sheet (TCS) formed during the growth phase. This dipolarization/current disruption starts in the near‐Earth plasma sheet and expands tailward. This “erosion” produces a negative Bz component at the earthward edge of the TCS. For large enough distances this contribution can eventually be dominant, thereby producing a negative Bz. In this interpretation the formation of an X‐point/X‐line is the consequence of the erosion of the currents in near‐Earth tail. Present observations give evidence for an association between increases in the ion velocity and small‐scale Alfvenic fluctuations. The plasma sheet electrons are heated via interaction with the waves. This heating is preferentially along the direction of the magnetic field. Large ion flow velocities coincide with wave observations.

Tail plasma sheet models derived from Geotail particle data

Simple analytical models have been derived for the first time, describing the 2‐D distribution (along and across the Earth's magnetotail) of the central plasma sheet (CPS) ion temperature, density, and pressure, as functions of the incoming solar wind and interplanetary magnetic field (IMF) parameters, at distances between 10 and 50 RE. The models are based on a large set of data of the Low‐Energy Particle (LEP) and Magnetic Field (MGF) instruments, taken by Geotail spacecraft between 1994 and 1998, comprising 7234 1‐min average values of the CPS temperature and density. Concurrent solar wind and IMF data were provided by the Wind and IMP 8 spacecraft. The accuracy of the models was gauged by the correlation coefficient (c.c.) R between the observed and predicted values of a parameter. The CPS ion density N is controlled mostly by the solar wind proton density and by the northward component of the IMF. Being the least stable characteristic of the CPS, it yielded the lowest c.c. RN = 0.57. The CPS temperature T, controlled mainly by the solar wind speed V and the IMF Bz, gave a higher c.c. RT = 0.71. The CPS ion pressure P was best controlled by the solar wind ram pressure Psw and by an IMF‐related parameter F = B⟂, where B⟂ is the perpendicular component of the IMF and θ is its clock angle. In a striking contrast with N and T, the model pressure P revealed a very high c.c. with the data, RP = 0.95, an apparent consequence of the force balance between the CPS and the tail lobe magnetic field. No significant dawn‐dusk asymmetry of the CPS was found beyond the distance 10 RE, in line with the observed symmetry of the tail lobe magnetic field. The plasma density N is lowest at midnight and increases toward the tail's flanks. Larger (smaller) solar wind ion densities and northward (southward) IMF Bz result in larger (smaller) N in the CPS. In contrast to the density N, the temperature T peaks at the midnight meridian and falls off toward the dawn/dusk flanks. Faster (slower) solar wind flow and southward (northward) IMF Bz result in a hotter (cooler) CPS. The CPS ion pressure P is essentially a function of only XGSM in the midtail (20–50 RE); at closer distances the isobars gradually bend to approximately follow the contours of constant geomagnetic field strength. For northward IMF conditions combined with a slow solar wind, the isobars remain quasi‐circular up to larger distances, reflecting a weaker tail current and, hence, more dipole‐like magnetic field.

Modeling the inner plasma sheet protons and magnetic field under enhanced convection

In order to understand the evolution of the protons and magnetic field in the inner plasma sheet from quiet to disturbed conditions, we incorporate a modified version of the Magnetospheric Specification Model (MSM) with a modified version of the Tsyganenko 96 (T96) magnetic field model to simulate the protons and magnetic field under an increasing convection electric field with two‐dimensional (2‐D) force balance maintained along the midnight meridian. The local time dependent proton differential fluxes assigned to the model boundary are a mixture of hot plasma from the mantle and cooler plasma from the low latitude boundary layer (LLBL). We previously used this model to simulate the inner plasma sheet under weak convection corresponding to a cross polar cap potential drop (ΔΦPC) equal to 26 kV and obtained 2‐D quiet time equilibrium for proton and magnetic field that agrees well with observations. We start our simulation for enhanced convection with this quiet time equilibrium and time‐independent boundary particle sources and increase ΔΦPC steadily from 26 to 146 kV in 5 hours. Simulations are also run separately to steady states by keeping ΔΦPC constant after it is increased to 98 and 146 kV. The magnitudes of proton pressure, number density, and temperature and their increase from quiet to moderate activity (ΔΦPC = 98 kV) are consistent with most observations. Our simulation at high activity (ΔΦPC = 146 kV) underestimates the observed pressure and temperature. This disagreement indicates possible dependence of the boundary particle sources on activity and possible effects of solar wind dynamic pressure enhancements that have not yet been included in our simulation. The simulated equatorial pressures and temperatures show stronger enhancement on the dusk side than on the dawn side as convection is increased, while density profiles show an increase on the dawn side and a decrease on the dusk side. The simulated proton flow speed at the equatorial plane increases with enhancing convection while the overall flow direction does not change significantly, a result of enhancement in both the earthward electric drift and the azimuthal diamagnetic drift. The equatorial magnetic field strength decreases more in the near‐Earth plasma sheet than at larger radial distances as ΔΦPC increases, resulting in an increasing flat radial profile with enhancing convection. The feedbacks from diamagnetic drift and magnetic fields to increasing convection are found to restrain the pressure increase. Based on the good agreement between our results and observations at moderate activity, our magnetic field indicates that the plasma and magnetic field in the plasma sheet can be in a state far from possible force balance inconsistency during periods of moderately enhanced convection. A scale analysis of our results shows that the frozen‐in condition E = −v × B is not valid in the inner plasma sheet for moderate activity.

Two‐dimensional quiet time equilibrium for the inner plasma sheet protons and magnetic field

In order to study the quiet time proton flow and magnetic field in the inner plasma sheet resulting from electric and magnetic drifts, we incorporate a modified version of the Magnetospheric Specification Model with a modified Tsyganenko 96 magnetic field model to self‐consistently simulate plasma sheet protons and magnetic fields with two‐dimensional force balance maintained along the midnight meridian. The resulting equatorial proton flows and pressures agree very well with observations quantitatively. Because of this agreement, the simulated force‐balanced magnetic field configuration should give more realistic plasma sheet magnetic field variations for quiet times than does the original T96 model. This 2‐D self‐consistent simulation reproduces the observed flows and pressures better than our previous 1‐D self‐consistent simulation. The improvement results from a more accurate modeling of the coupling between plasma and magnetic field in the 2‐D self‐consistent simulation.

Observation of a large density dropout across the magnetic field at 600 km altitude during the 6–7 April 2000 magnetic storm

The first Republic of China Satellite, ROCSAT‐1 at 600 km altitude observed two groups of equatorial spread‐F (ESF) plasma depletion structures located in either side of the midnight meridian during the 6–7 April 2000 magnetic storm recovery phase. In the group of postmidnight ESF irregularities, a large density dropout was observed in two consecutive ROCSAT orbits. This large density dropout structure is similar to the density depletion event observed by Defense Meteorological Satellite Program (DMSP) F9 spacecraft during the 13–14 March 1989 magnetic storm [Greenspan et al., 1991] except that the ROCSAT orbit traverses the magnetic field in the east‐west direction. The postmidnight group of depletions together with the large density dropout was observed to drift westward with the background plasma at ∼170 m s−1. We examine the properties of the large density dropout through the plasma data taken from the drift meter (DM) and retarding potential analyzer (RPA) sensors in three consecutive orbits. The apparent widening of the density depletion is explained by the different locations of the satellite within the depletion region. The extreme low‐density level inside the upwelling depletion indicates that of the nighttime F‐peak passed over the ROCSAT orbit at 600 km. The existence of NO+ ions inside the dropout is consistent with such an elevation of the F‐peak.

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.

Characteristics of magnetosheath plasma in the vicinity of the high-altitude cusp

The gas dynamic and global MHD models predict magnetosheath plasma and magnetic field properties near the magnetopause. Because of the way the models include the magnetic field, predictions from gas dynamic theory and MHD simulations will differ in regions and under conditions when the magnetosheath magnetic field plays an important role. This paper compares predictions from the gas dynamic model and an MHD simulation with observations from Interball Tail and Polar in the vicinity of the high-latitude magnetopause and cusp during a period of strongly northward IMF. In this region and under this condition, the magnetic field plays an important role in the process of plasma depletion and magnetic reconnection. All plasma characteristics except one are found to deviate in predictable ways from the gas dynamic model and are found generally to be more consistent with the MHD simulation. The notable exception is the plasma bulk flow velocity observed at the spacecraft nearest the noon–midnight meridian.