Magnetospheric Boundary Layer Research Articles

First simultaneous observations of flux transfer events at the high-latitude magnetopause by the Cluster spacecraft and pulsed radar signatures in the conjugate ionosphere by the CUTLASS and EISCAT radars

Abstract. Cluster magnetic field data are studied during an outbound pass through the post-noon high-latitude magnetopause region on 14 February 2001. The onset of several minute perturbations in the magnetospheric field was observed in conjunction with a southward turn of the interplanetary magnetic field observed upstream by the ACE spacecraft and lagged to the subsolar magnetopause. These perturbations culminated in the observation of four clear magnetospheric flux transfer events (FTEs) adjacent to the magnetopause, together with a highly-structured magnetopause boundary layer containing related field features. Furthermore, clear FTEs were observed later in the magnetosheath. The magnetospheric FTEs were of essentially the same form as the original "flux erosion events" observed in HEOS-2 data at a similar location and under similar interplanetary conditions by Haerendel et al. (1978). We show that the nature of the magnetic perturbations in these events is consistent with the formation of open flux tubes connected to the northern polar ionosphere via pulsed reconnection in the dusk sector magnetopause. The magnetic footprint of the Cluster spacecraft during the boundary passage is shown to map centrally within the fields-of-view of the CUTLASS SuperDARN radars, and to pass across the field-aligned beam of the EISCAT Svalbard radar (ESR) system. It is shown that both the ionospheric flow and the backscatter power in the CUTLASS data pulse are in synchrony with the magnetospheric FTEs and boundary layer structures at the latitude of the Cluster footprint. These flow and power features are subsequently found to propagate poleward, forming classic "pulsed ionospheric flow" and "poleward-moving radar auroral form" structures at higher latitudes. The combined Cluster-CUTLASS observations thus represent a direct demonstration of the coupling of momentum and energy into the magnetosphere-ionosphere system via pulsed magnetopause reconnection. The ESR observations also reveal the nature of the structured and variable polar ionosphere produced by the structured and time-varying precipitation and flow.Key words. Ionosphere (auroral ionosphere) Magentospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions)

Optical and particle signatures of magnetospheric boundary layers near magnetic noon: Satellite and ground‐based observations

In this paper we present a set of satellite and ground‐based observations suggesting that energetic magnetospheric electrons cannot be used as an unambiguous discriminator between open and closed field lines on the dayside. Using two data sets from the Defense Meteorological Satellite Program (DMSP) F13 and NOAA 12 satellites flying through dayside Type 1 cusp aurora (both close in time and space), we reach two apparently incompatible conclusions. Cusp/mantle precipitation, stepped cusp signatures, and antisunward convection in the DMSP F13 data set strongly suggest open magnetic field lines. On the other hand, NOAA 12 observed a mixture of magnetosheath and isotropic energetic particles. Trapped energetic electrons are traditionally regarded as being on closed flux. However, in addition to earlier proposed trapping on open field lines, we suggest that transmission lines connecting merging sites near the cusp in the Southern Hemisphere with the northern auroral ionosphere can be several tens of RE long. Alfvén wave transit times of several minutes may make it impossible to determine from satellite measurements in the ionosphere whether magnetic field lines threading low‐latitude boundary layer (LLBL) plasmas are open or closed. New research tools will be needed to unify understanding of complementary particle measurements from the DMSP and NOAA satellites.

Laboratory and Numerical Simulations of the Impulsive Penetration Mechanism

Plasma interaction at the interface between the magnetosheath and magnetosphere has been extensively studied during recent years. As a consequence various theoretical models have emerged. The impulsive penetration mechanism initially proposed by Lemaire and Roth as an alternative approach to the steady state reconnection, is a non-stationary model describing the processes which take place when a 3-D solar wind plasma irregularity interacts with the outer regions of the Earth's magnetosphere. In this paper we are reviewing the main features of the impulsive penetration mechanism and the role of the electric field in driving impulsive events. An alternative point of view and the controversy it has raised are discussed. We also review the numerical codes developed to simulate the impulsive transport of plasma across the magnetopause. They have illustrated the relationship between the magnetic field distribution and the convection of solar-wind plasma inside the magnetosphere and brought into perspective non-stationary phenomena (like instabilities and waves) which were not explicitly integrated in the early models of impulsive penetration. Numerical simulations devoted to these processes cover a broad range of approximations, from ideal MHD to hybrid and kinetic codes. The results show the limitation of these theories in describing the full range of phenomena observed at the magnetopause and magnetospheric boundary layers.

On the Origin of High-Latitude Boundary Layer of the Earth's Magnetosphere

Localization of the reconnection region at the dayside magnetopause is among the unsolved problems of magnetospheric physics. There are two alternative models, one of which predicts the reconnection at the equatorial magnetopause, and the other predicts the reconnection in the region where the magnetic field of the solar wind flowing around the magnetosphere is antiparallel to the geomagnetic field. The statistical analysis carried out for 53 INTERBALL-1crossings of the high-latitude magnetopause in a special coordinate frame invariant with respect to the interplanetary conditions shows that the model of a reconnection in antiparallel fields agrees well with the experimental data.

Lightning whistler waves in the high‐latitude magnetosphere

Whistler waves caused by lightning are known to penetrate the ionosphere with significant wave amplitudes. Recent rocket experiments have shown that lightning from sources as far as 1000 km from the ionospheric rocket subtrack are easily seen. The spectral energy density of lightning in the ionosphere often has significant power at frequencies near and below 1 kHz. Such low‐frequency whistler waves can propagate all the way to the magnetopause boundary layer if they begin in the cusp or polar cap. Lightning is common over high‐latitude continental land masses in the summer months which places this wave source function well within range of the cusp and polar cap. Ray tracing studies using a global three‐dimensional two‐fluid code are conducted to investigate the propagation of these whistler waves into the high latitude magnetosphere. The estimated, mapped lightning whistler wave amplitude is compared to wave intensity measurements from ISEE and AMPTE and supports the argument that falling tone whistler waves, seen by Geotail near the dayside magnetopause, may have been from intense lightning. It is found that the mapped whistler wave amplitude from lightning is comparable to the in situ wave amplitudes measured in the outer dayside magnetosphere and low latitude boundary layer.

Footprints of lobe reconnection observed in ionospheric electron density under steady northward IMF

The paper focuses on ionospheric signatures of lobe reconnection near magnetic noon under steady northward IMF. Enhanced Type 2 aurora observed by a meridian scanning photometer and reverse ion energy dispersion measured during a DMSP satellite pass provide evidence for the reconnection. Tomographic imaging of the spatial plasma distribution reveals key footprints of the processes, including the location of the adiaroic boundary separating the lobe cells from the closed LLBL precipitation, the dispersion of precipitating soft ions along sunward convecting field lines, and the possible presence of a Birkeland current sheet at the adiaroic boundary. The chain of ionospheric tomography receivers from Tromsø to Svalbard provides a powerful tool for investigation of the ionospheric response to magnetospheric boundary layer processes and their coupling to the ionosphere.

Magnetospheric plasma regimes identified using Geotail measurements: 1. Regime identification and distant tail variability

For many important scientific objectives, the analysis and interpretation of magnetospheric data sets require a sorting by primary plasma regimes. This sorting process is much more difficult in the distant magnetotail than for data sets sampled close to Earth. We have carried out systematic regime identifications for the entire deep‐tail portion of Geotail data at timescales down to ∼l min. Criteria were developed for selection of five basic plasma regimes: plasma sheet, lobe, magnetospheric boundary layer, magnetosheath, and solar wind. In addition to low‐energy plasma data central to the identification process, critical supplementary information derives from magnetic field and plasma wave data. We find that all basic plasma regimes remain physically distinct populations at all downtail distances sampled by the Geotail spacecraft. Preliminary automated separation of subregimes using a B versus δB method shows that plasma mantle intervals increase in probability of occurrence from 18% near Earth to about 50% near 205 RE. In contrast, plasma sheet intervals decrease overall with increasing tailward distance. Beyond about X = −100 RE, the frequency of short regime intervals steadily increases, overall variability increases, and the lobe and plasma sheet are progressively displaced by the boundary layer, here mostly plasma mantle. Our plasma regime identifications provide a critical baseline for the development of automated identification and a key resource for both Geotail data analysis and correlative studies between Geotail and other International Solar Terrestrial Physics mission data sets.

Magnetospheric plasma regimes identified using Geotail measurements: 2. Statistics, spatial distribution, and geomagnetic dependence

We investigate the spatial distribution, solar‐terrestrial correlation, and basic statistics of a long sequence of plasma regime identifications through the probability of observation P at downtail distances of ∼30 < xagsm < 210 RE, in an aberrated GSM coordinate system. The data are from Geotail spacecraft measurements obtained from October 1992 through October 1994. Earth's local plasma environment has been classified into five principal plasma regimes: PS, the plasma sheet, LB, the lobe (vacuum lobe, with only trace plasma content), and BL, the magnetospheric boundary layer, which together comprise the magnetosphere proper, and MS, the magnetosheath, and SW, the nearby solar wind. Eastman et al. [this issue] describe the process, rationale, and instrumentation used in the regime identifications. In the middle magnetotail (30 < xagsm < 100 RE): At geomagnetic activity index values Kp < 3o, (a) The average magnetospheric morphology appears nominal, with no large‐scale dawn‐dusk asymmetries; (b) PS is the regime most likely to be observed, and the spatial interrelationships of the PS, LB, BL, and MS appear similar to those commonly observed within −20 RE of Earth; that is, the magnetosphere is inside and the magnetosheath is outside; and (c) P(BL) increases gradually with increasing xagsm. At Kp ≥ 3o, (d) The centroid of P(PS) lies duskward of the XAGSM axis, producing a distinct dawn‐dusk asymmetry; (e) P(BL) increases sharply at xagsm > 60 RE and remains high tailward; and (f) At xagsm < 75 RE, MS is rarely observed for |YAGSM| < 20 RE. In the distant magnetotail (xagsm > 100 RE): (a) Conditions appear more chaotic. PS, BL, or MS can be observed anywhere inside the nominal magnetotail domain; (b) Compared to the middle magnetotail, PS and BL are observed more dawnward of the XAGSM axis and P(PS) is significantly lower (most distinctly at Kp > 3o); (c) P(PS) is centered dawnward of the XAGSM axis for both low and high Kp; and (d) At xagsm ≥ 170 RE, P(BL) is greater than P(PS) or P(MS), with P(MS) ≈ P(PS). P(LB) > 0, but low, out to xagsm ≥ 200 RE. In general, the downtail distributions of the magnetospheric plasma regimes depend more strongly on xagsm than on Kp, however the YAGSM distribution of P(PS) depends strongly with both xagsm and Kp. Magnetotail width varies weakly with xagsm and Kp.

Comment on “A quantitative assessment of energy storage and release in the Earth's magnetotail”, by D. N. Baker, T. I. Pulkkinen, M. Hesse, and R. L. McPherron

Journal of Geophysical Research: Space PhysicsVolume 103, Issue A8 p. 17729-17732 CommentariesFree Access Comment on “A quantitative assessment of energy storage and release in the Earth's magnetotail”, by D. N. Baker, T. I. Pulkkinen, M. Hesse, and R. L. McPherron Walter J. Heikkila, Walter J. HeikkilaSearch for more papers by this author Walter J. Heikkila, Walter J. HeikkilaSearch for more papers by this author First published: 01 August 1998 https://doi.org/10.1029/98JA01039Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat References Axford, W. I., C. O. Hines, A unifying theory of high-latitude geophysical phenomena and geomagnetic storms, Can. J. Phys., 39, 1433, 1961. Baker, D. N., T. I. Pulkkinen, M. Hesse, R. L. McPherron, A quantitative assessment of energy storage and release in the Earth's magnetotail, J. Geophys. Res., 102, 7159, 1997. Cole, K. D., On solar wind generation of polar geomagnetic disturbances, Geophys. J. R.A.S., 6, 103, 1961. Dungey, J. W., Interplanetary magnetic field and the auroral zones, Phys. Rev. Lett., 6, 47, 1961. Eastman, T. E., E. W. Hones Jr., S. J. Bame, J. R. Asbridge, The magnetospheric boundary layer: Site of plasma, momentum, and energy transfer from the magnetosheath into the magnetosphere, Geophys. Res. Lett., 3, 685, 1976. Heikkila, W. J., Magnetospheric topology of fields and current, Magnetospheric Currents, Geophys. Monogr. Ser., 28 T. Potemra, 208, AGU, Washington, D. C., 1984. Heikkila, W. J., Current sheet crossings in the distant magnetotail, Geophys. Res. Lett., 15, 299, 1988. Heikkila, W. J., Interpretation of recent AMPTE data at the magnetopause, J. Geophys. Res., 102, 2115, 1997a. Heikkila, W. J., Comment on “The alternative paradigm for magnetospheric physics” by E. N. Parker, J. Geophys. Res., 102, 9651, 1997b. Heikkila, W. J., Cause and effect at the magnetopause, Space Sci. Rev., 1998. Iijima, T., T. A. Potemra, Large-scale characteristics of field-aligned currents associated with substorms, J. Geophys. Res., 83, 599, 1978. Parker, E. N., The alternative paradigm for magnetospheric physics, J. Geophys. Res., 101, 10587, 1996. Phan, T.-D., G. Paschmann, The low-latitude dayside magnetopause and boundary layer for high magnetic shear, J. Geophys. Res., 101, 7801, 1995. Pulkkinen, T. I., D. N. Baker, C. J. Owen, J. A. Slavin, A model for the distant tail field: ISEE-3 revisited, J. Geomagn. Geoelectr., 48, 455, 1996. Slavin, J. A., E. J. Smith, D. G. Sibeck, D. N. Baker, R. D. Zwickl, S.-I. Akasofu, An ISEE-3 study of average and substorm conditions in the distant magnetotail, J. Geophys. Res., 90, 10875, 1985. Wangness, R., Magnetic energy, Electromagnetic Fields, 284, John Wiley, New York, 1986. Yamamoto, T., K. Shiokawa, S. Kokobun, Magnetic field structures of the magnetotail as observed by GEOTAIL, Geophys. Res. Lett., 21, 2875, 1994a. Yamamoto, T., A. Matsuoka, K. Tsuruda, H. Hayakawa, A. Nishida, M. Nakamura, S. Kokubun, Dense plasmas in the distant magnetotail as observed by GEOTAIL, Geophys. Res. Lett., 21, 2879, 1994b. Citing Literature Volume103, IssueA81 August 1998Pages 17729-17732 ReferencesRelatedInformation

The Geospace Modeling Program Grand Challenge

The first Geospace Modeling Program (GEM) campaign, the boundary layer campaign, focused on the cusp, the dayside magnetopause, and dayside magnetospheric boundary layers. One of the projects pursued during this campaign was a construction of synoptic maps of convective flows and particle regions within the polar ionosphere for different orientations of the interplanetary magnetic field. In 1996, the GEM steering committee recommended that these synoptic maps be used for data‐model comparisons to provide tests of model convection patterns, convection strengths, separatrix locations, boundary layers, and currents. The papers that follow this introductory paper describe results of the data‐model comparisons. This paper summarizes the data analysis approach and the results that were obtained that are important for the comparisons.

Dayside auroral hydrogen emission as an aeronomic signature of magnetospheric boundary layer processes

Doppler spectral profiles of dayside auroral hydrogen show sudden increases in precipitating proton energy by a broadening of the spectral profile towards shorter wave‐lengths. This coincides with the equatorward motion of the dayside aurora. A model of the expected spectral character and spatial distribution of the H emission shows that the H emission spectrum is a direct measure of the reconnection process at the dayside magnetopause, i.e., 1) the narrow, symmetric spectral profiles are a measure of the magnetospheric “velocity filter”, 2) the sudden spectral shift of the H emission profiles reflect changes of ion energy due to an increase in the rate of reconnection, 3) spectral H emission observations could be used to map the spatial distribution of the precipitating ion energy, the total reconnection rate, the reconnection sites on the magnetopause, and the direction and intensity of convection.

Energetic particle bursts in the predawn Jovian magnetotail

From September to October 1996 the Galileo spacecraft crossed through the distant predawn tail region of the Jovian magnetosphere. The Energetic Particles Detector (EPD) onboard Galileo recorded a series of energetic particle flow bursts in the region beyond 80 RJ to the apojove at 113 RJ. The events are similar in nature to an event observed with the hot plasma instrument (LECP) onboard Voyager 2. The individual events last for several hours and cover the whole energy range from 15 keV to 55 MeV. The majority of them show considerable intensity increases which are most prominent for heavy ions. The events exhibit high radially outward directed anisotropies suggesting strongly collimated radial outflowing ion beams. The Voyager event was observed beyond the corotation boundary within a magnetospheric boundary layer termed the magnetospheric wind region and consequently it was assumed that the underlying process is connected with a boundary layer instability. However, the Galileo observations show the bursts being embedded in a general corotation flow. It is thus suggested that the flow bursts are driven by an internal plasma sheet process possibly associated with a major re‐configuration of the Jovian magnetotail. A series of five very prominent flow bursts observed near apojove of the orbit occurred quasi‐periodically with a repetition period of about 2.6±0.2 days which is presumably related to an internal time constant of the Jovian magnetotail.

First results from the plasma composition spectrometer PROMICS-3 in the Interball project

Abstract. PROMICS-3 is a plasma experiment flown in the Russian project Interball. It performs three-dimensional (3D) measurements of ions in the energy range 4 eV–70 keV with mass separation and of electrons in the energy range 12 eV–35 keV. The Interball project consists of two main satellites, the Tail Probe and the Auroral Probe, each with one subsatellite. The Interball Tail Probe was launched on 3 August 1995, into a 65° inclination orbit with apogee at about 30 RE. Both main satellites carry identical PROMICS-3 instruments and thus direct comparisons of the particle distributions will be possible once the Auroral Probe is launched. Furthermore, PROMICS-3-Tail is the first instrument measuring the 3D ion distribution function in the magnetospheric boundary layers at high latitudes. In this paper we describe the PROMICS-3 instrument and show initial results from the Tail probe, measurements of the magnetosheath, plasma sheet, and ring current plasmas.

The INTERBALL-Tail ELECTRON experiment: initial results on the low-latitude boundary layer of the dawn magnetosphere

Abstract. The Toulouse electron spectrometer flown on the Russian project INTERBALL-Tail performs electron measurements from 10 to 26 000 eV over a 4<pi> solid angle in a satellite rotation period. The INTERBALL-Tail probe was launched on 3 August 1995 together with a subsatellite into a 65° inclination orbit with an apogee of about 30 RE. The INTERBALL mission also includes a polar spacecraft launched in August 1996 for correlated studies of the outer magnetosphere and of the auroral regions. We present new observations concerning the low-latitude boundary layers (LLBL) of the magnetosphere obtained near the dawn magnetic meridian. LLBL are encountered at the interface between two plasma regimes, the magnetosheath and the dayside extension of the plasma sheet. Unexpectedly, the radial extent of the region where LLBL electrons can be sporadically detected as plasma clouds can reach up to 5 RE inside the magnetopause. The LLBL core electrons have an average energy of the order of 100 eV and are systematically field-aligned and counterstreaming. As a trend, the temperature of the LLBL electrons increases with decreasing distance to Earth. Along the satellite orbit, the apparent time of occurrence of LLBL electrons can vary from about 5 to 20 min from one pass to another. An initial first comparison between electron- and magnetic-field measurements indicates that the LLBL clouds coincide with a strong increase in the magnetic field (by up to a factor of 2). The resulting strong magnetic field gradient can explain why the plasma-sheet electron flux in the keV range is strongly depressed in LLBL occurrence regions (up to a factor of \\sim10). We also show that LLBL electron encounters are related to field-aligned current structures and that wide LLBL correspond to northward interplanetary magnetic field. Evidence for LLBL/plasma-sheet electron leakage into the magnetosheath during southward IMF is also presented.

Traversal of the nightside magnetosphere at 10 to 15 Re during northward IMF

The nightside magnetosphere under the northward IMF condition is not flat at the distance of 10 to 15 Re but reveals a variety of interesting features. The magnetospheric boundary layer consists of the tailward flowing outer layer and the earthward flowing inner layer. In the dawn as well as the dusk flank regions there are large amplitude oscillations with periods of 8 to 15 min which show the character of the slow mode or the mirror mode waves. Moreover, around the midnight meridian the ions and electrons are injected earthward at intervals of 1 to 2 hours. The westward drift of the injected ions produces the energy‐dispersive ion signatures in the dusk region. The electric field Ey is positive on average at this distance while in the distant tail it has been found to be negative on average. These observations of Ey are compatible with the magnetic reconnection in the magnetotail when the twisting of the neutral sheet is taken into account.

Tearing instability, Kelvin‐Helmholtz instability, and magnetic reconnection

Two closely related macroscopic instabilities at the magnetospheric boundary layer are the tearing instability and the Kelvin‐Helmholtz (KH) instability. It has been suggested that a coupling of these modes may lead to larger reconnection rates and thus may explain magnetic flux transfer events at the dayside magnetopause. In the framework of two‐dimensional compressible MHD simulations we study the linear and nonlinear evolution of these instabilities. When the velocity of shear flow is below the Alfvén speed, it is verified that the tearing mode is stabilized by the flow shear and the reconnection rate is reduced. For super‐Alfvénic shear velocities the tearing mode ceases its growth, and the KH instability develops consistently with analytical results. A reasonably low resistivity has no significant influence on the growth of the KH mode. Only for nonlinear amplitudes of the KH mode is reconnection driven by the KH vortices. Thus our results do not confirm a linear interaction between the KH and the tearing mode in terms of vortex‐induced reconnection in two dimensions. Particularly, a reasonably small resistivity does not lower the threshold velocity for the KH instability to a level at which it could be excited for typical magnetopause conditions. Therefore the evolution of the KH mode (and vortex‐induced reconnection) with k vectors aligned with a southward interplanetary magnetic field component appears rather unrealistic for the subsolar magnetopause where the magnetosheath flow is slow.

The INTERBALL-Tail ELECTRON experiment: initial results on the low-latitude boundary layer of the dawn magnetosphere

The Toulouse electron spectrometer flown on the Russian project INTERBALL-Tail performs electron measurements from 10 to 26 000 eV over a 4<pi> solid angle in a satellite rotation period. The INTERBALL-Tail probe was launched on 3 August 1995 together with a subsatellite into a 65° inclination orbit with an apogee of about 30 RE. The INTERBALL mission also includes a polar spacecraft launched in August 1996 for correlated studies of the outer magnetosphere and of the auroral regions. We present new observations concerning the low-latitude boundary layers (LLBL) of the magnetosphere obtained near the dawn magnetic meridian. LLBL are encountered at the interface between two plasma regimes, the magnetosheath and the dayside extension of the plasma sheet. Unexpectedly, the radial extent of the region where LLBL electrons can be sporadically detected as plasma clouds can reach up to 5 RE inside the magnetopause. The LLBL core electrons have an average energy of the order of 100 eV and are systematically field-aligned and counterstreaming. As a trend, the temperature of the LLBL electrons increases with decreasing distance to Earth. Along the satellite orbit, the apparent time of occurrence of LLBL electrons can vary from about 5 to 20 min from one pass to another. An initial first comparison between electron- and magnetic-field measurements indicates that the LLBL clouds coincide with a strong increase in the magnetic field (by up to a factor of 2). The resulting strong magnetic field gradient can explain why the plasma-sheet electron flux in the keV range is strongly depressed in LLBL occurrence regions (up to a factor of \sim10). We also show that LLBL electron encounters are related to field-aligned current structures and that wide LLBL correspond to northward interplanetary magnetic field. Evidence for LLBL/plasma-sheet electron leakage into the magnetosheath during southward IMF is also presented.

Characterization of the dynamic variations of the dayside high‐latitude ionospheric convection reversal boundary and relationship to interplanetary magnetic field orientation

We present initial results from observations of the northern summer polar ionospheric convection reversal boundary using ground‐ and satellite‐based instrumentation. Ionospheric convection measurements obtained using the Sondrestrom radar are used to locate and observe the boundary. The flow around the reversal is compared to three different modeled flow patterns. The first is a shear reversal (oppositely directed flows with no flow across the boundary). The second is a shear reversal combined with uniform poleward flow. The final flow pattern observed is a rotational reversal, where flows gradually turn from one direction to the opposite over distances on the order of 200–400 km. The convection reversals observed were categorized into three different classes: (1) stationary and steady, (2) nonstationary, and (3) oscillating. A stationary and steady boundary remains at the same invariant latitude for long periods of time and demonstrates no observable motion. A nonstationary boundary will propagate northward or southward, generally remaining parallel to a line of invariant latitude. The oscillating reversal boundary (ORB) will have wave‐like motions of the local boundary location. A number of different reversals were classified and then studied further using other instrumentation, which include the Greenland coastal and MAGIC chains of magnetometers, the drift meter and particle precipitation instruments from the DMSP F9, F10, and F11 satellites, and interplanetary magnetic field (IMF) measurements from the IMP 8 satellite. A stationary and steady reversal has been observed during a time of magnitude variations in the By and Bz components of the IMF, but no sign changes in either of the components. The nonstationary reversals have been observed to be a response to both sign changes and magnitude changes with no sign changes in IMF components. Of the two ORBs reported here, one has been determined to be a ULF wave propagating along the convection reversal boundary, which maps to the magnetospheric low latitude boundary layer. The other ORB has little coherence between longitudinally spaced magnetometer stations, implying that the propagating waves are changing form between the stations.

Energetic ion and electron observations at Jupiter's dayside magnetopause: implications for magnetopause location and boundary coupling processes

An analysis is presented of the fluxes and anisotropies of energetic (MeV) ions and electrons measured by instruments of the COSPIN package during the first inbound traversal of the Jovian dayside magnetospheric boundary region by the Ulysses spacecraft in February 1992. The aim of the study is to elucidate the nature of the multiple structures observed and reported previously in magnetic field, plasma and wave data. The combined data suggest the presence of a layer of depleted magnetosheath density which lay outside but adjacent to the magnetopause, and a boundary layer which lay within. The width of each of these structures is inferred to be ∼4 R J, assuming an outward boundary expansion speed of ∼100 km s −1. The magnetic field rotated slowly (over ∼10 min) towards the magnetospheric orientation just outside the depletion layer, and subsequently (after ∼50 min) turned rapidly through a small angle (∼20°) to point almost due south at the magnetopause. The major changes in energetic particle properties between interplanetary and outer magnetospheric values took place at the latter boundary. Energetic ion fluxes decreased sharply by a factor of ∼2 from interplanetary to magnetospheric values, while energetic electron fluxes increased by about an order of magnitude. Detailed analysis of the ion distributions indicates that the magnetopause was locally closed at the time of the crossing, i.e. a tangential discontinuity, as expected from the small magnetic shear present. Outside the magnetopause the electron fluxes had previously increased from background values by about two orders of magnitude over a 2 h period as the magnetosphere approached, indicating an escaping magnetospheric population in the magnetosheath similar to that at Earth. These electrons generally streamed antiparallel to the field away from the subsolar region, opposite to the observed streaming direction of the interplanetary ions in the magnetosheath. Inside the magnetospheric boundary layer the energetic ions and electrons exhibited weaker variable counterstreaming along the field lines, suggesting an intermittent presence of open flux tubes which were connected through the magnetopause first to the south and then to the north of the spacecraft. It is suggested that these changes were associated with switches in the east-west magnetic field in the magnetosheath, a suggestion which is supported by the observed magnetic field perturbations and field-per-pendicular ion flows within the layer. The fact that the spin-averaged electron flux did not show significant changes despite the field-aligned streaming is suggested to be due to a sufficiently rapid cross-field drift of these particles. In the outer magnetosphere proper the ion and electron distributions finally approach isotropy.

Magnetopause crossings without a boundary layer

The microstructure of pristine magnetopause crossings has been analyzed by using high‐resolution particle and field data obtained by the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) and ISEE 2 spacecraft. These crossings are pristine in the sense that they exhibit no adjoining magnetospheric boundary layer, or, at most, a low‐density plateau. The CCE crossings include the low‐latitude near‐noon region not typically sampled by ISEE 2, which covers all other local time sectors in a complementary way. Magnetopause crossings without a boundary layer are found to occur to all local times, and such crossings constitute about 10% of all magnetopause crossings. Total pressure balance across the magnetopause is observed to within experimental errors; however, electron data, full‐energy composition measurements, and occasionally field stress are needed to fully evaluate pressure balance. The microstructure of the magnetopause current layer is also found to depend on local time. Crossings within about 1 hour local time of the noon meridian often exhibit very sharp density gradients on scale lengths down to a few plasma skin depths. These gradients are reduced for crossings farther from local noon such that, for cases near the dawn‐dusk meridian, the scale length for density gradients and the magnetopause current are roughly comparable. Magnetopause crossings without a boundary layer impose severe constraints on various theories of boundary layer formation. Pristine magnetopause crossings may be direct cuts through the diffusion region for reconnection. With this interpretation our results are in qualitative agreement with recent simulations of the diffusion region and associated turbulence by Drake et al. [1994], who propose the current convective instability as the dominant process for current transport at the magnetopause.